How to study the soft x-ray emission from the LB

In this setion we will disuss how to study the soft x-ray emission from the LB.

In the x-ray regime the ideal way to do studies on the nature of the LB plasma

physial state, or in general on other Galati hot plasmas, is to perform x-ray

shadow experiments. The existene of ISM x-ray shadows requires the presene of

ISMmaterialinbetween theemissionx-ray soureand anobserver. Forthe LB the

idealaseistostudythex-rayshadowsproduedbyloudsofneutralandmoleular

materialinside of the LB boundaries.

Thegure2.4hasadualpurpose: rsttoshowtheISMphotoeletriabsorption

ross-setion (

σ

), and seond, to show the assoiated mean free path (m.f.p.) of a photoninthatmedium. Thetoppanelofgure2.4showsthe eetiveabsorption

σ

perhydrogenatomforthe ISM,usingthe analytitoeientsfromMorrisonand

MCammon [143℄. Assuming a partile density of 1.0 m

−3

, we an ompute the

m.f.p. for eahphoton in the energy band of 0.1 to10.0 keV. Given that the m.f.p.

10 -19 10 -20 10 -21 10 -22 10 -23 10 -24

PhotonEnergy[keV℄

Cross Setion [m 2 ℄ C O Ne Mg Si S Fe 0.1 1 10 1 10 10 2 10 3 10 4 10 5

PhotonEnergy[keV℄

Mean F ree P ath [p ℄ C O Ne Mg Si S Fe

Figure 2.4: TheISMPhotoeletri

Absorption. The presene of material

in the ISM has as a onsequene the

absorptionofradiation, induingare-

dution ofthe radiation intensityof a

given soure. The probability of in-

teration between a given photon of

energy E and some partile is given

by the rosssetion

σ

of the partile. Onthetop plottheinterstellarphoto-

eletri absorption ross setions, al-

ulated by Morrison and MCammon

[143 ℄, as a funtion of energy, are de-

pited. Theserosssetionswerethen

usedtoalulatethemeanfreepathof

photons, assuminganaveragepartile

density

n0

= 1.0 cm

−3

, bottom plot.

most abundant atomi speies are alsopresented ineah of the panels.

The LBemissionisthoughttobeproduedinaavity,devoidofHi, lledwith

a

∼

10

6

K hot thin plasma and having an average radius of 100 p. Early all-sky

surveys,liketheWisonsinSurveyperformedbyMCammonetal.[107℄,hadshown

aremarkableonstany between the ount ratio of the B and Be bands. Given the

fat, that a photon of the Be-band has a

σ

about 4 times higher than a photon of the B-band, the onstant ount ratio implied the same physial origin for these

dierent photons.

If the eet of the absorption

σ

is ombined with the average partile density (n) along a given line of sight (l =

R

ds), then the optial depth of the medium

is obtained,

τ

=

σ

N

, where N is the olumn density along that partiular line of sight. The physial parameter

τ

allows us to alulate whether a given mediumis optiallythin or thik for a given photon. As anillustrativeexample: for a photon

of 1.0 keV in a medium with an assoiated olumn density of

10

22

m

−2

and with

σ

= 2.422

_×10−22

m

2

[143℄, we obtain

τ

of 2.422. This value means, that the radiationat 1.0 keV, emerging from this medium was redued to about 9 per ent

of its original value, sine I/I

0

=

e

−τ

, therefore, produing an x-ray shadow for

photons with an energy of 1.0keV. The absorption inreases as the photon energy

isdereased.

In gure 2.5 a simulation of an astrophysial x-ray shadow is presented. For

generating this simulated x-ray shadow, the following astrophysial model was as-

sumed. An unabsorbed thermal plasma, representing the LB, is shown in orange

olor. To this plasma atemperatureof 8.1

×10

5

Kwas assigned. The modelis also

omposedby twootherx-rayemissionomponents,but absorbed byagiven olumn

density. The rst of the former two omponents is a thermal plasma, representing

a Galati hot halo plasma, with a temperature of 1.5

×10

6

Figure 2.5: An ideal x-ray shadowexperiment ontheCXB. Inthis guretheeet of

anx-rayshadowexperiment,duetoamoleularloudinanidealastrophysialsituationis

presented. The astrophysial modelonsists oftwo thermal plasmas and anextragalati

x-rayomponent. TherstplasmaofthemodelistheLB(orange),andtheseondplasma

orrespondsto adistant haloplasma (red). A temperature of 8.1

×10

5

K and 1.5

×10

6

K

was attributed to the LB and halo plasmas, respetively. The power-law representing

the extragalati ontribution is shown in blue. The total spetrum is shown as a solid

blak line (when visible). The halo and extragalati x-ray emissions are subjeted to

absorption of intervening material of dierent olumn densities (

NH

). The

NH

of

10

19

to

10

20

m

−2

represents the Galati

NH

(an o-loud situation), and the

NH

of

10

21

to

1022

m

−2

represents the olumn density oftwo regions assoiated to an ideal interstellar

moleular loud (an on-loud situation). The x-ray absorption eet, due to intervening

neutral material, in partiular of the moleular loud, produes an x-ray shadow in the

ISM, permitting to disentangle the soft x-ray emission of the LB from the bakground.

Thephotonenergy is given inunits ofkeV.

olor. The seondisthe ontributionof the CXB,represented by apower-law, with

a spetral index of

Γ =

−1.4

, and shown as the blue line. The total spetrum is shown inblak (when visible).

The two absorbed x-ray omponents are then subjeted to dierent olumn

densities(

NH

). Columndensities ranging from

10

19

to

10

20

m

−2

represent typial

valuesfortheGalati

NH

andorrespondtoanon-shadoworo-loudsituation. As itan beseeninthe twotop plots,the LBemissionisstronglyontaminatedby the

the halo emission. Ifthe

NH

isinreased to values of

10

21

m

−2

photons,fromthe haloand extragalatiomponentsstarttobestronglyabsorbed.

Inreasing the

NH

to

10

22

m

−2

, all photons with energies

.

1.0

keV are severely absorbed by the intervening material. These two situations orrespond to an on-

loud observation, where an x-ray shadow is produed by a moleular interstellar

loudwith a

NH

gradient.

This is the ideal physial situation to study the emerging x-ray emission pro-

duedbytheLB.Therefore,weneedtostudy thex-rayemissiontowardsinterstellar

moleular louds with high

NH

, whih are inside of the LB or lose to its bound- aries, in order to be able to disentangle its ontribution from other Galati x-ray

emissions.

In document Mendes, Paulo (2010): The Soft X-Ray Background Towards Ophiuchus. Dissertation, LMU München: Fakultät für Physik (Page 60-63)