73 research outputs found
X-ray Localization of the Globular Cluster G1 with XMM-Newton
We present an accurate X-ray position of the massive globular cluster G1 by
using XMM-Newton and the Hubble Space Telescope (HST). The X-ray emission of G1
has been detected recently with XMM-Newton. There are two possibilities for the
origin of the X-ray emission. It can be either due to accretion of the central
intermediate-mass black hole, or by ordinary low-mass X-ray binaries. The
precise location of the X-ray emission might distinguish between these two
scenarios. By refining the astrometry of the XMM-Newton and HST data, we
reduced the XMM-Newton error circle to 1.5". Despite the smaller error circle,
the precision is not sufficient to distinguish an intermediate-mass black hole
and luminous low-mass X-ray binaries. This result, however, suggests that
future Chandra observations may reveal the origin of the X-ray emission.Comment: 4 pages, 2 figures; accepted for publication in Ap
Optical studies of the ultraluminous X-ray source NGC1313 X-2
NGC1313 X-2 was among the first ultraluminous X-ray sources discovered, and
has been a frequent target of X-ray and optical observations. Using the HST/ACS
multi-band observations, this source is identified with a unique counterpart
within an error circle of 0\farcs2. The counterpart is a blue star on the
edge of a young cluster of years amid a dominant old stellar
population. Its spectral energy distribution is consistent with that for a
Z=0.004 star with 8.5 about years old, or for an O7 V
star at solar metallicity. The counterpart exhibited significant variability of
mag between two F555W observations separated by
three months, reminiscent of the ellipsoidal variability due to the orbital
motion of this ULX binary.Comment: 21 pages, 7 figures, scheduled for the ApJ June 10, 2007, v662n 1
issu
X-ray Spectral Signatures of the Photon Bubble Model for Ultraluminous X-ray Sources
The nature of ultraluminous X-ray sources in nearby galaxies is one of the
major open questions in modern X-ray astrophysics. One possible explanation for
these objects is an inhomogeneous, radiation dominated accretion disk around a
black hole -- the so-called ``photon bubble'' model. While
previous studies of this model have focused primarily on its
radiation-hydrodynamics aspects, in this paper, we provide an analysis of its
X-ray spectral (continuum and possible edge and line) characteristics. Compton
reflection between high and low density regions in the disk may provide the key
to distinguishing this model from others, such as accretion onto an
intermediate mass black hole. We couple a Monte Carlo/Fokker-Planck radiation
transport code with the XSTAR code for reflection to simulate the photon
spectra produced in a photon bubble model for ULXs. We find that reflection
components tend to be very weak and in most cases not observable, and make
predictions for the shape of the high-energy Comptonizing spectra. In many
cases the Comptonization dominates the spectra even down to a few keV.
In one simulation, a \sim 9 \kev feature was found, which may be considered a
signature of photon bubbles in ULXs; furthermore, we make predictions of high
energy power-laws which may be observed by future instruments.Comment: Accepted for publication in the Astrophysical Journa
New insights into ultraluminous X-ray sources from deep XMM-Newton observations
The controversy over whether ultraluminous X-ray sources (ULXs) contain a new
intermediate-mass class of black holes (IMBHs) remains unresolved. We present
new analyses of the deepest XMM-Newton observations of ULXs that address their
underlying nature. We examine both empirical and physical modelling of the
X-ray spectra of a sample of thirteen of the highest quality ULX datasets, and
find that there are anomalies in modelling ULXs as accreting IMBHs with
properties simply scaled-up from Galactic black holes. Most notably, spectral
curvature above 2 keV in several sources implies the presence of an
optically-thick, cool corona. We also present a new analysis of a 100 ks
observation of Holmberg II X-1, in which a rigorous analysis of the temporal
data limits the mass of its black hole to no more than 100 solar masses. We
argue that a combination of these results points towards many (though not
necessarily all) ULXs containing black holes that are at most a few 10s of
solar mass in size.Comment: 5 pages, 2 figures, to appear in the proceedings of "The X-ray
Universe 2005", San Lorenzo de El Escorial (Spain), 26-30 September 200
Spectral State Transitions of the Ultraluminous X-ray Sources X-1 and X-2 in NGC 1313
X-ray spectral state transitions are a key signature of black hole X-ray
binaries and follow a well-defined pattern. We examined 12 XMM-Newton
observations of the nearby spiral galaxy NGC 1313, which harbors two compact
ultraluminous X-ray sources (ULXs), X-1 and X-2, in order to determine if the
state transitions in ULXs follow the same pattern. For both sources, the
spectra were adequately fitted by an absorbed power-law with the addition of a
low temperature (kT=0.1~0.3 keV) disk blackbody component required in 6 of the
12 observations. As the X-ray luminosity of X-1 increases to a maximum at
3x10^40 ergs/s, the power-law photon index softens to 2.5-3.0. This behavior is
similar to the canonical spectral state transitions in Galactic black hole
binaries, but the source never enters the high/soft or thermal dominant state
and instead enters the steep power-law state at high luminosities. X-2 has the
opposite behavior and appears to be in the hard state, with a photon index of
Gamma=1.7-2.0 at high luminosity, but can soften to Gamma=2.5 at the lower
luminosities.Comment: 5 pages, 2 figures, submitted to ApJL on May 26t
The dependence of the estimated luminosities of ULX on spectral models
Data from {\it Chandra} observations of thirty nearby galaxies were analyzed
and 365 X-ray point sources were chosen whose spectra were not contaminated by
excessive diffuse emission and not affected by photon pile up. The spectra of
these sources were fitted using two spectral models (an absorbed power-law and
a disk blackbody) to ascertain the dependence of estimated parameters on the
spectral model used. It was found that the cumulative luminosity function
depends on the choice of the spectral model, especially for luminosities ergs/s. In accordance with previous results, a large number () of the sources have luminosities ergs/s (Ultra-Luminous X-ray
sources) with indistinguishable average spectral parameters (inner disk
temperature keV and/or photon index ) with those of the
lower luminosities ones. After considering foreground stars and known
background AGN,we identify four sources whose minimum luminosity exceed
ergs/s, and call them Extremely Luminous X-ray sources (ELX). The
spectra of these sources are in general better represented by the disk black
body model than the power-law one. These ELX can be grouped into two distinct
spectral classes. Two of them have an inner disk temperature of keV and
hence are called ``supersoft'' ELX, while the other two have temperatures
keV and are called ``hard'' ELX. The estimated inner disk
temperatures of the supersoft ELX are compatible with the hypothesis that they
harbor intermediate size black holes, which are accreting at times
their Eddington Luminosity. The radiative mechanism for hard ELX, seems to be
Inverse Comptonization, which in contrast to standard black holes systems, is
probably saturated.Comment: Accepted for publication in Astrophysical Journal. 9 pages. Complete
long Tables 4 and 5 are given as tab4.tex and tab5.tex separatel
Elemental Abundances of Nearby Galaxies through High Signal-to-Noise XMM-Newton Observations of ULXs
(abridged) In this paper, we examined XMM Newton EPIC spectra of 14
ultra-luminous X-ray sources (ULXs)in addition to the XMM RGS spectra of two
sources (Holmberg II X-1 and Holmberg IX X-1). We determined oxygen and iron
abundances of the host galaxy's interstellar medium (ISM) using K-shell (O) and
L-shell (Fe) X-ray photo-ionization edges towards these ULXs. We found that the
oxygen abundances closely matched recent solar abundances for all of our
sources, implying that ULXs live in similar local environments despite the wide
range of galaxy host properties. Also, we compare the X-ray hydrogen column
densities (n_H) for 8 ULX sources with column densities obtained from radio H I
observations. The X-ray model n_H values are in good agreement with the H I n_H
values, implying that the hydrogen absorption towards the ULXs is not local to
the source (with the exception of the source M81 XMM1). In order to obtain the
column density and abundance values, we fit the X-ray spectra of the ULXs with
a combined power law and one of several accretion disk models. We tested the
abundances obtained from the XSPEC models bbody, diskbb, grad, and diskpn along
with a power law, finding that the abundances were independent of the thermal
model used. We comment on the physical implications of these different model
fits. We also note that very deep observations allow a breaking of the
degeneracy noted by Stobbart et al. (2006) favoring a high mass solution for
the absorbed grad + power law model.Comment: 18 pages, accepted to Ap
The hard X-ray spectral evolution in X-ray binaries and its application to constrain the black hole mass of ultraluminous X-ray sources
We investigate the relationship between the hard X-ray photon index
and the Eddington ratio () in six X-ray
binaries (XRBs) with well constrained black hole masses and distances. We find
that different XRBs follow different anti-correlations between and
when is less than a critical value, while and
generally follow the same positive correlation when is larger than the
critical value. The anti-correlation and the positive correlation may suggest
that they are in different accretion modes (e.g., radiatively inefficient
accretion flow (RIAF) and standard disk). We fit both correlations with the
linear least-square method for individual sources, from which the crosspoint of
two fitted lines is obtained. Although the anti-correlation varies from source
to source, the crosspoints of all sources roughly converge to the same point
with small scatter(), which may
correspond to the transition point between RIAF and standard accretion disk.
Motivated by the observational evidence for the similarity of the X-ray
spectral evolution of ultraluminous X-ray sources (ULXs) to that of XRBs, we
then constrain the black hole masses for seven ULXs assuming that their X-ray
spectral evolution is similar to that of XRBs. We find that the BH masses of
these seven luminous ULXs are around 10^{4}\msun, which are typical
intermediate-mass BHs (IMBHs). Our results are generally consistent with the BH
masses constrained from the timing properties (e.g., break frequency) or the
model fitting with a multi-color disk.Comment: accepted for publication in ApJ, 18 pages, 2 figures, Comments is
welcomed
Accretion Disk Spectra of the Brightest Ultra-luminous X-ray Source in M82
Emission spectra of hot accretion disks characteristic of advection dominated
accretion flow (ADAF) models are investigated for comparison with the brightest
ultra-luminous source, X-1, in the galaxy M82. If the spectral state of the
source is similar to the low luminosity hard state of stellar mass black holes
in our Galaxy, a fit to the {\it Chandra} X-ray spectrum and constraints from
the radio and infrared upper limits, require a black hole mass in the range of
9 \times 10^4 - 5 \times 10^5 \msun. Lower black hole masses (\la 10^4
\msun) are possible if M82 X-1 corresponds to the high luminosity hard state
of Galactic black hole X-ray binary sources. Both of these spectrally
degenerate hot accretion disk solutions lead to an intermediate mass black hole
interpretation for M82 X-1. Since these solutions have different spectral
variability with X-ray luminosity and predict different infrared emission, they
can be distinguished by future off axis {\it Chandra} observations or
simultaneous sensitive infrared detections.Comment: 17 pages, 2 figures; accepted by Ap
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