Be/X-ray Binaries in the SMC

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Spin period change and the magnetic fields of neutron stars in Be X-ray binaries in the SMC

Spin period change and the magnetic fields of neutron stars in Be X-ray binaries in the SMC

Abstract. We report on the long term average spin period, rate of change of spin period and X-ray luminosity during outbursts for 42 Be X-ray binary systems in the Small Mag- ellanic Cloud. We also collect and calculate parameters of each system and use this data to determine that all systems contain a neutron star which is accreting via a disc, rather than a wind, and that if these neutron stars are near spin equilibrium, then over half of them, including all with spin periods over about 100 seconds, have magnetic fields over the quantum critical level of 4.4×10 13 G. If these neutron stars are not close to spin equi-
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A spectroscopic study of high mass X ray binaries

A spectroscopic study of high mass X ray binaries

In its broad details, Cen X-3 is similar to SMC X-1 and LMC X-4, and has been tentatively associated with an anti-correlated pulse-period relationship (Corbet, 1984). As was shown in the study of SMC X-1 (chapter 3, this study), this class of system is eminently suited to optical study, since once spectroscopy has been performed, all the information needed to determine the component masses is in place, the proximity of the two stars making eclipses likely and the brightness of the x-ray source allow­ ing accurate pulse-timing. Three spectroscopic studies were published, both based on data accumulated in the early-to-late seventies - Osmer, Hiltner and Whelan (1975), OHW from hereon, published six low dispersion spectrograms which did not permit an orbital solution, HCCPW (1979) published a velocity curve with considerable scatter about the derived sinusoidal solution, while Mouchet, Ilovaisky and Chevalier (1980), MIC from hereon, were only able to demonstrate the existence oi positive and negative radial velocities, their solution based on just 10 spectrograms. All studies used inter­ mediate to low resolution configurations, using image tube/photocathode spectroscopy and photographic plates. MIC did not derive a semi-amplitude but stated that their observations were consistent with the value of 25 km s” ^ reported by HCCPW, on the basis of measurements of photospheric Hel lines. Interestingly, the Baimer line mea­ surements performed by HCCPW seem to show only a very marginal radial velocity variation. This is what might be expected from the “classical” picture of tidal-distortion and reflection-effect, where the true orbital variations are suppressed, rather than ex­ aggerated, as would be seen in a heating scenario dominated by a soft x-ray flux. In this respect, HCCPWs observations seem to be similar to those of SMC X-1 reported in this study.
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Deep Chandra Survey of the Small Magellanic Cloud. III. Formation Efficiency of High-mass X-Ray Binaries

Deep Chandra Survey of the Small Magellanic Cloud. III. Formation Efficiency of High-mass X-Ray Binaries

Our results are also in agreement with studies of the formation ef ficiency of massive Oe/Be stars in the Magellanic Clouds (e.g., Martayan et al. 2006 , 2007b ; Bonanos et al. 2009 , 2010 ), and the Milky Way (McSwain & Gies 2005 ). These works show a peak at ages of ∼20–50 Myr (Iqbal & Keller 2013 ), matching the age of maximum production of HMXBs at least at the metallicity of the SMC. This similarity could indicate that (a) the Be stars, the donor stars of Be-XRBs (the predominant HMXB population in the SMC), are the result of binary evolution (e.g., Porter & Rivinius 2003 , and references therein ), and/or (b) the larger mass-loss rates of Be stars through their equatorial winds (in comparison to the much weaker spherical stellar winds ) lead to an enhanced population of active XRBs (see Antoniou et al. 2010 ). However, only detailed population synthesis models account- ing for the complex orbital evolution and mass transfer in eccentric binaries (e.g., Dray 2006 ) can distinguish between these possibilities.
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X-ray studies of ultraluminous X-ray sources

X-ray studies of ultraluminous X-ray sources

One object in our sample had sufficient counts in its Chandra X-ray spectrum for us to analyse it separately. We found the spectrum of CXOU J024238.9-000055 to be very hard, with a power-law of photon index Γ = 0.75 ± 0.1 providing an acceptable fit to the data. This result is consistent with previous studies, which also found that the photon index is very hard (Smith & Wilson, 2003; Swartz et al., 2004; Berghea et al., 2008). Indeed, the photon index seems to be extremely hard for ULX spectra (Swartz et al., 2004), but instead may be consistent with the X-ray spectral properties of background AGN. Obviously, in order to distinguish between these two possibilities, the optical counterpart to the X-ray source needs to be identified by further optical observations. However, if the source is a genuine ULX residing in the host galaxy NGC 1068, an interesting interpretation is obtained. Smith & Wilson (2003) interpret the hard spectrum of this ULX as inverse Compton scattering of synchrotron emission in a jet. However, they also noted the presence of a broad feature that dips below the continuum level at ∼ 1.7 keV, and suggest that this may be a spurious feature resulting from an artefact in the gain table of the Chandra ACIS-S instrument. We have re-analysed the data with the latest Chandra calibration database that includes corrections for such features 23 , but still
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Advection‐Dominated Accretion and the Spectral States of Black Hole X‐Ray Binaries: Application to Nova Muscae 1991

Advection‐Dominated Accretion and the Spectral States of Black Hole X‐Ray Binaries: Application to Nova Muscae 1991

Within the limitations imposed by the uncertainties in the distance to each particular system and the mass of the accreting black hole, the five spectral states can be arranged in the following sequence of increasing total luminosity: quiescent state, low state, intermediate state, high state, and very high state. This order seems to be preserved when individual systems undergo state transitions and also when one binary is compared to another (provided the luminosity is expressed in Eddington units). However not all BHXBs display all the states, and in fact most systems spend the bulk of their time in a single state, making only brief excursions to other states. Cygnus X-1, for instance, is generally found in the low state, with occasional high state transitions, while LMC X-3 is nearly always in the high state (Tanaka & Lewin 1995). The transient sources (e.g. A0620-00, Nova Muscae, V404 Cyg) are the most interesting in this respect since they undergo dramatic outbursts during which their luminosities vary over many orders of magnitude. Consequently, some transients cycle through all five states. Nova Muscae 1991 (also known as GS 1124-683), for instance, went very quickly from quiescence to the very high state and then decayed via the high, the intermediate and low states back to the quiescent state (E94).
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Observation of femtosecond X-ray interactions with matter using an X-ray X-ray pump probe scheme

Observation of femtosecond X-ray interactions with matter using an X-ray X-ray pump probe scheme

Resolution in the X-ray structure determination of noncrystalline samples has been limited to several tens of nanometers, because deep X-ray irradiation required for enhanced resolution causes radiation damage to samples. However, theoretical studies predict that the femtosecond (fs) durations of X-ray free-electron laser (XFEL) pulses make it possible to record scattering signals before the initiation of X-ray damage processes; thus, an ultraintense X-ray beam can be used beyond the conventional limit of radiation dose. Here, we verify this scenario by directly observing femto- second X-ray damage processes in diamond irradiated with extraordinarily intense (∼10 19 W/cm 2 ) XFEL pulses. An X-ray pump–probe diffraction scheme was developed in this study; tightly focused double–5-fs XFEL pulses with time separations ranging from sub-fs to 80 fs were used to excite (i.e., pump) the diamond and characterize (i.e., probe) the temporal changes of the crystalline structures through Bragg reflection. It was found that the pump and probe diffraction intensities remain almost constant for shorter time separations of the double pulse, whereas the probe diffraction intensities decreased after 20 fs following pump pulse irradiation due to the X-ray –induced atomic displacement. This result indicates that sub-10-fs XFEL pulses enable conductions of damageless structural determinations and supports the validity of the theoretical predictions of ultraintense X-ray –matter interac- tions. The X-ray pump –probe scheme demonstrated here would be effective for understanding ultraintense X-ray –matter interac- tions, which will greatly stimulate advanced XFEL applications, such as atomic structure determination of a single molecule and generation of exotic matters with high energy densities. X-ray free-electron laser | pump –probe | femtosecond X-ray damage S ince W. C. Röntgen discovered X-rays emitted from vacuum
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Multiwavelength, Machine Learning, and Parallax Studies of X-ray Binaries in Three Local Group Galaxies

Multiwavelength, Machine Learning, and Parallax Studies of X-ray Binaries in Three Local Group Galaxies

Our HMXB sample comprises only ∼50 objects, and the uncertainties are still substantial. As such, though we can rule out a very strong spatial correlation or anti-correlation between HMXBs and spiral arms, we cannot use our result to distinguish between the scenarios listed above. Since we are unable to reject the null hypothesis that HMXBs are uncorrelated with spiral arms, our result is consistent with Bodaghee et al. (2012)’s analysis, which found that HMXBs are not spatially correlated with spiral arms. The scale at which the HMXB / SFR cor- relation breaks down (if at all) is not well-constrained. In nearby galaxies, the X-ray sources are typically studied by considering the integrated properties of the entire population (for exam- ple, X-ray luminosity function) and comparing to global parameters of the galaxy. Correlating XRBs with galactic structure is challenging since galaxies that are close enough to resolve on the desired scales require many fields in order to encompass the entire galaxy. In addition, contamination from X-ray sources in front of or behind the galaxy creates additional di ffi cul- ties. Swartz et al. (2003) investigated the relationship between the spiral arms of M81 and its X-ray source population, finding strong correlation between spiral arm position and X-ray source density. They note that brighter sources tend to be closer to spiral arms, attributable to the brightest and shortest-lived HMXBs being close downstream from their spiral arms. More recently, Kuntz et al. (2016) performed a deep Chandra survey of M51. This study also finds that X-ray sources are concentrated in spiral arms, though the distances to spiral arm midpoints are not presented. Both studies also found a non-trivial population of supernova remnants con- tributing to the total X-ray source population.
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A Magnetic Model for Low/Hard States Associated with Jets in Black Hole X Ray Binaries

A Magnetic Model for Low/Hard States Associated with Jets in Black Hole X Ray Binaries

Disk-corona model seems to be supported by a number of recent observations. For example, XMM-Newton ob- servations of GX 339-4 show that a broad iron line together with a dim, hot thermal component was present in its spectra during the hard state. This effect seems to be observed in a few other sources such as Cygnus X-1, and SWIFT J1753.5-0127 [6,7]. Recently, Reis, Miller & Fabian [8] studied the Chandra observation of XTE J1118+480 in the canonical LH state, and a thermal disk emission with a temperature of approximately 0.21 keV is found at greater than the 14 level, and they concluded that this thermal emission most likely originates from an
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New distance and depth estimates from observations of eclipsing binaries in the SMC

New distance and depth estimates from observations of eclipsing binaries in the SMC

Until a purely geometrical distance determination is feasible, Paczy´ nski (2001) con- sidered that detached EBs are the most promising distance indicators to the Magellanic Clouds. Four B-type EB systems belonging to the Large Magellanic Cloud (LMC) were accurately characterized in a series of papers by Guinan et al. (1998), Ribas et al. (2000b, 2002) and Fitzpatrick et al. (2002, 2003). More recently, from high resolution, high S/N spectra obtained with UVES at the ESO VLT, the analysis of eight more LMC systems was presented by Gonz´ alez et al. (2005). Harries et al. (2003, hereinafter HHH03) and Hilditch et al. (2005, hereinafter HHH05) have given the fundamental parameters of a total of 50 EB systems of spectral types O and B. The spectroscopic data were obtained with the 2dF multi-object spectrograph on the 3.9-m Anglo-Australian Telescope. This was the first use of multi-object spectroscopy in the field of extragalactic EBs. Let us also mention that the distances of an EB in M 31 (Ribas et al. 2005) and another in M 33 (Bonanos et al. 2006) were measured recently. Although the controversy about the distance to the Magellanic Clouds seems to be solved in favour of a mid position between the “short” and the “long” scales, distance data and line of sight depth remain vital for comparison with theoretical models concerning the three-dimensional structure and the kinematics of the SMC (Stanimirovi´ c et al. 2004).
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A Brief Review for Quasi Periodic Oscillations in Neutron Star Low Mass X Ray Binaries

A Brief Review for Quasi Periodic Oscillations in Neutron Star Low Mass X Ray Binaries

Sonic-point beat-frequency model—Considering the relation between falling velocity of accreting material and sound speed [69] proposed the sound-point model, which ascribes the frequency of upper kHz QPO as the Keplerian motion of accretion flow at a radius near the sonic point at the inner edge of accretion disk, whereas the lower frequency is the difference between the Keplerian frequency at a radius near the sonic point and the fundamental or first overtone of the NS spin frequency. The difference between twin frequencies is therefore close to (but not necessarily equal) the spin frequency of NS. The amplitudes of QPOs at the sonic-point Keple- rian frequency and at the beat frequency depend on the strength of magnetic field and accretion rate, and hence one or both of these QPOs may sometimes be undetectable. The sound-point model is consistent with the mag- netic field strengths, accretion rates, and scattering optical depths inferred from previous modeling of the X-ray spectra and rapid X-ray variability of Z and atoll sources. It explains naturally the frequencies of kHz QPOs, the similarity of these frequencies in sources with different accretion rates and magnetic fields, the high coherence and large amplitudes, and the steep increase of amplitude with photon energy. However, it was found that the frequency separation between twin signals does not exactly equal the spin frequency. Accordingly, this model was modified, by considering that although the stellar spin interacts with the orbital motion of accreting gas at the sonic radius with a frequency equaling the sonic-point beat frequency. The X-ray oscillations are produced by interaction between accretion flow and NS surface, and their frequencies are therefore affected by the flow from the sonic radius to the NS surface [70]. With more and more data released for kHz QPOs, the beat-fre- quency mechanism cannot match the observations.
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Optical and X-ray studies of HEAO-1 X-ray sources

Optical and X-ray studies of HEAO-1 X-ray sources

The association of Galactic X-ray sources with Be stars was first suggested by Maraschi, Treves and van den Heuvel (1976) when they identified 4 such objects. By the time of Rappaport and van den Heuvel's (1982) review paper, another 8 such systems had been added, and clear observational tendencies had become apparent. All are hard X-ray sources (>3 keV), and half of them exhibit X-ray pulsations ranging from 69 ms to 835 s, which is interpreted as the spin period of an accreting magnetic neutron star (X-ray pulsar). With the exception of X Per (4U0352+30), which is possibly the closest Be X-ray binary, all of the twelve sources are highly variable, or transient, by a factor of 10-100 times. X Per is unusual in that it shows a persistent steady X-ray flux, and has a low inferred X-ray luminosity (-1 0 34 ergs s_1; eg Corbet 1986). Such a low level 'quiescent' luminosity (in the range 1032 to 1034 ergs s_1), also applicable for another close system y Cas (2S0053+60), is explained in terms of the standard stellar wind accretion theory of Davidson and Ostriker (1973). This interpretation has subsequently been supported by observations of blue shifted UV resonance lines, confirming the presence of a stellar wind. However, for the majority of variable or transient Be sources, this theory is inadequate in explaining the much higher luminosities during the X-ray high states of activity, which reach 1038 to 1039 ergs s”1. The rotation induced equatorial mass ejection, which is a defining characteristic of a Be star (eg Slettebak 1988), has become the natural explanation for the higher X-ray luminosities. Not only does this mechanism result in an increased mass loss rate, but its highly variable nature adequately accounts for the transient characteristics of the X-ray emission. Rappaport and van den Heuvel (1982) have shown that the neutron star accretion rate expected from Be-type mass ejection is large enough to power the observed X-ray luminosity. The often observed periodic X-ray outbursts (eg every -17 days in A0538-66) are consistent with periastron passage of the neutron star near the Be companion.
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CHROMOSOME STRUCTURE X. AN X-RAY EXPERIMENT

CHROMOSOME STRUCTURE X. AN X-RAY EXPERIMENT

D, quartet stage (only one of the two chromo- somes which separate in the second division is shown). E, metaphase in first division of the pollen grain. Another multiplicat[r]

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A century of X-ray crystallography and 2014 international year of X-ray crystallography

A century of X-ray crystallography and 2014 international year of X-ray crystallography

for many different areas of science and technology. The double helix structure of DNA molecule de- termined by X-ray crystallography was the epochal discovery of the 20 th century. Molecular genetics and related disciplines emerged from that discov- ery. X-ray crystallography has revolutionized our understanding of molecular biology and has assist- ed in development of molecular medicine and new drugs. Design of catalysts essential in syntheses of various compounds based on the principals of „green‟ chemistry within the context of sustainable ecology, is on the front line of chemical crystallog- raphy. The preparation of a wide selection of novel materials such as alloys, ceramics, fibres, plastic materials, detergents, design of systems for pro- duction of „green energy‟ such as batteries, fuel and solar cells is guided by structural characteriza- tion by X-ray diffraction of materials used in these technologies. On the front lines of novel materials are magnetic materials, materials for molecular electronics, and potential materials for quantum computers, being at present close to a science fic- tion. X-ray crystallography has been also estab- lished as a method of choice in geosciences includ- ing mineralogy, as well as in archaeology. The in- volvement of X-ray crystallography in all these fields of science and technology advocates for its high impact on a wide area of research and de- clares it as a highly interdisciplinary science. In short, crystallography defines the shape of our modern world.
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Calculation of x-ray spectra emerging from an x-ray tube. Part I. electron penetration characteristics in x-ray targets.

Calculation of x-ray spectra emerging from an x-ray tube. Part I. electron penetration characteristics in x-ray targets.

there are x-ray tubes in use within healthcare that possess anodes made of elements that are not high-Z materials. The majority of such tubes are used in mammography. The tube potentials used in these cases are less than 50 kVp, outside of the range investigated here. If the approach adopted here were to be repeated and applied to mammographic energies and lower Z materials, careful consideration would have to 545

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Neutron Stars and NuSTAR: A Systematic Survey of Neutron Star Masses in High Mass X ray Binaries & Characterization of CdZnTe Detectors for NuSTAR

Neutron Stars and NuSTAR: A Systematic Survey of Neutron Star Masses in High Mass X ray Binaries & Characterization of CdZnTe Detectors for NuSTAR

hand-in-hand with astronomical motivation to make better and better satellites, 1,2 which are many orders of magnitude more sensitive than these pioneering experiments (Table 1.1). These developments in technology warrant a bit more attention to place NuSTAR in context. The most basic elements of an X-ray telescope are the light gathering aperture and the detectors. Early X-ray experiments had very little directionality. Solar X-rays were first discovered by a crude pinhole camera payload on board a Naval Research Laboratory rocket, with no means to collimate or focus the incoming radiation. The next evolutionary step was to add collimators in front of the detectors. Collimators enable basic imaging of the sky in the same way a single dish radio telescope does: by pointing and imaging one field of view at a time. They are still used primarily for X-ray timing, for example in the Proportional Counter Array on board the Rossi X-ray Timing Explorer (RXTE; Jahoda et al., 1996). Finer imaging is achieved by using coded aperture masks—which uses
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Grimm, Hans-Jakob
  

(2003):


	X-ray binaries in the Milky Way and other galaxies.


Dissertation, LMU München: Fakultät für Physik

Grimm, Hans-Jakob (2003): X-ray binaries in the Milky Way and other galaxies. Dissertation, LMU München: Fakultät für Physik

The vertical distribution outside the bulge (Fig. 3.4) is significantly broader than that of HMXBs and includes a number of sources at high galactic z. A formal fit to the observed distribution with an exponential law results in a large scale height of 950 ± 130 pc, which is close to the value of 710 pc obtained by van Paradijs & White (1995) for NS LMXBs. However, due to presence of a tail of sources at |z| > 1.5−2 kpc, the observed z-distributions cannot be adequately described by a simple exponential law. As only three out of nine sources at |z| > 2 kpc are located in globular clusters, this tail of high-z sources cannot be solely due to the globular cluster component. A possible mechanism – a kick received by a compact object during the SN explosion, was considered e.g. by van Paradijs & White (1995). The relatively small number of high-z sources does not allow one to determine the shape of their distribution based on the data only. In order to account for the high-z sources and the LMXB sources in globular clusters we chose to include in the spatial distribution of LMXBs the spheroid component described by a de Vaucouleurs profile (Eq. (3.3)). Note that a de Vaucouleurs profile correctly represents the distribution of globular clusters. The overall vertical distribution can be adequately represented by a sum of an exponential law with a scale height of 410 +100 −80 pc and a de Vaucouleurs profile with the parameters given in Table 3.1. The spheroid component represented by the de Vaucouleurs profile contains a ∼ 25% of the total number of LMXBs. Note, that this number is by a factor of ∼ 2–3 larger than the mass fraction of the stellar spheroid in the standard Galaxy model. The enhanced fraction of the spheroid component is generally consistent with the fact, that the number of X-ray sources per unit mass is ∼ 100 times higher in the globular clusters than in the Galactic disk and 12 out of 104 LMXBs in our sample are globular cluster sources. The angular resolution of the ASM instrument does not permit to study in detail the very central region of the Galaxy which is characterised by the highest volume and surface density of X-ray binaries. Based on GRANAT/ART-P data having significantly better sensitivity and angular resolution, Grebenev et al. (1996) showed that the distribution of the surface density of X-ray binaries in the central 8 ◦ × 8 ◦ of the Galaxy is consistent with
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Measurements of X ray selected AGN and novel superconducting X ray detectors

Measurements of X ray selected AGN and novel superconducting X ray detectors

standard deviation. Two to three iterations are typically required, with the biggest shift applied after the initial matching. As shown in Table 2, the typical standard deviation on this astrometric correction is ∼ < 0.3 00 , comparable to the accuracy of the optical astrometry. For fields with multiple optical images we modify this algorithm slightly. Some images cover an area with few (or one) X-ray sources, which, if we were to follow the procedure described above, could lead to incorrect offsets and matches. Instead, for a given Chandra field, we first take each optical image and find the closest match within a 4 00 radius of all Chandra source positions that fall within the image. We record these astrometric differences. For X-ray sources falling on multiple optical images, we use the data from the image with the deepest limiting magnitude. As before, we use this list of multi-image astrometric differences to calculate an average Chandra to USNO offset, again eliminating sources greater than 1.5σ from the mean offset. This astrometric correction is then applied to all Chandra positions for the SEXSI field considered. A second pass at optical identifications is then made with the OAA-dependent matching radius, using the larger of 1.5 00 and PSF/3. Of the 998 SEXSI sources with optical coverage, 655 used a 1.5 00 search radius, while 343 used the PSF/3 search radius.
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X Ray specs

X Ray specs

advertisements that populated the back of pulp magazines from the 1920s to the 1960s. The implication that a technological enhancement of vision could be turned toward sexual voyeurism had been part of the first responses to Röntgen’s discovery, as the Punch poem quoted earlier suggests. In the pulp magazines, the adverts for X- Ray Specs promised ‘See the bones in your hand! See through clothes!’ They slotted into the world of fantastical hyper-masculine enhancement. In 1922, Charles Atlas, who started out as an under- nourished Italian immigrant who grew up in Brooklyn, began his mail order body building business, offering to turn nerdy weaklings into butch hard bodies. The famous ads with a weakling having sand kicked in his face, only to return as a triumphant Hercules with sculpted muscles and a surplus of sexual magnetism, entered into the everyday discourse of popular culture. By 1942 there were four hundred thousand people on the Atlas course, and he had a mail order business that employed 27 secretaries to answer the mail. These ads appeared in detective pulps like Black Mask, whose hard-boiled, tough-guy he-men offered ‘a prophylactic toughness that was organized around the rigorous suppression of affect.’ [11] In the science fiction pulps, there was a
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Time-dependent search for neutrino emission from X-ray binaries with the ANTARES telescope

Time-dependent search for neutrino emission from X-ray binaries with the ANTARES telescope

X-ray binaries are binary systems composed of a compact object (neutron star (NS) or stellar mass black hole (BH) candidate) and a companion non-degenerate star. Due to the strong gravitational attraction, matter expelled from the companion is accreted by the compact object. Depending on the mass of the companion star and the process of matter accretion, X- ray binaries are separated into two classes: Low-Mass X-ray Binaries (LMXB) which contain an evolved companion star of spectral class later than B transferring matter to the compact object through Roche lobe overflows; and High-Mass X-ray Binaries (HMXB) consisting of a massive O or B star developing intense stellar winds, a fraction of which is accreted by the compact object. While some of these objects are seen as persistent sources, most of them exhibit occasional outbursts, making them transient sources, in particular in the radio and X-ray domains.
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X-ray and optical studies of soft x-ray transients in M31

X-ray and optical studies of soft x-ray transients in M31

This Chapter started with a theoretical discussion of the expected light curve behavior, and followed with the observational data (obtained from Chandra, XMM-Newton and Swift) for com- parison. The transients used in the sample were selected on the basis of their bright outburst luminosity, and hence were each expected to show an initial exponential decay on the basis of the KR98 model. However, only two showed such an effect: the others were more consistent with a linear decay, suggesting a relatively weakly irradiated disk, in contrast to the observed X-ray luminosity. This suggests that the KR98 result (based on an α-accretion disk model) is too simple, and should be expanded to incorporate, for example, an ADAF. Clearly, however, the dataset should be expanded. This would involve increasing both the number of sources, and the sampling of their light curves. The latter is important because sparse sampling may result in important behavior being overlooked (such as an exponential early phase). In addition, greater sampling will allow us place better constraints on where the state changes occur (e.g. high-soft to low-hard), as a function of L x . All of these issues will be gradually addressed over time as M31 continues to be monitored
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