1,329 research outputs found
Soft X-ray Emission from the Spiral Galaxy NGC 1313
The nearby barred spiral galaxy NGC 1313 has been observed with the PSPC
instr- ument on board the ROSAT X-ray satellite. Ten individual sources are
found. Three sources (X-1, X-2 and X-3 [SN~1978K]) are very bright (~10^40
erg/s) and are unusual in that analogous objects do not exist in our Galaxy. We
present an X-ray image of NGC~1313 and \xray spectra for the three bright
sources. The emission from the nuclear region (R ~< 2 kpc) is dominated by
source X-1, which is located ~1 kpc north of the photometric (and dynamical)
center of NGC~1313. Optical, far-infrared and radio images do not indicate the
presence of an active galactic nucleus at that position; however, the compact
nature of the \xray source (X-1) suggests that it is an accretion-powered
object with central mass M >~ 10^3 Msun. Additional emission (L_X ~ 10^39
erg/s) in the nuclear region extends out to ~2.6 kpc and roughly follows the
spiral arms. This emission is from 4 sources with luminosity of several x 10^38
erg/s, two of which are consistent with emission from population I sources
(e.g., supernova remnants, and hot interstellar gas which has been heated by
supernova remnants). The other two sources could be emission from population II
sources (e.g., low-mass \xray binaries). The bright sources X-2 and SN~1978K
are positioned in the southern disk of NGC~1313. X-2 is variable and has no
optical counterpart brighter than 20.8 mag (V-band). It is likely that it is an
accretion-powered object in NGC~1313. The type-II supernova SN~1978K (Ryder
\etal 1993) has become extra- ordinarily luminous in X-rays 13 years
after optical maximum.Comment: to appear in 10 Jun 1995 ApJ, 30 pgs uuencoded compressed postscript,
25 pgs of figures available upon request from colbert, whole preprint
available upon request from Sandy Shrader ([email protected]),
hopefully fixed unknown problem with postscript fil
Observational Evidence for Intermediate-Mass Black Holes in Ultra-luminous X-ray Sources
Evidence is mounting that some Ultra-luminous X-ray sources (ULXs) may
contain accreting intermediate-mass black holes (IMBHs). We review the current
observational evidence for IMBH-ULXs. While low-luminosity ULXs with L_X <~
10^39.5 erg/s (assuming isotropic emission) are consistent with mildly X-ray
beamed high-mass X-ray binaries, there are a considerable number of ULXs with
larger X-ray luminosities that are not easily explained by these models. Recent
high-S/N XMM X-ray spectra are showing an increasing number of ULXs with ``cool
disks'' -- accretion disks with multi-color blackbody inner disk temperatures
kT_in ~ 0.1-0.2 keV, consistent with accreting IMBHs. Optical emission-line
studies of ULX nebulae provide useful measurements of X-ray energetics, and can
thus determine if the X-rays are emitted isotropically. Analysis of an optical
spectrum of the Ho II ULX nebulae implies an X-ray energy source with ~10^40
erg/s is present, suggesting an isotropically-emitting IMBH. The spatial
coincidence of ULXs with dense star clusters (young clusters and globular
clusters) suggests that IMBHs formed in these clusters could be the compact
objects in the associated ULXs. Quasi-periodic oscillations and frequency
breaks in XMM power-density spectra of ULXs also suggest that the black hole
masses are more consistent with IMBHs than stellar-mass black holes. Since
_all_ of these ULXs with evidence for IMBHs are high-luminosity ULXs, i.e., L_X
>~ 10^40 ergs, we suggest that this class of ULXs is generally powered by
accreting IMBHs.Comment: Invited review talk at the Tenth Marcel Grossmann Meeting on General
Relativity, Rio de Janeiro, July 20-26, 2003. Proceedings edited by M.
Novello, S. Perez-Bergliaffa and R. Ruffini, World Scientific, Singapore,
2005. Full resolution version of this paper available at
http://blackhole.phys.cua.edu/Colbert_MGX.pd
A Deep ROSAT HRI Observation of NGC 1313
We describe a series of observations of NGC 1313 using the ROSAT HRI with a
combined exposure time of 183.5 ksec. The observations span an interval between
1992 and 1998; the purpose of observations since 1994 was to monitor the X-ray
flux of SN1978K, one of several luminous sources in the galaxy. No diffuse
emission is detected in the galaxy to a level of ~1-2x10^37 ergs/s/arcmin^-2. A
total of eight sources are detected in the summed image within the D_25
diameter of the galaxy. The luminosities of five of the eight range from
\~6x10^37 to ~6x10^38 erg/s; these sources are most likely accreting X-ray
binaries, similar to sources obseved in M31 and M33. The remaining three
sources all emit above 10^39 erg/s. We present light curves of the five
brightest sources. Variability is detected at the 99.9% level from four of
these. We identify one of the sources as an NGC 1313 counterpart of a Galactic
X-ray source. The light curve, though crudely sampled, most closely resembles
that of a Galactic black hole candidate such as GX339-4, but with considerably
higher peak X-ray luminosity. An additional seven sources lie outside of the
D_25 diameter and are either foreground stars or background AGN.Comment: 18 pages, 9 figures; accepted AJ, scheduled for November 200
A First Estimate Of The X-Ray Binary Frequency As A Function Of Star Cluster Mass In A Single Galactic System
We use the previously-identified 15 infrared star-cluster counterparts to
X-ray point sources in the interacting galaxies NGC 4038/4039 (the Antennae) to
study the relationship between total cluster mass and X-ray binary number. This
significant population of X-Ray/IR associations allows us to perform, for the
first time, a statistical study of X-ray point sources and their environments.
We define a quantity, \eta, relating the fraction of X-ray sources per unit
mass as a function of cluster mass in the Antennae. We compute cluster mass by
fitting spectral evolutionary models to K_s luminosity. Considering that this
method depends on cluster age, we use four different age distributions to
explore the effects of cluster age on the value of \eta and find it varies by
less than a factor of four. We find a mean value of \eta for these different
distributions of \eta = 1.7 x 10^-8 M_\sun^-1 with \sigma_\eta = 1.2 x 10^-8
M_\sun^-1. Performing a \chi^2 test, we demonstrate \eta could exhibit a
positive slope, but that it depends on the assumed distribution in cluster
ages. While the estimated uncertainties in \eta are factors of a few, we
believe this is the first estimate made of this quantity to ``order of
magnitude'' accuracy. We also compare our findings to theoretical models of
open and globular cluster evolution, incorporating the X-ray binary fraction
per cluster.Comment: 20 pages, 6 figures, accepted by Ap
- …