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An X ray search for active cores in Local Group galaxies


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Local G roup G alaxies


Zhiyun Zang


A Thesis subm itted to

The University of Dubhn

for the degree of

Doctor in Philosophy

D epartm ent of Physics

University of Dublin

Trinity College

oC: <z>


This thesis has not been subm itted as an exercise for a degree a t any other University. Except the following listed articles and where otherwise stated , the work described herein has been carried out by the au th o r alone. This thesis may be borrowed or copied upon request w ith the permission of the Librarian, University of Dublin, Trinity College. The copyright belongs jointly to the University of Dublin, Dunsink O bservatory and Zhiyun Zang.

R O S A T P SP C observations of three Sculptor Group galaxies: N G C 55, N G C 247 and N G C 300

Z. Zang, R. S. Warwick and E. J. A. Meurs, 1997, Irish A. J., 24, 45. R O S A T H R I and P S P C observations of M32

Z. Zang and E. J. A. Meurs, 1998, in The Stellar C ontent of Local Group Galaxies, lAU Symp. 192, Cape Town, South Africa, eds. P. W hitelock and R. Cannon, p334.

The case fo r a low-level active nucleus in M32 Z. Zang and E. J. A. Meurs, 1999, New A ., in press.

X-ray candidates fo r a population o f nuclear cores in Local Group galaxies E. J. A. Meurs and Z. Zang, 1999, in Black holes in Binaries and Galactic Nuclei, ESO Workshop, Garching, Germany, in press.

Signature of A uthor

Zhiyun Zang


Sum m ary

The nuclei of galaxies are sometimes sites of very energetic process, resulting in high levels of electrom agnetic radiation being em itted. Such active galactic nuclei (AGNs) are thought to be powered by accretion of mass onto a centrally collapsed object, a superniassive black hole of typically 10® — 10® solar masses. W hen gravitational energy of the accreting m aterial is liberated it is radiated away, causing the nucleus to appear very bright. The extrem e m anifestation of this phenom enon is probably seen in the famous Quasi Stellar Objects.

In this thesis, a dedicated search for X-ray emission from the cores in the Local G roup galaxies is performed to establish w hether low levels of nuclear activity in these nearest galaxies may be found. The d a ta are from ROSAT HRI or PSPC pointing observations which are available for 21 Local G roup galaxies in the centre of the observing fields, and for another 3 in the outer area, covering 70% of all Local G roup members. The HRI d a ta are p articular useful, since the nuclear source may be b etter resolved from the HRI images.

M31 is the brightest galaxy of the group, in which a source has been detected coincident with its nucleus. M32, a satellite galaxy of M31, is a small Elliptical with a clear central concentration. The HRI observation in 1994 shows a little extension

2.7") of the X-ray source in M32, PSPC spectral models give higher absorption columns than the G alactic one; no choice can be made between Power Law or Therm al Brem sstrahlung models. In the view of the small source extension and still uncertain central or non-central position of the source, conceivable interpretations are either a Supernova R em nant or a mini-AGN. An ASCA spectrum and a radio upper limit suggest th a t a low-level AGN is a more attractive explanation.


w ith help of d a ta from other wavebands and from new high-energy measurements. Among the rest of the Local G roup galaxies, no sources are detected in their central area. The upper limits of the X-ray luminosities derived for the central regions show th a t most dwarf ElUpticals has little X-ray emission {Lx < 10^^ ergs s “ ^). The results on the Local Group members suggest th a t galaxies w ith to tal blue m agnitude Lb — 10* Lq^b or more may have some central X-ray emission at the level of the central object in the Milky Way.

For th e sake of comparison, ROSAT observation of the next nearest group of galaxies, the Sculptor Group, were also examined. Among 5 large members of Sculptor G roup galaxies, NGC 253 is a well-known starb u rst galaxy; NGC 7793 has some diffuse emission detected all over the galaxy b u t w ithout significant enhance­ m ent near the centre position. For NGC 55, 247 and 300 no sources are detected in the central regions, down to X-ray lum inosity L x — 10^® ergs s“ ^.

As seen from the example of M32, the X-ray luminosities of the central X-ray sources are several orders of m agnitude less th an typical Seyfert galaxies, which could be due to a lower black hole mass {e.g. ~ 10'* Mq) or a lower accretion rate. For such a low mass accretion rate, the stan d ard th in disk model for accretion is probably not applicable and the model for A dvection-D om inated Accretion Flows provides possible solutions, in which very little energy is radiated away during the accretion process. Masses for the central condensed objects have been determ ined for the Milky Way and M32. In both cases the accretion rate is extrem ely low and well into th e expected Advection D om inated Accretion Flow regime.


Acknowledgem ents

Firstly, I would like to thank my supervisor, Professor Evert Meurs, for all his help

and advise during these years. I am very grateful to him for proof-reading this thesis

as it was being w ritten and providing me with invaluable com m ents on it. Thanks

are also due to Dr. Sara McMurry, my internal supervisor in Physics D epartm ent

of TCD , for all her help. I would like to th an k Dr. L aura Norci for her advise on

m any aspects of my research work during these years. I am very grateful to Michael

C arr for his comments on p art of my transfer report which was th e startin g point of

this thesis. I am also very grateful to Dr. Stefan Ddbereiner (M PE, Garching) for

correcting one of the ROSAT d a ta sets for this study. I would like to acknowledge

th e financial support given to me by the Dublin In stitu te for Advanced Studies

and by Forbairt under the Basic Research G ran t Scheme. This research is based

m ainly on the d a ta from ROSAT Public D ata Archive and th e M IDAS/EXSAS

software. The Starlink and the XANADU are also used for d a ta reduction. The

databases from NED, SIMBAD, ADS and Skyview are used as well to obtain the

various astronom ical informations. Thanks to all the people who make those services

available. This research is carried out at D unsink Observatory, Dublin, and I am

very grateful to everyone a t the O bservatory who helped me during these years,

especially Wai Ming Tai, Mike Smyth and John Cunniffe who created the com puter

system ‘giacobini’ and kept it running for me. I am very grateful to the Head of the

Observatory, Professor Evert Meurs, for the possibility to stay in th e Observatory,

otherwise it would not have been possible to sta rt this study. I would like to thank

all my friends for their encouragement they gave me during these years. Finally, I

would like to thank my family, and especially my wife, Xiaoqin Wu and my daughter,


AGN Active G alactic Nuclei

ASCA Advanced Satellite for Cosmology and A strophysics AXAF Advanced X-ray A stronom y Facility

BH Black Hole

CDS Strasbourg astronom ical D ata Centre EXSAS Extended Scientific Analysis System

FOV Field of View

HR Hardness R atio

HRI High Resolution Imager HST Hubble Space Telescope IPC Imaging Proportional Counter

LINER Low-Ionization Nuclear Emission-line Region

MC Magellanic Cloud

MDO Massive D ark O bject

ML M aximum Likelihood

MFC M onitor Proportional Counter

M PE Max Planck In stitu te for E x traterrestrial Physics NED NASA/IPAC Extragalactic D atabase

P E T Photon Event Table PSF Point Spread Function

PSPC Position Sensitive Proportional C ounter ROSAT Rontgen Satellite

SASS Standard Analysis Software System

SIMBAD Set of Identifications, M easurements and Bibliography for Astronomical D ata

SNR Supernova Rem nant

XMM X-ray M ulti-M irror


C ontents

D e c la r a tio n ii

S u m m a ry iii

A c k n o w le d g e m e n ts v

A b b r e v ia tio n s v i

1 I n tr o d u c tio n 1

2 A n o v e rv ie w o f th e L ocal G rou p o f g a la x ie s 4

3 K n o w n X -ra y P r o p e r tie s o f L ocal G rou p g a la x ie s 8

3.1 X-ray o b s e rv a tio n s ... 8

3.1.1 Einstein o b serv atio n s... 9

3.1.2 ROSAT o b serv atio n s... 10

3.2 M31 ... 13

3.3 M 3 3 ... 14

4 R O S A T o b servation s o f L ocal G roup g a la x ies 16 4.1 In tro d u c tio n ... 16

4.2 General X-ray data analysis for ROSAT o b s e rv a tio n s ... 18

4.3 M31 and M 3 3 ... 21

4.4 NGC 147 and 1 8 5 ... 24

4.5 IC 1613 ... 28

4.6 NGC 6822 ... 31

4.7 WLM ... 35

4.8 NGC 205 ... 37

4.9 Sextans A ...39

4.10 F o r n a x ... 44


4.12 Pegasus ...50

4.13 C a r i n a ... 52

4.14 S c u l p t o r ...54

4.15 Sextans ...56

4.16 Draco ... 57

4.17 IC 10 and the o t h e r s ... 60

5 M 32 62 5.1 In tr o d u c tio n ... 62

5.2 X-ray d a t a ... 63

5.3 Analysis of reprocessed PSPC observation ...66

5.3.1 M o rp h o lo g y ... 66

5.3.2 Spectrum ... 66

5.4 Analysis of HRI f r a m e s ... 72

5.4.1 Source detection ... 72

5.4.2 M o rp h o lo g y ... 73

5.4.3 X-ray position versus optical positions ... 76

5.4.4 The other HRI o b s e rv a tio n ... 77

5.5 D is c u s s io n ... 79

5.5.1 Residual emission close to the centre of M32? ...79

5.5.2 PSPC and HRI fluxes ... 81

5.5.3 N ature of the M32 source ... 82

6 T he p op u lation o f nuclear cores 85 6.1 In tr o d u c tio n ... 85

6.2 Results on cores from ROSAT o b se rv a tio n s... 86

6.2.1 Overview ... 86

6.2.2 Comments on d e te c tio n s ...87

6.2.3 Comments on no n-detection s... 89

6.2.4 X-ray Luminosities and galaxy m o rp h o lo g y ... 90

6.3 D is c u s sio n ... 91

6.3.1 M G Ns ...91

6.3.2 MDOs and B H s ... 93

6.3.3 Lim itations and further work ...93

6.3.4 S u m m a r y ... 95



7.2 NGC 5 5 ...98

7.2.1 PSPC o b se rv a tio n ... 98

7.2.2 HRI o b se rv a tio n ... 98

7.3 NGC 247 ... 102

7.3.1 PSPC o b serv atio n ... 102

7.3.2 HRI o b serv atio n ...103

7.4 NGC 300 104 7.4.1 PSPC o b serv atio n ... 104

7.4.2 HRI o b serv atio n ...104

7.5 NGC 7793 ... 105

7.6 D is c u s sio n ... 107

7.6.1 X-ray bright sources and lu m in o s itie s ... 107

7.6.2 X-ray emission from the c e n t r e s ...108


2.1 Projection of the galaxies in the Local Group onto a convenient plane. 6

4.1 ROSAT HRI image of the bulge in M 3 1 ... 23

4.2 ROSAT HRI contour plot of NGC 147 overlaid on optical image . . . 25

4.3 ROSAT HRI contour plot of NGC 185 overlaid on optical image . . . 27

4.4 ROSAT HRI image of IC1613 X-1 30

4.5 ROSAT contour plots of NGC 6822’s central 4' x 4' a r e a ... 34

4.6 ROSAT HRI contour plot of WLM overlaid on optical i m a g e ... 37

4.7 ROSAT HRI contour plot of NGC 205 overlaid on optical image . . . 40

4.8 ROSAT PSPC contour plot of Sextans A ... 42

4.9 ROSAT HRI contour plot of Sextans A overlaid on optical image . . 43

4.10 ROSAT PSPC contour plot of F o rn a x ...44

4.11 ROSAT PSPC contour plots of NGC 3 1 0 9 ...49

4.12 ROSAT PSPC contour plot of P e g a s u s ... 50

4.13 ROSAT PSPC contour plot of C a r i n a ... 52

4.14 ROSAT PSPC contour plot of S c u lp to r ... 56

4.15 ROSAT PSPC contour plot of S e x t a n s ... 58

4.16 ROSAT PSPC contour plot of D r a c o ... 60

5.1 ROSAT PSPC observations w ith M32 in the f i e l d ... 65

5.2 Contour plots of the X-ray source associated with M 3 2 ... 67

5.3 ROSAT PSPC ring profiles of M 3 2 ... 68

5.4 ROSAT PSPC spectra of M32 fitted with T herm al B rem sstrahlung and Power Law m o d e ls ... 70

5.5 ROSAT HRI contour plot of M32 overlaid on optical im a g e ... 74

5.6 ROSAT HRI ring profiles of M 3 2 ... 75

5.7 ROSAT HRI radial profiles of two sources in the sam e field of view as M 3 2 ... 75

5.8 ROSAT HRI integrated counts of M32 within 16" r a d i u s ...76



5.10 ROSAT HRI radial profiles of two sources in the field of 6009311i . . . 78 5.11 Source position affected by an additional source for various count

ratios from 12:12 to 1 2 : 1 ... 79 5.12 Examples for differences among positions given by local, m ap and ML

detection m e t h o d ... 80 5.13 X-ray brightness history of the M32 s o u r c e ... 81

6.1 Histogram of luminosities of Local G roup galaxy cores ...91 6.2 S tatus of detections of nuclear sources in Local Group members . . . 92 6.3 Local Group galaxies in L x — Lb diagram ... 94 7.1 NGC 55 ROSAT PSPC (hard band) image overlaid on the optical


2.1 Local Group g a la x i e s ... 5

3.1 Einstein sources in Local G roup g a l a x i e s ... 11

3.2 ROSAT PSPC C h a ra c te ris tic s ... 12

3.3 ROSAT HRI C h a ra c te ris tic s ... 12

4.1 ROSAT observation log of small Local G roup g a la x ie s ... 17

4.2 ROSAT HRI point sources of M31 in central a r e a ... 22

4.3 Possible point sources w ithin ROSAT HRI field of NGC 1 4 7 ... 24

4.4 Possible point sources w ithin ROSAT HRI field of NGC 1 8 5 ... 26

4.5 Point sources within ROSAT HRI field of IC 1 6 1 3 ... 29

4.6 Sources and count rates w ithin ROSAT PS PC field of NGC 6822 . . . 32

4.7 Sources in two ROSAT HRI observations of NGC 6822 ... 33

4.8 Sources and Count R ates w ithin ROSAT HRI field of WLM ... 36

4.9 Sources within ROSAT HRI field of NGC 205 ... 38

4.10 Possible optical counterparts w ithin ROSAT HRI field of NGC 205 . 39 4.11 Sources and count rates w ithin ROSAT P S PC field of Sextans A . . . 41

4.12 Sources and count rates within ROSAT HRI field of Sextans A . . . . 41

4.13 Sources within ROSAT PSPC field of F o r n a x ... 45

4.14 Sources and count rates within ROSAT PS PC field of NGC 3109: I . 47 4.15 Sources and count rates within ROSAT PS PC field of NGC 3109: II . 48 4.16 Sources and count rates within ROSAT PS PC field of Pegasus . . . . 51

4.17 Sources and count rates within ROSAT P S PC field of C a r in a ...53

4.18 Sources and count rates w ithin ROSAT PS PC field of Sculptor . . . . 55

4.19 Sources and count rates w ithin ROSAT PS PC field of Sextans . . . . 57

4.20 Sources and count rates w ithin ROSAT PS PC field of D r a c o ... 59

5.1 ROSAT observation log of M 3 2 ... 64

5.2 The spectral fitting param eters of M 3 2 ... 69



6.1 X-rays from the cores of the Local G roup g a la x i e s ... 88

7.1 Sculptor group g a la x ie s ... 97

7.2 ROSAT observation log of Sculptor Group g a l a x i e s ...97

7.3 ROSAT HRI point sources in the field of NGC 5 5 ...100

7.4 ROSAT HRI point sources in the field of NGC 247 ... 102

7.5 ROSAT HRI point sources in the field of NGC 300 ... 105



A ctive galactic nuclei

The active galaxies, as a distinct from normal galaxies, were discovered by Carl Seyfert. He found th a t some spiral galaxies possessed extraordinary bright and point-like nuclei. On short exposure photographs, these nuclei looked like stars but spectroscopic studies revealed they possessed strong and broad emission lines (Seyfert 1943). We use the term of AGN (Active G alactic Nuclei) to define these objects because it is the very central region of a galaxy th a t is active, not the entire galaxy.


C H A P T E R 1. IN TR O D U C TI O N 2

The weak activity in the G alactic nucleus suggests th a t all galaxies may have some form of nuclear activity. It is therefore of great interest to establish the preva­ lence of nuclear activity of any level among the galaxies. This is not only im p o rtan t for the dynamics and evolution of galaxies in general, it is also relevant to astro- physical and cosmological questions such as the faint tail of the AGN lum inosity function and, given the ubiquity of the smaller galaxies, the ionization flux in the universe and the expected contributions to the X-ray background {e.g. Griffiths and Padovani 1990).

The link between nuclear activity and X-ray emission is very well established. Probably, X-ray emission is one of the b etter ways to assess nuclear activity in galaxies, also when in other wavebands like optical there is no obvious central core. The Local Group galaxies are the nearest specimen to be examined and therefore allow low levels of nuclear activity to be detected. The ROSAT HRI d a ta (see 3.1) are particularly aj)propriate for an investigation of the nuclear regions of the Local G roup galaxies. The com bination of high spatial resolution of the detector and small distance of the targets is very advantageous. W hen no HRI d a ta are available, ROSAT PSPC d a ta are an alternative.

O rganization o f th is th esis

Following this introduction. Chapter 2 gives an overview of the Local G roup galaxies, including their optical contents and commonly used param eters.

The Chapter 3 summarizes the known X-ray properties of th e Local G roup galaxies. After a brief history of X-ray astronomy, the two m ajor X-ray imaging observatories th a t have flown until now, E instein and ROSAT, are described with characteristics of their detectors, and the observations of two large system s of the Local Group, M31 and MSS, are sum m arized in the end.


follow and their X-ray emission properties based on th e ROSAT d a ta are presented. Chapter 5 is all about the galaxy M32. This chapter is based on an article which has been accepted by “New Astronomy'^ and therefore is m ostly self-contained. It includes a review of the known X-ray properties of th e galaxy, then the improved

results of the ROSAT PSPC and HRI data, and a discussion of the possible natu re of the X-ray source in this galaxy.

Chapter 6 summarizes the X-ray results obtained for th e central regions of the Local Group galaxies for which X-ray d a ta are available. The overall X-ray charac­


C hapter 2

A n overview o f th e Local Group of


T h e Local G roup was first recognized by H ubble in th e early days of ex tra g alac tic research when distances to galaxies were first being m easured. T h ere was a d istin ct difference betw een those galaxies th a t resolved easily in to s ta rs an d those th a t did no t, im plying th a t th e M ilky Way is p a rt of a sm all local clu ster of galaxies, th a t condensed o ut of th e general expansion of th e U niverse. Now m ore th e n 30 m em bers have been recognized for th is co ndensation (Table 2.1, W h itin g et al. 1997a), which is called T he Local G roup (LG).

T h e M31 an d M ilky W ay galaxies are th e m o st lu m ino us an d m assive of the group. M31 is a H ubble Sb type, w ith a lum in ou s large bulge of old stars, sur­ roun ded by a less lum inous disk w ith gas, d u st a n d young sta rs, a rran g ed in spiral arm s. Its d iam eter is ~ 60 kpc and its m ass ~ 7 x 10^^ M q. W ith in it th e re are a b o u t 300 globular clusters, 400 open clusters, su p ern o v a re m n a n ts (SN Rs), and o th e r com ponents. T he M ilky W ay galaxy is a S b /S c ty p e , w ith som ew hat less- conspicuous central bulge and a b righ ter disc an d looser arm s th a n M31. It has a d ia m ete r of ~ 40 kpc and a m ass of 5 x 10® Mq. M33 is a n o th e r spiral galaxy (Sc) of th e group, b u t sm aller {d ~ 14 kpc) an d fain ter th a n th e above two. It contains m any blue sta rs in its com plex, thick sp iral arm s, a n d has several g ia n t H II regions.


Mag-Table 2.1: Local G roup galaxies

Name « 1 9 5 0h m


o t Type D{kpc) M v

Pegasus DDO 216 23 26.1 + 14 28 Irr 1800 -1 4 .6

Sextans B DDO 70 09 57.4 +05 34 Irr 1300 -1 4 .3

Sextans A DDO 75 10 08.6 - 0 4 28 Irr 1300 -1 4 .2 NGC 3109 DDO 236 10 00.8 - 2 5 55 Irr 1260 -1 5 .8

1C 10 00 17.7 +59 01 Irr 1250 -1 7 .6

SagDlG 19 27.9 - 1 7 47 Irr 1150 -1 1 .0

Antlia 10 01.8 - 2 7 05 dE3 1150 -1 0 .7

WLM DDO 221 23 59.4 - 1 5 45 Irr 940 -1 4 .0

Tucana 22 38.5 - 6 4 41 dE5 900 -9 .6

EGB0427+63 U G C -A 92 04 27.4 +63 30 Irr 800 -1 0 .0

M33 NGC 598 01 31.1 +30 24 Sc 795 -1 8 .9

And 1 00 43.0 +37 44 dEO 790 -1 1 .7

And 111 00 32.6 +36 12 dE6 790 -10 .2

1C 1613 DDO 8 01 02.2 +01 51 Irr 765 -1 4 .9

LGS 3 01 01.2 +21 37 Irr/d E 760 - 9 .7

M31 NGC 224 00 40.0 +40 59 Sb 725 -21.1

M32 NGC 221 00 40.0 +40 36 E2 725 -1 6 .4

NGC 205 00 37.6 +41 25 E5 725 -1 6 .3

NGC 185 00 36.2 +48 04 E3 620 -1 5 .3

NGC 147 DDO 3 00 30.5 +48 14 E4 598 -1 4 .8

And 11 01 13.5 +33 09 dE3 587 -1 1 .7

NGC 6822 DDO 209 19 42.1 - 1 4 56 Irr 540 -1 6 .4

Phoenix 01 49.0 - 4 4 42 Irr 390 -9 .9

Leo I DDO 74 10 05.8 +12 33 dE3 270 -1 2 .0

Leo 11 DDO 93 11 10.8 +22 26 dEO 230 -1 0 .2

Fornax 02 37.8 - 3 4 44 dE3 131 -1 3 .0

Sextans 10 10.6 -0 1 24 dE4 90 -1 0 .0

Carina 06 40.4 - 5 0 55 dE4 87 - 9 .2

Sculptor 00 57.6 - 3 3 58 dE 78 -1 0 .7

Draco DDO 228 17 19.2 +57 58 dE3 76 - 8 .6

Ursa Minor DDO 199 15 08.2 +67 23 dE5 69 - 8 .9

SMC 00 51.0 - 7 3 06 Irr 58 -1 6 .2

LMC 05 24.0 - 6 9 48 Irr 49 -18.1

Sagittarius 18 51.9 - 3 0 30 dE7 24 -1 4 .0



ellanic Clouds (MCs): the bigger one, the Large M agellanic Cloud (LMC), is only 49 kpc away and the Small Magellanic Cloud (SMC) is not much farther. Because of this they can be studied in great detail. We have inform ation about nearly every kind of stars and interstellar objects which help us understanding stellar evolution and the extragalactic distance scale. The LMC has a giant H II region, 30 Doradus, a huge complex of massive stars, stars in the process of form ation, gas and dust.

There are four m oderately bright elliptical galaxies which are companions to M3L Two very close companions are M32 and NGC 205, bo th of which are seen superim posed on the outer p arts of M31. M32 is a nearly circular galaxy w ith a


-Se;< ♦ S e x t a n s

1 1 1 1 1 1

t a n s B A

1 ' 1 r—

- .NGC 3 1 0 9 •Antlia

• E G B 0 4 2 7 + 6 2

• T u c a n a o




• P h o e n ix ° »IC 1 6 1 3 •NGC 6 8 2 2


•1C 1 0


1 r

‘ P e g a s u s

1 1 1 1 1

- 1 0 0 0 0 1000

P r o j e c t e d d i s t a n c e ( k p c )


population of exclusively very old stars, whereas NGC 205 is more elongated in shape and contains a small population of young stars, w ith accom panying dust and gas. The other two companions, NGC 147 and NGC 185, are som ew hat fainter and more d istan t from M31. Besides the few galaxies m entioned here, the Local Group contains a still larger num ber of dwarf galaxies (see Table 2.1). Figure 2.1 shows a diagram of Local Group galaxies, projected onto a plane (W hiting et al. 1997b).


C hapter 3

K now n X-ray P ro p erties o f Local

Group galaxies


X -ray o b serv a tio n s

The optical astronom y has been established for hundreds of years, while other wave­

lengths of study are products of this century. Radio astronom y started in the 1930s,

followed by ultraviolet(U V ), infrared(IR) and X-ray astronom y in the 1960s. The

gam m a-ray astronom y is m aking progress. Sim ilar as for UV and gam m a-ray mea­

surem ents, X-ray observations m ust be undertaken using space-borne telescopes.

In the early years, the sky was searched for sources of high energy radiation using

rocket-borne proportional counters and scintillators. The first X-ray source other

th an th e Sun was detected in 1962, known as Sco X-1 (Giacconi et al. 1962, Gursky

et al. 1963), followed by the detection of the C rab Nebula, a well-known young

supernova rem nant in our galaxy (Clark 1965, Peterson et al. 1966).

By the end of 1970, ab o u t 50 cosmic X-ray sources had been discovered, most

of them accretion-powered X-ray binaries in the galactic plane. Two of these were

identified with stellar optical objects. The diffuse X-ray background was discovered

and determ ined to be uniform to 5-10%. Six supernova rem nants had been identified

and X-rays had been detected coming from the LMC, th e active galaxies M87 and


The first surveying astronom ical X-ray satellite, Uhuru, was launched on 12 De­

cember 1970 with two sets of conventional proportional counters on board (Giacconi

et al. 1971). It was able to detect X-ray sources 10 tim es fainter th a n the faintest

ones detectable on earlier flights. The 4U (fourth Uhuru) catalogue contains 339

sources (Forman et al 1978).

The study of X-ray properties of norm al galaxies as a class was made possible

by the Einstein Observatory, which was launched in November 1978 (Giacconi et

al. 1979). Before then only 4 Local G roup galaxies had been detected in X-rays:

the Milky Way, M31, and the Magellanic Clouds. A dozen of Local G roup galaxies

were observed with Einstein am ong ab o u t 100 galaxies observed in to tal. The most

detailed work on individual X-ray sources in galaxies and th eir identifications has

been done in the Local G roup as well.

3 .1 .1

E in s te in o b se r v a tio n s

Einstein was the first fully imaging X-ray telescope pu t into space (B radt et al. 1992),

w ith an angular resolution of a few arcseconds, a field-of-view of tens of arcm inutes,

and a sensitivity ~ 1000 tim es greater th an any mission before it. The telescope had

sensitivity over the approxim ate energy range 0.2-3.5 keV. The satellite contained

a high resolution X-ray telescope and focal plane assembly capable of positioning

the focus a t one of four instrum ents: a high resolution im aging detector (HRI), a

broader field imaging proportional counter (IPC ), a solid state spectrom eter (SSS),

and a Bragg crystal spectrom eter (FPC S). It also contained a m onitor proportional

counter (M FC) aligned w ith the telescope, as well as a broad band filter spectrom eter

(BBFS) and an objective grating spectrom eter (OGS) to be used w ith the imaging


The IPC had a field of view (FOV) of 75' x 75', with a sp atial resolution of ~ 1'.

The effective detector area was ~ 100 cm^, and the time resolution was 63 ms. The

energy range was 0.4-4.0 keV, and the background count rate was 10 ^ cts s ^

The HRI had a 25' diam eter FOV, w ith a spatial resolution of 2" w ithin 5' of the



The tim e resolution was 8 ms. The energy range was 0.15-3.0 keV. The background was ~ 5 X 10“ ^ cts arcm in” ^ s“ ^ The HRI had no inherent spectral resolution, but spectral studies could be performed using interchangeable broad band filters and an objective grating.

The Einstein observations of Local G roup and other galaxies have been reviewed comprehensively by Helfand (1984) and Fabbiano et al. (1989). Table 3.1 lists the sources in Local Group galaxies detected by the Einstein observatory.


R O SA T observations

Launched in June 1990, ROSAT (Rontgen-Satellite) is the first mission to perform a deep sky survey with an imaging telescope of the whole sky in soft X-rays (0.1-2 keV) and the EUV. It is also used for detailed observations of selected sources, in order to study spatial structure, spectra and tim e variability. In this Pointing Mode the sensitivity will be at least two times larger than th a t of former missions.


Galaxy Total

Lx (ergs s ^)

No. of sources Interlopers SNR Binaries Unidentified in galaxies young Pop I older population

LMC 6 . 6 X 10^* 102(52) ~ 46(~ 19) 32(21) 7(6) 2(2) ~ 15(~ 4)

SMC 6.1 X 10^^ 57 ~ 14 ~ 12 1 0 ~ 30

M31 3.6 X 10^9 117 ~ 6 2 ~ 26 ~ 23G C +~ 60

-M32 5.4 X 10^^ 1 - - - 1

M33 1.1 X 10^^ 17 ~ 3 1 ~ 12 0

-IC 1613 - 0 - - - -

-NGC 6822 1 X 10^^ 2 ~ 1 ~ 1 - -

-NGG 205 < 9 X 10^® 0 - - - -

-Ursa Minor < 3.2 X 10^^ 3 - - - - < 3

Maffei 1^ 1 X 10^9 < 3 - - - - extended

Milky Way ~ 3 X 10^^ ~ 125 - ~ 10 ~ 40 8 G C + ~ 67

-t From Fabbiano e-t al. (1989).



T able 3.2; ROSAT P S P C C h arac teristics

W indow size 8 cm (diam eter of c irc u lar a p e rtu re ) F ield of view 2° (diam eter)

G as m ix tu re 65% argon, 15% m eth an e, 20% xenon O p eratin g pressure 1.466 b ar a t 22 °C

E nergy resolution 43% a t 0.93 keV

S p atial resolution 300 Aim ( ~ 25") a t 1 keV (F W H M ) E n tran ce window 1 /j,m polypropylene

S up po rt grid tran sm issio n 72% (on average)

T h e ROSAT high -resolution im ager (HRI) is very sim ilar to th e E in stein O b ser­ vatory HRI and com prises tw o cascaded m icrochannel p la tes (M C P s) w ith a crossed grid position read o u t system . T he d etecto r perform ance is briefly su m m a rized in Table 3.3. Values of th e q u a n tu m efficiency and background q u o te d here are based on in-flight and grou nd perform ance of th e HRI. Tw o significant changes from th e E instein in stru m en t th a t have affected th e perform ance of th e d e te c to r are; the su b stitu tio n of C sl for M gF2 as the d etecto r p h o to c ath o d e, a n d th e in tro d u c tio n of a th in alum in um -co ated p lastic m em brane (the “ele c tro sta tic shield” ) n e a r th e front M C P (in ad d itio n to th e U V /Io n shield which is s itu a te d fa rth e r in fro n t of th e d etecto r).

T able 3.3; RO SAT H RI C h arac teristics F ield of View 38' (square)

S p a tia l R esolution - 4" (FW H M ) Q u a n tu m Efficiency 30% a t 1 keV W indow Transm ission 75% a t 1 keV

B ackground 1.0 X 10“ ^ i n t e r n a l

(counts a rc m in ” ^ s~^) 1 00 X 1—( o 1 external^ 6 .9 ( 3 .5 - 1 3 .8 ) X 1 0 - 4 XRB^ 3.8 X 10 3 T y pical T o tal T em po ral R esolution 61 /iis




X-rays from M31 (NGC 224) were first seen in the d a ta from a rocket launched in

1973 (Bowyer et al. 1973), which was confirmed by the Uhuru and Ariel V all-sky

surveys. The luminosity was calculated as Lx 2 x 10^^ ergs s ^ ^ The Einstein

IPC revealed 30 bright sources in the outer spiral arms and an unresolved cluster

of sources in the nuclear region. The HRI resolved this cluster into 117 individual

sources {cf. C ram pton et al. 1984). L ater Trinchieri and Fabbiano (1991) reported 108 point sources in total, using 4 IP C pointings and several HRI pointings. Of

these sources, 19 have been identified w ith globular clusters (Sargent et al. 1977),

and 4 more sources could also be associated w ith globular clusters (C ram pton et al.

1984); about 30 sources are associated with the disc and the spiral arms; 2 sources

are associated with bright SNRs.

The sources in the spiral arm s correlate well with the distribution of neutral

hydrogen, so they are likely to be associated with Population I stars. However, most

of th e X-ray sources in the dom inant central cluster are instead likely to belong

to an older stellar population. The lum inosity of these sources is in the range

of G alactic low-mass binaries, and some variability has been reported consistent

with the hypothesis of them being powered by accretion onto a com pact object

(McKechnie et al. 1984).

Supper et al. (1997) reported the result of the first M31 survey w ith the ROSAT

PSPC performed in July 1991. 369 individual sources were detected w ithin the

~ 6.3 deg^ field investigated. Their lum inosities range from 3 x 10^^ ergs s"^ to

2 X 10^® ergs s “ ^ (0.1-2.4 keV). Of these sources, 43 have been tentatively identified with foreground sources, 29 w ith globular clusters, 17 with SNRs, 3 with other

galaxies (including M32), and 3 with radio sources. A comparison w ith the Einstein

source list (Trinchieri and Fabbiano, 1991) confirms 65 Einstein sources, 15 of which

appear to be variable. In addition, 6 faint possible and 3 bright transients were

discovered. 327 ROSAT sources are new whereas 43 Einstein sources are not seen



T he RO SAT HRI observation of th e cen tral region of M31 revealed 86 sources

w ithin ~ 17' of the nucleus of M31 (P rim in i et al. 1993), 18 of th em are identified

w ith globular clusters. Diffuse em ission is also d e te c te d , th e am o u n t is a b o u t 6 x

10^® ergs s “ ^ (0.2-4.0 keV) w ithin 5' of th e nucleus.



M33 (N GC 589) was observed by E instein , w ith b o th th e H R I an d th e IP C and

M FC . T he H RI observation (M arket an d R allis 1983) d etecte d 8 X -ray sources,

m ost of them associated w'ith P o p u latio n I trac ers. A lth o u g h th e b rig h test source

(the nucleus, nam ely M33 X-8) was seen to be variable, th e sim ultaneous M P C

observations indicated no such v ariability (Peres et al. 1989), suggesting th a t th e

variatio n existed only below 1.2 keV. No v aria b ility was seen w ith EX O SA T eith er

(G ottw ald et al. 1987). In ad d itio n to th e b rig h t nuclear source and several o th e r

brigh t point-like features seen in th e disc and th e arm s of M 33, T rinchieri et al.(1988)

also rep o rt the detectio n of low surface brig htness, diffuse em ission, th o u g h t to be

the result of th e integrated co n trib u tio n of several lower lum inosity sources and

possibly diffuse hot gas.

Long et al.(1996) repo rted recently th e ir resu lts of a long exposure RO SAT P S P C

observation of M33. They found 37 sources w ith in 15' of th e nucleus, m ost of th e m

associated w ith P o pu latio n I tracers, an d several of th e m tim e-v ariable. B oth spiral

arm s are very ap p are n t in X-rays. All of th e sources seen by E in stein are seen by

ROSAT. T h e brigh test source is again th e nucleus a n d acco u n ts for alm ost 2 /3 of

th e to ta l source flux. T he ASCA observations (T akano et al. 1994) are only able to

place a te n ta tiv e 10 percent as an u p p er lim it to th e tim e v aria b ility of this source.

Takano et al. also found th a t th e nuclear sp ectru m is significantly softer th a n typ ical

AGN sp ectra, and suggest th a t the nucleus m ay be a close b in a ry system co ntain in g

a black hole of 10 Mq.

T h e diffuse em ission detected by T rinchieri et al. (1988) is read ily a p p a re n t in


is-sion is softer th a n th e faint p o in t sources an d th e SN R s, an d is well fitted w ith a

B rem sstrah lu ng sp ectru m w ith k T ~ 0.4 keV an d l og ~ 20.6.

M33 has also been stu d ied w ith th e RO SA T H RI (S chulm an an d B regm an 1995),

revealing 27 sources w ithin 17.5' of th e nucleus, 12 of w hich h ad also been d etected

by E instein. 3 of th e d etecte d sources ap p ea r to be coincident w ith g ian t H II

regions, 7 w ith SNRs an d 2 m ay be co rrelated w ith holes in th e n e u tra l hydrogen

layer. T h e diffuse em ission is also d etected w ith in 11' of th e nucleus of M33, the


Chapter 4

ROSAT observations o f Local

Group galaxies


In tro d u ctio n

To investigate the occurrence of nuclear activity in galaxies to the lowest possible levels, a dedicated search is being carried out w ith d a ta from the ROSAT HRI, or alternatively PSPC, for nuclear sources in members of the Local G roup of galaxies around the Milky Way. The use of the HRI may be particularly useful in order to distinguish any nuclear source from other conceivable sources nearby, like bright SNRs, X-ray binaries, etc.

Two Local Group galaxies, M31 and M33, {cf. 3.2 and 3.3) have been observed previously with the ROSAT PS PC and HRI a t different spatial resolution and cor­

respondingly different areas and different dates. These two galaxies are useful for

developing our d ata analysis techniques through comparison w ith the results pub­ lished for them.

Besides 4 of the larger systems, M31, M33, LMC and SMC, m any of th e Lo­

cal Group galaxies have been observed w ith the ROSAT PSPC or HRI or both. Table 4.1 lists those galaxies which are a t the centre of th e XRT Field of View

(FOV). All the d ata are from the ROSAT d a ta archive a t the Max Planck In stitu te


ROSAT Observation Request (ROR) num ber are stored as two separate observa­ tions at two separate exposure times, and the second p a rt of the observation may have started long after the first part. A part from the listing above there are three galaxies (namely Tucana, Ursa Minor and Sagittarius) which are also in a PSPC FOV with fairly large offset.

In this chapter, the general d a ta analysis procedures are described, followed by two long exposure HRI observations for M31 and M33, after which th e detailed results for each of the above small Local G roup galaxies are presented. M32 however follows next in a separate chapter.

Table 4.1: ROSAT observation log of small Local G roup galaxies

Name O ther Name RO R No. Instrum ent S ta rt D ate Exposure (ksec)

M 32 NGC 221 600600 HRI 1994.07.19 12.6

NGC 205 M 110 600816 HRI 1996.08.04 28.1

NGC 147 DDO 3 400744 HRI 1995.01.19 14.7

NGC 185 600743 HRI 1995.01.19 21.0

IC 10 600902 HRI 1995.01.18 32.6+38.5

IC 1613 600689 HRI 1995.01.18 3.8+22.7

WLM DDO 221 600814 HRI 1996.05.30 66.9+21.4

NGC 6822 DDO 209 600815 HRI 1995.10.19 4.9+52.8

600148 PSPC 1992.04.28 6.8

Sextans A DDO 75 600116 HRI 1991.12.10 11.5+18.3

600139 PS PC 1991.11.23 6.7

NGC 3109 DDO 236 600174 P SPC 1992.05.26 18.5

600385 P SPC 1992.11.18 13.3

Pegasus DDO 216 600143 P SPC 1992.06.12 8.8

Fornax 600003 P SPC 1992.01.31 10.6

Sextans 600184 P SPC 1992.05.29 1.9

Carina 600185 PSPC 1992.04.05 8.7

Sculptor 600378 P S PC 1992.07.01 9.8




G eneral X -ray d ata analysis for R O SA T ob­


T h e ROSAT X -ray telescope (X RT) consists of th ree focal d etecto rs, two of them are P S P C and the th ird is HRI. For all of th e raw d a ta a s ta n d a rd d a ta processing (SASS) is perform ed before release. D uring th is process th e basic events inform atio n for each photo n first undergoes various corrections an d n o rm alizatio n s. T h e ph oton event files co ntain ing this in fo rm atio n form , to g e th er w ith housekeeping an d a ttitu d e d a ta , the basic in p u t for all fu rth e r analysis.

W ith these d ata-sets, th e S ta n d a rd P rocessing co n stru cts im ages of th e observed sky regions, searches th e images for point-like sources and d eterm in es th e ir positions, and scans th e SIM BAD catalo gu e for possible o ptical c o u n te rp a rts, etc. All these results are d istrib u te d to th e p rin cip al inv estigators (P is) a n d th e re a fte r en ter the ROSAT archive.

O f course, th e S tan d ard P rocessing only provides a first tre a tm e n t of th e ob­ servational d a ta . M uch m ore detailed analysis has to be done for various scientific objectives. We use EX SA S/M ID A S (Z im m erm ann et al. 1998) as a basic analysis tool, along w ith o th er widely used packages such as X S P E C , F T ools etc. T h e fol­ lowing procedures have been used to analyse th e X -ray d a ta for th e Local G roup galaxies:

• Im ages. A pply dead tim e correction, v ig n e ttin g co rrection , m ask to exclude edges of th e d etecto r and ribs for th e P S P C .


used to determine the spectrum for individual source.

• Source detection. We accept sources till a lower minimum likelihood of ex­

istence in the maximum-likelihood (ML hereafter) detection, as a trade-off

between aiming at completeness and introducing spurious detections. In the

ML detection, the likelihood of existence is defined as —ln{l — P), where P is

the probability of existence of the source (c/. Cruddace et al. 1988, Zimrner-

mann et al. 1998). The start positions for the ML detection are the results

from map and local detection. We use the minimum likelihood of existence 6

to 10 which corresponds to 3 to 4 Gaussian a.

• Astrometry. Find optical counterparts with accurate positions and improve

X-ray positioning, usually HRI data will be used if available; considered useful

only if any sources are found near the centre of the galaxy.

• Search for diffuse eini.ssion. Annuli around centre of the galaxy; also contour

plots for image with different smoothing parameters.

• Determination of characteristics of individual sources. Positions, extended­

ness, flux, and if possible, spectra and/or hardness ratios, and variability.

The following definitions and remarks apply to all data analysis results unless


• Galaxy optical parameters. The coordinates found from with SIMBAD and

NED. The distances used are those listed in Table 2.1. The D2 5, R25 and P A

are from the RC3 catalogue (de Vaucouleurs et al. 1991).

• Sources detected. Source numbering is arbitrary, mostly in the order of de­

creasing declination; the coordinates listed are the X-ray results from the ML

detection transferred to the equinox J2000.0; the count rate and its error are

background, vignetting and dead time corrected, the error is calculated with l a

errors in counts determined from the ML method, however the source counts



of a source is also quoted as ML; th e off-axis angle is th e offset to th e center

of th e field of view (FOV ).

• Source identification. Searches for co u n te rp a rts were done w ith in a few arcsecs

away from the H RI position an d a few ten s of arcsecs away from th e P S P C

position, in view of th e FW H M of th e P S F (see below). Sources are taken

from various catalogues and p apers, m ost can be found in th e C D S /S IM B A D

d atabase; the nam es follow those in SIM BAD if available; th e co ord in ates

quoted are FK 5 if from SIM BAD in stead of o riginal ones; th e B m a g n itu d e

and sp ectral typ e are also q uo ted if available. T h e H ubble G u ide S ta r C atalog

(GSC hereafter) version 1.2 is always checked w ith in th e H R I FO V and w ith in

a q u a rte r of th e P S P C FO V {i.e. h alf box size).

• Point Spread F unction (P S F, also called p o in t response fun ctio n as P R F ).

Defined as to be th e (norm alized) p h o to n d istrib u tio n in th e focal plane caused

by a celestial X -ray point source a t infinity.

• X -ray image. T h e H RI im ages are form ed by binn ing th e P h o to n E vent Tables

(P E T ) a t 5 arcsecs, th e P S P C im ages a t 15 arcsecs, corresp on ding to th e ir

typical resolution. For contour plots in a large p a r t of FO V , th e H RI im age

is sm oothed w ith 3-pixel size G aussians; th e P S P C im age is d ead tim e and

v ig n ettin g corrected, and sm oo th ed w ith 2-pixel size G au ssian s over larger


• O ptical image. D ig ital Sky Survey (DSS) is used, w hich co n tain s m ostly th e

POSS; o btain ed from H EA SA R C Skyview.

• H ardness R atios (H Rs). O nly for P S P C . O ften calcu lated for sources w ith a

sm all num ber of counts, which is n ot su ita b le to fit a sp e c tra l m odel. Tw o

HRs are defined as:

H Ri = and H R2 =

w here A ( ll- 4 1 ) , B (5 2-201), C(52~90), an d D (91-201) are co un ts sum m ed over


roughly to the energy band A(0.11-0.42 keV), B(0.52-2.02 keV), C(0.52-0.91

keV), and D(0.91-2.02 keV); the errors of the HRs are calculated again from

the l a error of the counts.

• Variability measurement. Defined as \L2 — L i \ / + where a is the error

of L.

• Upper limit. Calculated at 95.4% confidence level, corresponding to 2 Gaussian



M 31 and M 33

As discussed in Chapter 3, M31 and M33 are the two Local Group galaxies th at were

well-studied at X-rays before, besides the Magellanic Clouds. We restrict ourselves

to analysis of tw'o long exposure HRI observations for these galaxies.

Searching the ROSAT data archive within 57' radius around M31, we find nearly

90 data sets available, spanning 8 years from 1990. Among them about a dozen

are observed with the HRI, and most of the PSPC d ata sets are for the second

survey, conducted during 1992 and 1993, with exposure times of only a few thousand

seconds, (c/. Figure 5.1 for field centres of these observations.)

Primini et al. (1993) and Supper et al. (1997) published the results for an HRI

observation in 1990 (ROR 150006) and the first PSPC survey in 1991, respectively.

VVe present here some results from a later HRI observation in January 1996 (ROR

600780) for comparison. This data set has the longest exposure time (85.4 ksec)

among those focusing on the central area of M31.

Within about 5' of radius from the field center, 61 sources are detected with

M L > 10. Table 4.2 lists their position and count rates. Comparing with the

previous HRI detection, 20 more sources are found, most of them are faint ones,

though. There are also a few sources th at were detected before but have disappeared

in this data set.


Table 4.2: ROSAT HRI point sources of M31 in central area

Source 02000 52000 C o u n t R ate

ML Off-axis No. h m s 0 / N

10“ ® s - i /


the nuclear source (No.25). (See Figure 4.1.) These d a ta do even not resolve the

source which was found previously in 150006 between No.25, 19 and 20. The nuclear

source has a count rate of 17.56 ± 0.48 x 10“ ^ s“ \ which is obviously contam inated

by the diffuse emission because the source position is clearly away from th e peak

(Figure 4.1). We restrict the cut radius as 9" from the peak position (G aussian


- j - 1 1



+ 5B

+ 2 9


+ 22i|61

I mmm

4 -3 3






- I - 59

_j_ 58

Figure 4.1: ROSAT HRI image of M31 on a field of 8.5' x 8.5' and centered a t th e

HRI field centre. The image has 3" x 3" pixels and has been sm oothed with 1-pixel

size Gaussians. The logarithm of intensity is displayed. The source num bers are as



center), the count rate will be 7.72 ± 0.32 x 10“ ^ s~^, which is com parable w ith the

count rate (6.60 ± 0.49 x 10“ ^ s“ ^) derived by the same way from the d a ta observed

6 years ago (ROR 150006h). The variability m easurem ent is 1.5.

The source search for M33 from the ROSAT HRI archive d a ta (ID 600020h),

observed in 1992, reveals 20 sources w ith M L > 10. 2 of them are resolved from

the nuclear source which is thought to be extended. All of th e rem aining 18 sources

are also detected by Schulman and Bregman (1995, Table 1). Among 9 undetected

sources th a t had been reported by Schulman and B regm an, 4 have lower ML levels

between 10 and 6.


NGC 147 and 185

NGC 147 and NGC 185 are two elliptical galaxies th a t are situ ated about 100 kpc

from M31. NGC 147 has a predom inantly old stellar population > 12 Gyr and 4

known globular clusters (Ford 1977). Most of th e stars in NGC 185 are also old,

but it probably contains a SNR (Gallagher 1984) and some OB stars. There are 6

globular clusters in the field. In the X-ray band, B ran d t et al. (1997) have reported

the results of ROSAT HRI observations which were carried out in January 1995.

Table 4.3: Possible point sources within ROSAT HRI field of NGC 147

Source No.

0^2000 h m s

^2000 0 f I t

Count R ate 10-3 s - i ML

Off-axis t

1 00 32 43.63 48 39 10.9 0.92±0.34 7.2 9.8

2 00 34 35.21 48 37 50.0 3.15±0.92 7.6 15.6

3 00 32 58.79 48 37 25.2 0.58±0.24 7.1 7.2

4 00 32 10.76 48 33 40.9 1.28±0.42 8.5 10.6

5 00 34 46.41 48 32 25.5 2.27±0.68 8.5 15.6

6 00 32 28.28 48 31 55.4 2.61±0.46 62.1 7.4

7 00 33 39.64 48 30 10.7 0.57±0.23 8.4 4.6

8 00 33 05.45 48 28 37.8 0.70±0.27 7.0 2.2

9 00 32 32.48 48 27 44.9 0.66±0.27 7.8 7.1


The to tal exposure tim e for this NGC 147 ROSAT HRI observation was about

14.7 ksec. We detect 11 sources w ithin 16' from th e field centre w ith M L > 7,

among which are 2 sources w ith M L > 10. Table 4.3 lists all source positions w ith

: ‘ 0

' - . o :

■ ■ ■ t ' h '

-' ■

-r ^ o ‘^ r

“ '®CP/S5

-' =


■■ .


Figure 4.2: ROSAT HRI contour plot of NGC 147 overlaid on optical image from

the POSS. The centre of the galaxy is marked w ith a cross and the D2 5 ellipse (from

the RC3 catalogue) is drawn in. The background level in th e HRI image is ~ 0.4

cts pix“ ^ The contour levels are 0.48, 0.56, 0.64, 0.72 and 0.8 cts pix“ ^. The HRI



their count rates and M aximum-Likelihood levels.

When we overlay the X-ray image on the optical one (DSS), we find th a t source

No.8 lies w ithin the £>25 ellipse of the galaxy, bu t there is no source coincident w ith

the centre (Figure 4.2). We scanned SIMBAD and the Guide S tar C atalogue (GSC),

bu t no catalogued optical object was found at the position of any of these X-ray

sources, and none of the known globular clusters either. Nevertheless, the brightest

X-ray source in the HRI region, No.6, may have a much fainter 15"*) optical

counterpart th an would be included in the GSC, as there is a visible spot in the

Digital Sky Survey.

From constructing a radial brightness profile for the known nuclear position,

we find no evidence for diffuse emission centred on NGC 147. Most of the X-ray

sources are probably unrelated foreground or background sources. The upper lim it

for the centre of the galaxy is about 0 .2 2 x 10“ ^ which corresponds to a flux of

2.1 X 10'^'* ergs cm “ ^ s “ ^ and a lum inosity of 0.8 x 10^® ergs s “ ^ (0.1-2.5 keV) at

the distance of 598 kpc, assum ing a power law model with photon index of 2 and

Galactic column 1.2 x 10^^ cm"^.

Table 4.4: Possible point sources w ithin ROSAT HRI field of NGC 185

Source No.


h m s


0 / II

C ount R ate

1 0-^ s -i ML



1 00 40 39.49 48 32 07.7 1.58±0.50 7.2 14.5

2 00 40 25.05 48 26 44.0 2.69±0.59 16.4 16.0

3 00 39 02.24 48 24 13.5 1.39±0.28 39.3 3.9

4 00 38 21.80 48 24 09.5 1.11±0.27 25.0 7.0

5 00 39 03.87 48 23 22.3 1.23±0.41 7.2 13.2

6 00 38 27.76 48 17 55.1 0.52±0.19 8.1 5.5

7 00 39 24.98 48 15 41.4 0.43±0.18 7.1 6 .6

8 00 38 41.77 48 13 49.6 0.60±0.21 8.8 7.1

NGC 185 was observed for 20.1 ksec during th e same dates as NGC 147. Using

the same m ethod we detect 8 X-ray sources which are listed in Table 4.4, am ong

which 3 sources with M L> 10. One source which can be seen in the left (East) of the


is m asked o u t in the detection routine. F igure 4.3 shows th e H RI con tou rs overlaid

on th e optical image.

C om pared w ith th e optical im age, all sources lay in th e o u te r area of th e galaxy;

none of th em is coincident w ith known glob ular clu sters or sta rs etc. from th e






0 . <D


F igure 4.3: ROSAT HRI contour p lo t of N G C 185 overlaid on o p tical im age covering

whole H RI area. T he centre of th e galaxy is m arked w ith a cross and th e D2 5 ellipse

(from th e RC3 catalogue) are m arked w ith cross an d solid ellipse is draw n in. T h e

background level in HRI im age is ~ 0.6 cts p ix ~ ^ T h e con tou r levels are 0.68, 0.76,

0.84 and 0.92 cts p ix “ \ T he H RI im age has 5" x 5" pixels an d is sm o o th ed w ith



BAD catalogue. However, some of them are probably coincident with fainter coun­

terparts. Near Nos. 3 and 8, there are objects visible in the image of the POSS

survey. The one near No.3 is catalogued in the GSC 1.2 as a star (GSC 0325300517,

02000 = 0^39'"2.60®, ^2000 = 48°24'10.8", Pm ag= 13.51). The other one is abo u t 10"

from source No.8, with m agnitude > 15.

Around the central position, there are lower contours which may be a hint of

diffuse emission. However, from constructing a radial brightness profile for the

nuclear position, we find no clear evidence for diffuse emission centred on NGC 147.

The upper lim it for the centre of the galaxy is ab o u t 0.41 x 10“ ^ s“ ^ Using the

same model as for NGC 147, this corresponds to a flux of 3.9 x 10“ ^^ ergs cm “ ^

and a luminosity of 1.5 x 10^® ergs s“ ^ (0.1-2.5 keV) a t th e distance of 620 kpc.

Comparing the upper limits w'ith those derived by B ran d t et al. (1997), our value

for NGC 185 is similar to their 0.46 x 10'^ s~^, bu t for NGC 147, is only one-third

of their 0.60 x 10“^ We checked a t other central positions near the ones from

various catalogues (SIMBAD and NED), and a t such positions closeby the upper

limit count rates may reach upto 4.1 x 10“ ^ s“ \ thus differences like these can easily

occur between different analyses.

4 .5

IC 1613

IC 1613 is a dw arf irregular (dl) galaxy which is among a few such galaxies observed

with Einstein. The X-ray emission associated w ith this galaxy is m ainly from a

single source (namely IC 1613 X-1) which is ab o u t 5.6' away from the centre of the

galaxy and probably to be identified w ith a cluster of galaxies (Eskridge 1995). The

Einstein IPC received a flux of f x = 2.20 x 10“ ^^ ergs cm~^ s“ ^ (0.2-4.0 keV and

for the Brem sstrahlung model w ith kT = 5 keV).

The ROSAT HRI observation was carried out during June and July 1995. An

earlier HRI observation in January 1995 has only 3.8 ksecs, which we do not use

here. W ithin the HRI field of the second observation which has 22.7 ksec of exposure


T able 4.5; P o in t sources w ithin R O SA T H R I field of IC 1613

Source 0 1 2 0 0 0 ^ 2 0 0 0 C o u n t R a te Off-axis

h m s o i l ! 10^ ^ '

1 01 04 47.96 + 02 19 29.7 1.51+ 0.46 7.7 15.7

2 01 04 50.96 + 02 12 59.5 0.74+ 0.25 8.2 9.2

3 01 04 49.95 + 02 10 31.9 0 .54 + 0.20 8.3 7.1

4 01 05 02.46 + 02 08 42.5 2 .11+ 0.33 83.5 4.5

5 01 05 00.01 + 0 2 02 45.9 0 .64+ 0.22 7.7 1.6

6 01 05 01.46 + 02 02 44.5 0 .39+ 0.15 8.7 1.5

7 01 04 52.27 + 02 02 25.3 0 .46 + 0 .1 7 9.7 3.1

8 01 05 02.07 + 02 02 05.2 0 .6 3+ 0 .2 0 12.9 2.1

9 01 05 04.72 + 02 01 53.9 0.36+ 0 .1 5 7.5 2.4

10 01 05 00.35 + 0 2 01 49.1 0.3 7+ 0 .15 7.9 2.4

11 01 05 01.79 + 0 2 01 35.4 1.27+0.31 8.3 2.6

12 01 05 01.50 + 02 01 24.1 1.25+0.30 9.5 2.8

13 01 04 40.80 + 02 01 07.7 1.98+0.32 68.2 6.2

14 01 04 10.22 +01 59 21.9 1.67+0.43 12.9 13.9

15 01 05 16.06 +01 59 12.2 1.23+ 0.27 29.2 6.1

16 01 05 43.30 +01 58 39.2 0.9 2+ 0.30 8.4 11.6

17 01 05 14.96 +01 57 49.9 0 .67+ 0.22 9.7 7.1

18 01 04 11.04 +01 55 36.2 1.67+0.50 7.9 15.4

Table 4.5 lists all sources d etected w ith in th e H R I field w ith th e ir count rates. To

com pare w ith th e PO SS survey, th e sources Nos. 1-4 are lo c ated w ith in th e b o u n d ­

aries of th e o p tical galaxy, b u t have no obvious o p tical c o u n te rp a rts. Source No. 15

is situ a ted in th e o u te r area, p ro b ab ly coincident w ith an o p tical o b je ct listed in th e

G uide S tar C atalog ue (C SC 0001900392, ^ 2 0 0 0 = l'^5”^15.94^ ^ 2 0 0 0 = +1°59'14.7",

P m ag= 11.42) th a t is only 4 arcsec away from th e X -ray p o sitio n . Source N o.4

is not far from th e o ptical positions of IC 1613 as given by SIM B A D and N ED ,

and has been identified as a supernova rem n a n t (SNR) by Lozinskaya et al. (1998).

T he count ra te of th is SN R is (2.1 ± 0.3) x 10“ ^ s“ ^. For th e G alactic colum n

density of N h = 2.8 x 10^® cm “ ^, a pow er law m odel w ith p h o to n index = 2.0

gives an un ab so rb ed flux f x = 1-23 ± 0.18 x 10“ ^“* ergs c m “ ^ s “ ^ an d a lum inosity



The Einstein source IC 1613 X-1 is also detected w ith the ROSAT HRI. Our

HRI image (Figure 4.4) shows th a t m ost of the X -1 region has been resolved into

individual sources which are listed in Table 4.5 as Nos. 5-12. However th e central

part of X-1 has not been resolved completely and is indeed reminiscent of X-ray

IC 1613 X-1




Figure 4.4: ROSAT HRI image of IC1613 X-1 on a field of 14' x 14' and centered

at the X -1 position, marked w ith a big cross. The image has 5" x 5" pixels and has

been sm oothed with a 2-pixel size Gaussians. IC 1613 itself is located North-W est

of X-1 where the big circle indicates its optical position in the NED (c/. Palum bo


emission from a cluster of galaxies. We integrate the counts within 3.5' radius, th at

gives a count rate of 0.0312±0.0012 s“ ^ Using a Bremsstrahlung model with kT =

5 keV, we get an unabsorbed flux f x = 1.51 ± 0.06 x 10“ ^^ ergs cm“^ s“ ^ (0.1-2.4


We also derived upper limits to the X-ray flux for several optical positions of IC

1613 in various catalogues (from SIMBAD and NED). The upper limit count rate

is from 1.9 x 10~'* to 4.1 x 10“ "^ s“ ^ Using the same conversion factor as above the

flux f x ~ (1-1 — 2.4) X 10~^^ ergs cm“^ s“ ^ in the 0.1-2.4 keV band, the luminosity

L x = ( 0 .8 - 1.7) X 10^® ergs s'^ .


N G C 6822

NGC 6822 was previously observed with Einstein (Markert and Donahue 1985).

Eskridge and White (1997) summarized the Einstein HRI and ROSAT PSPC results

and gave details of the source in the bar of the galaxy. This source coincides with

the emission-line object Ho 12. W ith only 75 ROSAT PSPC counts, a RS model fit

is attem pted th at gives kT~0.56 keV, while a BE model gives kT~0.06 keV. They

argue that this object may be either an HH region or a SNR, but the variability

of the X-ray source over 13 years indicates th a t the X-rays are not due to direct

emission from a SNR.

Our own PSPC data analysis yields 15 sources with M L > 10, listed in Table 4.6.

The brightest source (No. 15) may be associated with a bright star, HR 7496 (F5IV,

5.95™) with coordinates (0 : 2 0 0 0 = 19*43"*33.53^, ^ 2 0 0 0 = —15°28'12.4"), which is only

a few arc-seconds aw'ay. This source is a supersoft source with HRi < 0. No.3 is

partially obscured by the window support structure, its count rate may be under­

estimated. It is not far (about 7") from the star SAO 162956 (G5, 10.45'").

The source on the bar of NGC 6822 mentioned above is No. 10. Its 80 counts

is too little to fit any spectral model. We calculate the hardness ratios of all the

detected sources and also list these in Table 4.6.



Table 4.6: Sources and count rates within ROSAT PSPC field of NGC 6822

Src No.


h m s o ^2000 1 n

Cnt Rate

10-3 s -i HRi HRs ML


1 19 43 13.31 -14 15 49.4 22.0±4.2 0.72±0.12 0.11±0.15 16.9 40.9

2 19 45 46.55 -14 18 26.6 14.1±3.0 0.54±0.14 0.56±0.16 13.4 31.8

3 19 45 35.89 -14 27 46.7 88.3±4.2 0.90±0.02 0.24±0.05 966.3 22.2

4 19 45 05.85 -14 36 38.7 9.5±1.3 0.75±0.13 0.25±0.15 100.0 11.5



Table 2.1: Local Group galaxies
Figure 2.1: Projection of the galaxies in the Local Group onto a convenient plane.
Table 4.1 lists those galaxies which are at the centre of the XRT Field of View
Figure 4.8: ROSAT PSPC contour plot of Sextans A, central 40' x 40' area. The


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