416 research outputs found
X-ray emission from star-forming galaxies - I. High-mass X-ray binaries
Based on a homogeneous set of X-ray, infrared and ultraviolet observations
from Chandra, Spitzer, GALEX and 2MASS archives, we study populations of
high-mass X-ray binaries (HMXBs) in a sample of 29 nearby star-forming galaxies
and their relation with the star formation rate (SFR). In agreement with
previous results, we find that HMXBs are a good tracer of the recent star
formation activity in the host galaxy and their collective luminosity and
number scale with the SFR, in particular, Lx~2.6 10^{39} SFR. However, the
scaling relations still bear a rather large dispersion of ~0.4 dex, which we
believe is of a physical origin. We present the catalog of 1057 X-ray sources
detected within the ellipse for galaxies of our sample and construct the
average X-ray luminosity function (XLF) of HMXBs with substantially improved
statistical accuracy and better control of systematic effects than achieved in
previous studies. The XLF follows a power law with slope of 1.6 in the
logLx~35-40 luminosity range with a moderately significant evidence for a break
or cut-off at Lx~10^{40} erg/s. As before, we did not find any features at the
Eddington limit for a neutron star or a stellar mass black hole. We discuss
implications of our results for the theory of binary evolution. In particular
we estimate the fraction of compact objects that once upon their lifetime
experienced an X-ray active phase powered by accretion from a high mass
companion and obtain a rather large number, fx~0.2 (0.1 Myr/tau_x) (tau_x is
the life time of the X-ray active phase). This is ~4 orders of magnitude more
frequent than in LMXBs. We also derive constrains on the mass distribution of
the secondary star in HMXBs.Comment: 23 pages, 14 figures, 5 tables, MNRAS - Accepted 2011 September 2
Statistical properties of the combined emission of a population of discrete sources: astrophysical implications
We study the statistical properties of the combined emission of a population
of discrete sources (e.g. X-ray emission of a galaxy due to its X-ray binaries
population). Namely, we consider the dependence of their total luminosity
L_tot=SUM(L_k) and of fractional rms_tot of their variability on the number of
sources N or, equivalently, on the normalization of the luminosity function. We
show that due to small number statistics a regime exists, in which L_tot grows
non-linearly with N, in an apparent contradiction with the seemingly obvious
prediction =integral(dN/dL*L*dL) ~ N. In this non-linear regime, the
rms_tot decreases with N significantly more slowly than expected from the rms ~
1/sqrt(N) averaging law. For example, for a power law luminosity function with
a slope of a=3/2, in the non-linear regime, L_tot ~ N^2 and the rms_tot does
not depend at all on the number of sources N. Only in the limit of N>>1 do
these quantities behave as intuitively expected, L_tot ~ N and rms_tot ~
1/sqrt(N). We give exact solutions and derive convenient analytical
approximations for L_tot and rms_tot.
Using the total X-ray luminosity of a galaxy due to its X-ray binary
population as an example, we show that the Lx-SFR and Lx-M* relations predicted
from the respective ``universal'' luminosity functions of high and low mass
X-ray binaries are in a good agreement with observations. Although caused by
small number statistics the non-linear regime in these examples extends as far
as SFR<4-5 Msun/yr and log(M*/Msun)<10.0-10.5, respectively.Comment: MNRAS, accepted for publicatio
Lx-SFR relation in star forming galaxies
We compare the results of Grimm et al. (2003) and Ranalli et al. (2003) on
the Lx-SFR relation in normal galaxies. Based on the Lx-stellar mass dependence
for LMXBs, we show, that low SFR (SFR<1 Msun/year) galaxies in the Ranalli et
al. sample are contaminated by the X-ray emission from low mass X-ray binaries,
unrelated to the current star formation activity.
The most important conclusion from our comparison is, however, that after the
data are corrected for the ``LMXB contamination'', the two datasets become
consistent with each other, despite of their different content, variability
effects, difference in the adopted source distances, X-ray flux and star
formation rate determination and in the cosmological parameters used in
interpreting the HDF-N data. They also agree well, both in the low and high SFR
regimes, with the predicted Lx-SFR dependence derived from the parameters of
the ``universal'' HMXB luminosity function. This encouraging result emphasizes
the potential of the X-ray luminosity as an independent star formation rate
indicator for normal galaxies.Comment: revised, accepted for publication in MNRAS Letter
Origin of the Galactic ridge X-ray emission
We analyze a map of the Galactic ridge X-ray emission (GRXE) constructed in
the 3-20 keV energy band from RXTE/PCA scan and slew observations. We show that
the GRXE intensity closely follows the Galactic near-infrared surface
brightness and thus traces the Galactic stellar mass distribution. The GRXE
consists of two spatial components which can be identified with the bulge/bar
and the disk of the Galaxy. The parameters of these components determined from
X-ray data are compatible with those derived from near-infrared data. The
inferred ratio of X-ray to near-infrared surface brightness I(3-20 keV) (1e-11
erg/s/cm2/deg2)/I_(3.5micron)(MJy/sr)=0.26+/-0.05, and the ratio of X-ray to
near-infrared luminosity L_(3-20 keV)/L_(3-4 micron)=(4.1+/-0.3)e-5. The
corresponding ratio of the 3-20 keV luminosity to the stellar mass is
L_x/M_Sun=
(3.5\pm0.5) 10^{27} erg/s, which agrees within the uncertainties with the
cumulative emissivity per unit stellar mass of point X-ray sources in the Solar
neighborhood, determined in an accompanying paper (Sazonov et al.). This
suggests that the bulk of the GRXE is composed of weak X-ray sources, mostly
cataclysmic variables and coronally active binaries. The fractional
contributions of these classes of sources to the total X-ray emissivity
determined from the Solar neighborhood data can also explain the GRXE energy
spectrum. Based on the luminosity function of local X-ray sources we predict
that in order to resolve 90% of the GRXE into discrete sources a sensitivity
limit of ~10^{-16} erg/s/cm2 (2--10 keV) will need to be reached in future
observations.Comment: 12 pages, 9 figures. Submitted to A&
Ni abundance in the core of the Perseus Cluster: an answer to the significance of resonant scattering
Using an XMM-Newton observation of the Perseus cluster we show that the
excess in the flux of the 7-8 keV line complex previously detected by ASCA and
BeppoSAX is due to an overabundance of Nickel rather than to an anomalously
high Fe He/Fe He ratio. This observational fact leads to the
main result that resonant scattering, which was assumed to be responsible for
the supposed anomalous Fe He/Fe He ratio, is no longer required.
The absence of resonant scattering points towards the presence of significant
gas motions (either turbulent or laminar) in the core of the Perseus cluster.Comment: 29 pages, 10 bw figures, accepted for publication in the
Astrophysical Journa
Super-Eddington outburst of V4641 Sgr
X-ray transients provide unique opportunity to probe accretion regimes of at
a vastly different accretion rates. We analyze a collection of the RXTE
observations (Galactic Center scans, ASM monitoring and a pointed observation)
of enigmatic transient source high mass X-ray binary V4641 Sgr and argue that
they broadly support the hypothesis that giant September 1999 outburst was
associated with an episode of super-Eddington accretion onto the black hole.
During the outburst an extended optically thick envelope/outflow has been
formed around the source making the observational appearance of V4641 Sgr in
many aspects very similar to that of SS433. These results suggest that objects
like V4641 Sgr and SS433 indeed represent the class of objects accreting matter
at a rate comparable or above Eddington value and the formation of an
envelope/outflow is a generic characteristic of supercritical accretion.
When the accretion rate decreased the envelope vanished and the source short
term variability and spectral properties started to resemble those of other
galactic black hole candidates accreting at a rate well below the Eddington
value. Interestingly that during this phase the source spectrum was very
similar to the Cygnus X-1 spectrum in the low state inspite of more than order
of magnitude larger X-ray luminosity.Comment: 10 pages, 12 figures. Accepted in Astronomy & Astrophysic
Population synthesis of accreting white dwarfs: II. X-ray and UV emission
Accreting white dwarfs (WDs) with non-degenerate companions are expected to
emit in soft X-rays and the UV, if accreted H-rich material burns stably. They
are an important component of the unresolved emission of elliptical galaxies,
and their combined ionizing luminosity may significantly influence the optical
line emission from warm ISM. In an earlier paper we modeled populations of
accreting WDs, first generating WD with main-sequence, Hertzsprung gap and red
giant companions with the population synthesis code \textsc{BSE}, and then
following their evolution with a grid of evolutionary tracks computed with
\textsc{MESA}. Now we use these results to estimate the soft X-ray
(0.3-0.7keV), H- and He II-ionizing luminosities of nuclear burning WDs and the
number of super-soft X-ray sources for galaxies with different star formation
histories. For the starburst case, these quantities peak at Gyr and
decline by orders of magnitude by the age of 10 Gyr. For stellar
ages of ~10 Gyr, predictions of our model are consistent with soft X-ray
luminosities observed by Chandra in nearby elliptical galaxies and He II
4686 line ratio measured in stacked SDSS spectra of retired
galaxies, the latter characterising the strength and hardness of the UV
radiation field. However, the soft X-ray luminosity and
He~II~4686 ratio are significantly overpredicted for stellar
ages of Gyr. We discuss various possibilities to resolve this
discrepancy and tentatively conclude that it may be resolved by a modification
of the typically used criteria of dynamically unstable mass loss for giant
stars.Comment: 13 pages, 12 figures, MNRAS accepte
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