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Kirpal Nandra,
Didier Barret,
Xavier Barcons,
Andy Fabian,
Jan-Willem den Herder,
Luigi Piro,
Mike Watson,
Christophe Adami,
James Aird,
Jose Manuel Afonso, [......],
Joern Wilms,
Michael Wise,
Diana Worrall,
Andrew Young,
Luca Zampieri,
Jean In't Zand,
Silvia Zane,
Andreas Zezas,
Yuying Zhang,
Irina Zhuravleva
[show abstract]
[hide abstract]
ABSTRACT: This White Paper, submitted to the recent ESA call for science themes to
define its future large missions, advocates the need for a transformational
leap in our understanding of two key questions in astrophysics: 1) How does
ordinary matter assemble into the large scale structures that we see today? 2)
How do black holes grow and shape the Universe? Hot gas in clusters, groups and
the intergalactic medium dominates the baryonic content of the local Universe.
To understand the astrophysical processes responsible for the formation and
assembly of these large structures, it is necessary to measure their physical
properties and evolution. This requires spatially resolved X-ray spectroscopy
with a factor 10 increase in both telescope throughput and spatial resolving
power compared to currently planned facilities. Feedback from supermassive
black holes is an essential ingredient in this process and in most galaxy
evolution models, but it is not well understood. X-ray observations can
uniquely reveal the mechanisms launching winds close to black holes and
determine the coupling of the energy and matter flows on larger scales. Due to
the effects of feedback, a complete understanding of galaxy evolution requires
knowledge of the obscured growth of supermassive black holes through cosmic
time, out to the redshifts where the first galaxies form. X-ray emission is the
most reliable way to reveal accreting black holes, but deep survey speed must
improve by a factor ~100 over current facilities to perform a full census into
the early Universe. The Advanced Telescope for High Energy Astrophysics
(Athena+) mission provides the necessary performance (e.g. angular resolution,
spectral resolution, survey grasp) to address these questions and revolutionize
our understanding of the Hot and Energetic Universe. These capabilities will
also provide a powerful observatory to be used in all areas of astrophysics.
06/2013;
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[show abstract]
[hide abstract]
ABSTRACT: Non-solar composition of the donor star in ultra-compact X-ray binaries may
have a pronounced effect on the fluorescent lines appearing in their spectra
due to reprocessing of primary radiation by the accretion disk and the white
dwarf surface. We show that the most dramatic and easily observable consequence
of the anomalous C/O abundance, is the significant, by more than an order of
magnitude, attenuation of the Ka line of iron. It is caused by screening of the
presence of iron by oxygen - in the C/O dominated material the main interaction
process for a E ~ 7keV photon is absorption by oxygen rather than by iron,
contrary to the solar composition case. Ionization of oxygen at high mass
accretion rates adds a luminosity dependence to this behavior - the iron line
is significantly suppressed only at low luminosity, log(LX) less than 37-37.5,
and should recover its nominal strength at higher luminosity. The increase of
the EW of the Ka lines of carbon and oxygen, on the other hand, saturates at
rather moderate values. Screening by He is less important, due to its low
ionization threshold and because in the accretion disk it is mostly ionized.
Consequently, in the case of the He-rich donor, the iron line strength remains
close to its nominal value, determined by the iron abundance in the accretion
disk. This opens the possibility of constraining the nature of donor stars in
UCXBs by means of X-ray spectroscopy with moderate energy resolution.
02/2013;
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Mark J. Burke,
Somak Raychaudhury,
Ralph P. Kraft,
Thomas J. Maccarone,
Nicola J. Brassington,
Martin J. Hardcastle,
Jouni Kainulainen,
Kristin A. Woodley,
Joanna L. Goodger,
Gregory R. Sivakoff, [......],
Stephen S. Murray,
Mark Birkinshaw,
Judith H. Croston,
Daniel A. Evans, Marat Gilfanov,
Andres Jordan,
Craig L. Sarazin,
Rasmus Voss,
Diana M. Worrall,
Zhongli Zhang
[show abstract]
[hide abstract]
ABSTRACT: We present a spectral investigation of X-ray binaries in NGC 5128 (Cen A),
using six 100 ks Chandra observations taken over two months in 2007. We divide
our sample into thermally and non-thermally dominated states based on the
behavior of the fitted absorption column, and present the spectral parameters
of sources with L >2x10^37 erg/s. The majority of sources are consistent with
being neutron star low mass X-ray binaries (NS LMXBs) and we identify three
transient black hole (BH) LMXB candidates coincident with the dust lane, which
is the remnant of a small late-type galaxy. Our results also provide tentative
support for the apparent `gap' in the mass distribution of compact objects
between ~2-5 Msol.
We propose that BH LMXBs are preferentially found in the dust lane, and
suggest this is because of the younger stellar population. The majority
(~70-80%) of potential Roche-lobe filling donors in the Cen A halo are >12 Gyr
old, while BH LMXBs require donors >1 Msol to produce the observed peak
luminosities. This requirement for more massive donors may also explain recent
results that claim a steepening of the X-ray luminosity function with age at Lx
>= 5x10^38 erg/s for the XB population of early-type galaxies; for older
stellar populations, there are fewer stars >1 Msol, which are required to form
the more luminous sources.
02/2013;
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Akos Bogdan,
William R. Forman,
Mark Vogelsberger,
Herve Bourdin,
Debora Sijacki,
Pasquale Mazzotta,
Ralph P. Kraft,
Christine Jones, Marat Gilfanov,
Eugene Churazov,
Laurence P. David
[show abstract]
[hide abstract]
ABSTRACT: Luminous X-ray gas coronae in the dark matter halos of massive spiral
galaxies are a fundamental prediction of structure formation models, yet only a
few such coronae have been detected so far. In this paper, we study the hot
X-ray coronae beyond the optical disks of two normal massive spirals, NGC1961
and NGC6753. Based on XMM-Newton X-ray observations, hot gaseous emission is
detected to ~60 kpc - well beyond their optical radii. The hot gas has a
best-fit temperature of kT~0.6 keV and an abundance of ~0.1 Solar, and exhibits
a fairly uniform distribution, suggesting that the quasi-static gas resides in
hydrostatic equilibrium in the potential well of the galaxies. The bolometric
luminosity of the gas in the (0.05-0.15)r_200 region (r_200 is the virial
radius) is ~6e40 erg/s for both galaxies. The baryon mass fractions of NGC1961
and NGC6753 are f_b~0.1, which fall short of the cosmic baryon fraction. The
hot coronae around NGC1961 and NGC6753 offer an excellent basis to probe
structure formation simulations. To this end, the observations are confronted
with the moving mesh code Arepo and the smoothed particle hydrodynamics code
Gadget. Although neither model gives a perfect description, the observed
luminosities, gas masses, and abundances favor the Arepo code. Moreover, the
shape and the normalization of the observed density profiles are better
reproduced by Arepo within ~0.5r_200. However, neither model incorporates
efficient feedback from supermassive black holes or supernovae, which could
alter the simulated properties of the X-ray coronae. With the further advance
of numerical models, the present observations will be essential in constraining
the feedback effects in structure formation simulations.
12/2012;
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[show abstract]
[hide abstract]
ABSTRACT: For the first time, we have systematically explored the population of
discrete X-ray sources in the outskirts of early-type galaxies. Based on a
broad sample of 20 galaxies observed with Chandra we detected overdensity of
X-ray sources in their outskirts. The overdensity appears as halos of resolved
sources around the galaxies. These halos are broader than the stellar light,
extending out to at least ~ 10 Re (Re is the effective radius). These halos are
composed of sources fainter than ~5E38 erg/s, whereas the more luminous sources
appear to follow the distribution of the stellar light, suggesting that the
excess source population consists of neutron star binaries. Dividing the galaxy
sample into four groups according to their stellar mass and specific frequency
of globular clusters, we find that the extended halos are present in all groups
except for the low-mass galaxies with low globular cluster content. We propose
that the extended halos may be comprised of two independent components,
low-mass X-ray binaries (LMXBs) located in globular clusters (GCs), which are
known to have a wider distribution than the stellar light, and neutron star
(NS) LMXBs kicked out of the main body of the parent galaxy by supernova
explosions. The available deep optical and X-ray data of NGC 4365 support this
conclusion. For this galaxy we identified 60.1+/-10.8 excess sources in the
4-10 Re region of which ~ 40% are located in GCs, whereas ~ 60% are field
LMXBs. We interpret the latter as kicked NS LMXBs. We discuss the implications
of these results for the natal kick distributions of black holes and neutron
stars.
11/2012;
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Mark J. Burke,
Somak Raychaudhury,
Ralph P. Kraft,
Nicola J. Brassington,
Martin J. Hardcastle,
Joanna L. Goodger,
Gregory R. Sivakoff,
William R. Forman,
Christine Jones,
Kristin A. Woodley, [......],
Jouni Kainulainen,
Mark Birkinshaw,
Judith H. Croston,
Daniel A. Evans, Marat Gilfanov,
Andres Jordan,
Craig L. Sarazin,
Rasmus Voss,
Diana M. Worrall,
Zhongli Zhang
[show abstract]
[hide abstract]
ABSTRACT: We report the discovery of a bright X-ray transient, CXOU J132527.6-430023,
in the nearby early-type galaxy NGC 5128. The source was first detected over
the course of five Chandra observations in 2007, reaching an unabsorbed
outburst luminosity of 1-2*10^38 erg/s in the 0.5-7.0 keV band before returning
to quiescence. Such luminosities are possible for both stellar-mass black hole
and neutron star X-ray binary transients. Here, we attempt to characterize the
nature of the compact object. No counterpart has been detected in the optical
or radio sky, but the proximity of the source to the dust lanes allows for the
possibility of an obscured companion. The brightness of the source after a >100
fold increase in X-ray flux makes it either the first confirmed transient
non-ULX black hole system in outburst to be subject to detailed spectral
modeling outside the Local Group, or a bright (>10^38 erg/s) transient neutron
star X-ray binary, which are very rare. Such a large increase in flux would
appear to lend weight to the view that this is a black hole transient. X-ray
spectral fitting of an absorbed power law yielded unphysical photon indices,
while the parameters of the best-fit absorbed disc blackbody model are typical
of an accreting ~10 Msol black hole in the thermally dominant state.
02/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We investigate the dependence of the low-mass X-ray binary (LMXB) population
in early-type galaxies on stellar age, by selecting 20 massive nearby
early-type galaxies from the Chandra archive occupying a relatively narrow
range of masses and spanning a broad range of ages, from 1.6 Gyr to more than
10 Gyrs, with the median value of 6 Gyrs. With the ~ 2000 X-ray point sources
detected in total, we correlated the specific number of LMXBs in each galaxy
with its stellar age and globular cluster (GC) content. We found a correlation
between the LMXB population and stellar age: older galaxies tend to possess
about ~50% more LMXBs (per unit stellar mass) than the younger ones. The
interpretation of this dependence is complicated by large scatter and a rather
strong correlation between stellar age and GC content of galaxies in our
sample. We present evidence suggesting that the more important factor may be
the evolution of the LMXB population with time. Its effect is further amplified
by the larger GC content of older galaxies and correspondingly, the larger
numbers of dynamically formed binaries in them. We also found clear evolution
of the X-ray luminosity function (XLF) with age, that younger galaxies have
more bright sources and fewer faint sources per unit stellar mass. The XLF of
LMXBs in younger galaxies appears to extend significantly beyond E39 erg/s.
Such bright sources seem to be less frequent in older galaxies. We found that 6
out of ~ 12 (ultra-) luminous sources are located in GCs.
02/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We explore the nature of unresolved X-ray emission in a broad sample of
galaxies of all morphological types based on archival Chandra data. After
removing bright compact sources, we study L_X/L_K luminosity ratios of
unresolved emission, and compare them with the Solar neighborhood values. We
conclude that unresolved emission is determined by four main components, three
of which were known before: (i) The population of faint unresolved sources
associated with old stellar population. In early-type galaxies, their 2-10 keV
band luminosity scales with the stellar mass with L_X/L_K = (3.1\pm 0.9) x
10^27 erg/s/L_K,sun; (ii) The ISM with kT ~ 0.2-0.8 keV present in galaxies of
all types. Because of the large dispersion in the gas content of galaxies, the
size of our sample is insufficient to obtain reliable scaling law for this
component; (iii) The population of unresolved young stars and young stellar
objects in late-type galaxies. Their 2-10 keV band luminosity scales with the
star-formation rate with L_X/SFR = (1.7\pm0.9) x 10^38 (erg/s)/(M_sun/yr); (iv)
In four old and massive Virgo ellipticals (M49, M60, M84, NGC4636) we find
anomalously high X-ray emission in the 2-10 keV band. Its presence has not been
recognized before and its nature is unclear. Although it appears to be stronger
in galaxies having stronger ISM component, its existence cannot be explained in
terms of an extrapolation of the warm ISM spectrum. Association with Virgo
cluster of galaxies suggests that the excess emission may be due to
intracluster gas accreted in the gravitational well of a massive galaxy. We
investigate this and other possibilities.
06/2011;
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[show abstract]
[hide abstract]
ABSTRACT: Based on the archival data from the Chandra observations of nearby galaxies,
we study different sub populations of low-mass X-ray binaries (LMXBs) -
dynamically formed systems in globular clusters (GCs) and in the nucleus of M31
and (presumably primordial) X-ray binaries in the fields of galaxies. Our aim
is to produce accurate luminosity distributions of X-ray binaries in different
environments, suitable for quantitative comparison with each other and with the
output of population synthesis calculations. Our sample includes seven nearby
galaxies (M31, Maffei 1, Centaurus A,M81, NGC 3379, NGC 4697, and NGC 4278) and
the Milky Way, which together provide relatively uniform coverage down to the
luminosity limit of E35 erg/s. In total we have detected 185 LMXBs associated
with GCs, 35 X-ray sources in the nucleus of M31, and 998 field sources of
which ~ 365 are expected to be background AGN. We combine these data, taking
special care to accurately account for X-ray and optical incompleteness
corrections and the removal of the contamination from the cosmic X-ray
background sources, to produce luminosity distributions of X-ray binaries in
different environments to far greater accuracy than has been obtained
previously. We found that luminosity distributions of GC and field LMXBs differ
throughout the entire luminosity range, the fraction of faint (log(Lx) < 37)
sources among the former being ~ 4 times less than in the field population. The
X-ray luminosity function (XLF) of sources in the nucleus of M31 is similar to
that of GC sources at the faint end but differs at the bright end, with the M31
nucleus hosting significantly fewer bright sources. We discuss the possible
origin and potential implications of these results.
03/2011;
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[show abstract]
[hide abstract]
ABSTRACT: Although there is a nearly universal agreement that type Ia supernovae are
associated with the thermonuclear disruption of a CO white dwarf, the exact
nature of their progenitors is still unknown. The single degenerate scenario
envisages a white dwarf accreting matter from a non-degenerate companion in a
binary system. Nuclear energy of the accreted matter is released in the form of
electromagnetic radiation or gives rise to numerous classical nova explosions
prior to the supernova event. We show that combined X-ray output of supernova
progenitors and statistics of classical novae predicted in the single
degenerate scenario are inconsistent with X-ray and optical observations of
nearby early type galaxies and galaxy bulges. White dwarfs accreting from a
donor star in a binary system and detonating at the Chandrasekhar mass limit
can account for no more than ~5% of type Ia supernovae observed in old stellar
populations.
03/2011;
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Zhiyuan Li,
Christine Jones,
William R. Forman,
Ralph P. Kraft,
Dharam V. Lal,
Rosanne Di Stefano,
Lee R. Spitler,
Shikui Tang,
Q. Daniel Wang, Marat Gilfanov,
and Mikhail Revnivtsev
[show abstract]
[hide abstract]
ABSTRACT: Based on new and archival Chandra observations of the Sombrero galaxy (M 104 = NGC 4594), we study the X-ray emission from its nucleus and the extended X-ray emission in and around its massive stellar bulge. We find that the 0.3-8 keV luminosity of the nucleus appears constant at ~2.4 × 1040 erg s-1, or ~10–7 of its Eddington luminosity, on three epochs between 1999 December and 2008 April, but drops by a factor of two in the 2008 November observation. The 2-6 keV unresolved emission from the bulge region closely follows the K-band starlight and most likely arises from unresolved stellar sources. At lower energies, however, the unresolved emission reaches a galactocentric radius of at least 23 kpc, significantly beyond the extent of the starlight, clearly indicating the presence of diffuse hot gas. We isolate the emission of the gas by properly accounting for the emission from unresolved stellar sources, predominantly cataclysmic variables and coronally active binaries, whose quasi-universal X-ray emissivity was recently established. We find a gas temperature of ~0.6 keV with little variation across the field of view, except for a lower temperature of ~0.3 keV along the stellar disk. The metal abundance is not well constrained due to the limited counting statistics, but is consistent with metal enrichment by Type Ia supernovae (SNe Ia). We measure a total intrinsic 0.3-2 keV luminosity of ~2 × 1039 erg s-1, which corresponds to only 1% of the available energy input by SNe Ia in the bulge, but is comparable to the prediction by the latest galaxy formation models for disk galaxies as massive as Sombrero. However, such numerical models do not fully account for internal feedback processes, such as nuclear feedback and stellar feedback, against accretion from the intergalactic medium. On the other hand, we find no evidence for either the nucleus or the very modest star-forming activities in the disk to be a dominant heating source for the diffuse gas. We also show that neither the expected energy released by SNe Ia nor the expected mass returned by evolved stars is recovered by observations. We argue that in Sombrero a galactic-scale subsonic outflow of hot gas continuously removes much of the "missing" energy and mass input from the bulge region. The observed density and temperature distributions of such an outflow, however, continue to pose challenges to theoretical studies.
The Astrophysical Journal 03/2011; 730(2):84. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We study X-ray to K-band luminosity ratios (L_X/L_K) of late-type galaxies in the 0.3-0.7 keV energy range. From the Chandra archive, we selected nine spiral and three irregular galaxies with point source detection sensitivity better than 5 x 10^36 erg/s in order to minimize the contribution of unresolved X-ray binaries. In late-type galaxies cold gas and dust may cause significant interstellar absorption, therefore we also demanded the existence of publicly available HI maps. The obtained L_X/L_K ratios vary between (5.4-68) x 10^27 erg/s/L_K,sun exceeding by factor of 2-20 the values obtained for gas-poor early-type galaxies. Based on these results we constrain the role of supersoft X-ray sources as progenitors of type Ia supernovae (SNe Ia). For majority of galaxies the upper limits range from ~3% to ~15% of the SN Ia frequency inferred from K-band luminosity, but for a few of them no meaningful constraints can be placed. On a more detailed level, we study individual structural components of spiral galaxies: bulge and disk, and, for grand design spiral galaxies, arm and interarm regions. Comment: 10 pages, 2 tables, 6 figures, accepted for publication in MNRAS, minor changes
09/2010;
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[show abstract]
[hide abstract]
ABSTRACT: We present a study of discrete X-ray sources in and around the bulge-dominated, massive Sa galaxy, Sombrero (M104), based on new and archival Chandra observations with a total exposure of ~200 ks. With a detection limit of L X 1037 erg s–1 and a field of view covering a galactocentric radius of ~30 kpc (115), 383 sources are detected. Cross-correlation with Spitler et al.'s catalog of Sombrero globular clusters (GCs) identified from HST/ACS observations reveals 41 X-ray sources in GCs, presumably low-mass X-ray binaries (LMXBs). Metal-rich GCs are found to have a higher probability of hosting these LMXBs, a trend similar to that found in elliptical galaxies. On the other hand, the four most luminous GC LMXBs, with apparently super-Eddington luminosities for an accreting neutron star, are found in metal-poor GCs. We quantify the differential luminosity functions (LFs) for both the detected GC and field LMXBs, whose power-law indices (~1.1 for the GC-LF and ~1.6 for field-LF) are consistent with previous studies for elliptical galaxies. With precise sky positions of the GCs without a detected X-ray source, we further quantify, through a fluctuation analysis, the GC-LF at fainter luminosities down to 1035 erg s–1. The derived index rules out a faint-end slope flatter than 1.1 at a 2σ significance, contrary to recent findings in several elliptical galaxies and the bulge of M31. On the other hand, the 2-6 keV unresolved emission places a tight constraint on the field LF, implying a flattened index of ~1.0 below 1037 erg s–1. We also detect 101 sources in the halo of Sombrero. The presence of these sources cannot be interpreted as galactic LMXBs whose spatial distribution empirically follows the starlight. Their number is also higher than the expected number of cosmic active galactic nuclei (52 ± 11 [1σ]) whose surface density is constrained by deep X-ray surveys. We suggest that either the cosmic X-ray background is unusually high in the direction of Sombrero, or a distinct population of X-ray sources is present in the halo of Sombrero.
The Astrophysical Journal 09/2010; 721(2):1368. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We investigate unresolved X-ray emission from M31 based on an extensive set of archival XMM-Newton and Chandra data. We show that extended emission, found previously in the bulge and thought to be associated with a large number of faint compact sources, extends to the disk of the galaxy with similar X-ray to K-band luminosity ratio. We also detect excess X-ray emission associated with the 10-kpc star-forming ring. The L_X/SFR ratio in the 0.5-2 keV band ranges from zero to ~1.8 x 10^38 (erg/s)/(M_sun/yr), excluding the regions near the minor axis of the galaxy where it is ~1.5-2 times higher. The latter is likely associated with warm ionized gas of the galactic wind rather than with the star-forming ring itself. Based on this data, we constrain the nature of Classical Nova (CN) progenitors. We use the fact that hydrogen-rich material, required to trigger the explosion, accumulates on the white dwarf surface via accretion. Depending on the type of the system, the energy of accretion may be radiated at X-ray energies, thus contributing to the unresolved X-ray emission. Based on the CN rate in the bulge of M31 and its X-ray surface brightness, we show that no more than ~10 per cent of CNe can be produced in magnetic cataclysmic variables, the upper limit being ~3 per cent for parameters typical for CN progenitors. In dwarf novae, >~90-95 per cent of the material must be accreted during outbursts, when the emission spectrum is soft, and only a small fraction in quiescent periods, characterized by rather hard spectra. Comment: 11 pages, 10 figures, 4 tables, accepted for publication in MNRAS
02/2010;
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[show abstract]
[hide abstract]
ABSTRACT: We aim to place upper limits on the combined X-ray emission from the population of steady nuclear-burning white dwarfs in galaxies. In the framework of the single-degenerate scenario these systems are believed to be likely progenitors of Type Ia supernovae. From the Chandra archive, we selected normal early-type galaxies with the point source detection sensitivity better than 10^37 erg/s to minimize the contribution of unresolved low-mass X-ray binaries. The galaxies, contaminated by emission from ionized ISM, were identified based on the analysis of radial surface brightness profiles and energy spectra. The sample was complemented by the bulge of M31 and the data for the solar neighborhood. To cover a broad range of ages, we also included NGC3377 and NGC3585. Our final sample includes eight gas-poor galaxies for which we determine L_X/L_K ratios in the 0.3-0.7 keV energy band. In computing the L_X we included both unresolved emission and soft resolved sources with the color temperature of kT_bb <= 200 eV. We find that the X/K luminosity ratios are in the range of (1.7-3.2) x 10^27 erg/s/L_K,sun. The data show no obvious trends with mass, age, or metallicity of the host galaxy, although a weak anti-correlation with the Galactic NH appears to exist. It is much flatter than predicted for a blackbody emission spectrum with temperature of ~50-75 eV, suggesting that sources with such soft spectra contribute significantly less than a half to the observed X/K ratios. However, the correlation of the X/K ratios with NH has a significant scatter and in the strict statistical sense cannot be adequately described by a superposition of a power law and a blackbody components with reasonable parameters, thus precluding quantitative constraints on the contribution from soft sources. (abbr.) Comment: 9 pages, 5 figures, 4 tables, accepted for publication in Astronomy and Astrophysics
02/2010;
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[show abstract]
[hide abstract]
ABSTRACT: There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in a compact binary system. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 10(7) years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of approximately 30-50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate.
Nature 02/2010; 463(7283):924-5. · 36.28 Impact Factor
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ABSTRACT: We demonstrate that unresolved X-ray emission from the bulge of M31 is composed of at least three different components: (i) Broad-band emission from a large number of faint sources – mainly accreting white dwarfs and active binaries, associated with the old stellar population, similar to the Galactic Ridge X-ray emission of the Milky Way. (ii) Soft emission from ionized gas with temperature of about ~ 300 eV and mass of ~ 4 × 106 M⊙. The gas distribution is significantly elongated along the minor axis of the galaxy suggesting that it may be outflowing in the direction perpendicular to the galactic disk. The shadows cast on the gas by spiral arms and the 10-kpc star-forming ring confirm large off-plane extent of the gas. (iii) Hard unresolved emission from spiral arms, most likely associated with protostars and young stellar objects located in the star-forming regions.
Proceedings of the International Astronomical Union 10/2009; 5:296 - 296.
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Rasmus Voss, Marat Gilfanov,
Gregory R. Sivakoff,
Ralph P. Kraft,
Andrés Jordán,
Somak Raychaudhury,
Mark Birkinshaw,
Nicola J. Brassington,
Judith H. Croston,
Daniel A. Evans,
William R. Forman,
Martin J. Hardcastle,
William E. Harris,
Christine Jones,
Adrienne M. Juett,
Stephen S. Murray,
Craig L. Sarazin,
Kristin A. Woodley,
and Diana M. Worrall
[show abstract]
[hide abstract]
ABSTRACT: We study the X-ray luminosity function (XLF) of low-mass X-ray binaries (LMXB) in the nearby early-type galaxy Centaurus A, concentrating primarily on two aspects of binary populations: the XLF behavior at the low-luminosity limit and the comparison between globular cluster and field sources. The 800 ksec exposure of the deep Chandra VLP program allows us to reach a limiting luminosity of ~8 × 1035 erg s–1, about ~2-3 times deeper than previous investigations. We confirm the presence of the low-luminosity break of the overall LMXB XLF at log(LX ) 37.2-37.6, below which the luminosity distribution follows a dN/d(ln L) ~ const law. Separating globular cluster and field sources, we find a statistically significant difference between the two luminosity distributions with a relative underabundance of faint sources in the globular cluster population. This demonstrates that the samples are drawn from distinct parent populations and may disprove the hypothesis that the entire LMXB population in early-type galaxies is created dynamically in globular clusters. As a plausible explanation for this difference in the XLFs, we suggest an enhanced fraction of helium-accreting systems in globular clusters, which are created in collisions between red giants and neutron stars. Due to the four times higher ionization temperature of He, such systems are subject to accretion disk instabilities at 20 times higher mass accretion rate and, therefore, are not observed as persistent sources at low luminosities.
The Astrophysical Journal 07/2009; 701(1):471. · 6.02 Impact Factor
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ABSTRACT: The purpose of this study is to explore the population of X-ray point sources in the bulge of M31, with the primary goal to contrast properties of various subpopulations, such as persistent and transient sources, primordial LMXBs and dynamically formed ones. Based on the data from 26 archival Chandra observations we study the source content and properties of various subpopulations of X-ray sources to a maximum distance of 12 arcmin from the centre of M31. A study of the spatial distribution and the luminosity function of the X-ray sources shows that the distribution of primordial LMXBs is consistent with the distribution of the K-band light and that their luminosity function flattens below ~10^{37} erg/s to the dN/dL proportional to 1/L law in agreement with the behaviour found earlier for LMXBs in the Milky Way and in Cen A. The luminosity function of dynamically formed LMXBs shows a prominent fall-off below log(L_X)<36.5. Although the statistics is insufficient to claim a genuine low-luminosity cut-off in the luminosity function, the best fit powerlaw with a slope of -0.6+-0.2 is significantly flatter than the dN/dL proportional to 1/L law. We found 28 transient X-ray sources. Their spatial distribution follows the distribution of the persistent LMXBs within the accuracy allowed by the limited number of transients. Comment: 15 pages, accepted for publication in A&A revised version includes a comparison of the LF of LMXBs in globular clusters in M31 and in the Galaxy
10/2006;
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Rasmus Voss, Marat Gilfanov,
Gregory R. Sivakoff,
Ralph P. Kraft,
Andres Jordan,
Somak Raychaudhury,
Mark Birkinshaw,
Nicola J. Brassington,
Judith H. Croston,
Daniel A. Evans,
William R. Forman,
Martin J. Hardcastle,
William E. Harris,
Christine Jones,
Adrienne M. Juett,
Stephen S. Murray,
Craig L. Sarazin,
Kristin A. Woodley,
Diana M. Worrall
The Astrophysical Journal, v.701, 471-480 (2009).
