198 research outputs found
The Orbit and Position of the X-ray Pulsar XTE J1855-026 - an Eclipsing Supergiant System
A pulse timing orbit has been obtained for the X-ray binary XTE J1855-026
using observations made with the Proportional Counter Array on board the Rossi
X-ray Timing Explorer. The mass function obtained of ~16Mo together with the
detection of an extended near-total eclipse confirm that the primary star is a
supergiant as predicted. The orbital eccentricity is found to be very low with
a best fit value of 0.04 +/- 0.02. The orbital period is also refined to be
6.0724 +/- 0.0009 days using an improved and extended light curve obtained with
RXTE's All Sky Monitor. Observations with the ASCA satellite provide an
improved source location of R.A. = 18h 55m 31.3s}, decl. = -02o 36' 24.0"
(2000) with an estimated systematic uncertainty of less than 12". A
serendipitous new source, AX J1855.4-0232, was also discovered during the ASCA
observations.Comment: Accepted for publication in the Astrophysical Journa
High Resolution Infrared Imaging of the Compact Nuclear Source in NGC4258
We present high resolution imaging of the nucleus of NGC4258 from 1 micron to
18 microns. Our observations reveal that the previously discovered compact
source of emission is unresolved even at the near-infrared resolution of about
0.2 arcsec FWHM which corresponds to about 7 pc at the distance of the galaxy.
This is consistent with the source of emission being the region in the
neighborhood of the purported 3.5*10^7 M_sun black hole. After correcting for
about 18 mags of visual extinction, the infrared data are consistent with a
F_nu \propto nu^(-1.4+/-0.1) spectrum from 1.1 micron to 18 micron, implying a
non-thermal origin. Based on this spectrum, the total extinction corrected
infrared luminosity (1-20 micron) of the central source is 2*10^8 L_sun. We
argue that the infrared spectrum and luminosity of the central source obviates
the need for a substantial contribution from a standard, thin accretion disk at
these wavelengths and calculate the accretion rate through an advection
dominated accretion flow to be Mdot \sim 10^(-3) M_sun/yr. The agreement
between these observations and the theoretical spectral energy distribution for
advection dominated flows provides evidence for the existence of an advection
dominated flow in this low luminosity AGN.Comment: 21 pages, 5 figures, Appearing in Mar 2000 ApJ vol. 53
Bridging the gap between stellar-mass black holes and ultraluminous X-ray sources
The X-ray spectral and timing properties of ultraluminous X-ray sources
(ULXs) have many similarities with the very high state of stellar-mass black
holes (power-law dominated, at accretion rates greater than the Eddington
rate). On the other hand, their cool disk components, large characteristic
inner-disk radii and low characteristic timescales have been interpreted as
evidence of black hole masses ~ 1000 Msun (intermediate-mass black holes). Here
we re-examine the physical interpretation of the cool disk model, in the
context of accretion states of stellar-mass black holes. In particular, XTE
J1550-564 can be considered the missing link between ULXs and stellar-mass
black holes, because it exhibits a high-accretion-rate, low-disk-temperature
state (ultraluminous branch). On the ultraluminous branch, the accretion rate
is positively correlated with the disk truncation radius and the bolometric
disk luminosity, while it is anti-correlated with the peak temperature and the
frequency of quasi-periodic-oscillations. Two prototypical ULXs (NGC1313 X-1
and X-2) also seem to move along that branch. We use a phenomenological model
to show how the different range of spectral and timing parameters found in the
two classes of accreting black holes depends on both their masses and accretion
rates. We suggest that ULXs are consistent with black hole masses ~ 50-100
Msun, moderately inefficiently accreting at ~20 times Eddington.Comment: 11 pages, accepted for publication in Astrophysics and Space Science.
Based on work presented at the Fifth Stromlo Symposium, Australian National
University, Dec 200
X-ray Signatures of an Ionized Reprocessor in the Seyfert galaxy Ton S 180
We discuss the hard X-ray properties of the Seyfert galaxy Ton S 180, based
upon the analysis of ASCA data. We find the X-ray flux varied by a factor ~2 on
a time scale of a few thousand seconds. The source showed significantly higher
amplitude of variability in the 0.5-2 keV band than in the 2-10 keV band. The
continuum is adequately parameterized as a Gamma ~ 2.5 power-law across the
0.6--10 keV band . We confirm the recent discovery of an emission line of high
equivalent width, due to Fe K-shell emission from highly-ionized material.
These ASCA data show the Fe line profile to be broad and asymmetric and
tentatively suggest it is stronger during the X-ray flares, consistent with an
origin from the inner parts of an accretion disk. The X-ray spectrum is complex
below 2 keV, possibly due to emission from a blend of soft X-ray lines, which
would support the existence of an ionized reprocessor, most likely due to a
relatively high accretion rate in this source.Comment: 24 pages, 8 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journa
A diagnosis on torque reversals in 4U 1626-67
Several X-ray pulsars have been observed to experience torque reversals,
which provide important observational clues to the interaction between the
neutron star magnetic field and the accretion disk. We review the current
models proposed for the torque reversals and discuss their viability based on
the observations of the quasi-periodic oscillations (QPOs) in 4U 1626-67. Most
of these models seem to be incompatible with the evolution of the QPO
frequencies if they are interpreted in terms of the beat frequency model. We
suggest that winds or outflows from the neutron star and the accretion disk may
play an important role in accounting for the spin-down in disk-fed neutron
stars.Comment: 7 pages, accepted for publication in A&
Multi-band study of NGC 7424 and its two newly-discovered ULXs
We have studied the face-on, barred spiral NGC 7424 (site of the rare Type
IIb SN 2001ig) with Chandra, Gemini and the Australia Telescope Compact Array.
After giving revised X-ray colours and luminosity of the supernova, here we
focus on some other interesting sources in the galaxy: in particular, our
serendipitous discovery of two ultraluminous X-ray sources (ULXs). The brighter
one (~ 10^{40} erg/s) has a power-law-like spectrum with photon index Gamma ~
1.8. The other ULX shows a spectral state transition or outburst between the
two Chandra observations, 20 days apart. Optical data show that this ULX is
located in a young (age ~ 7-10 Myr), bright complex rich with OB stars and
clusters. An exceptionally bright, unresolved radio source (0.14 mJy at 4.79
GHz, implying a radio luminosity twice as high as Cas A) is found slightly
offset from the ULX (~ 80 pc). Its radio spectral index alpha ~ -0.7 suggests
optically-thin synchrotron emission, either from a young supernova remnant or
from a radio lobe powered by a ULX jet. An even brighter, unresolved radio
source (0.22 mJy at 4.79 GHz) is found in another young, massive stellar
complex, not associated with any X-ray sources: based on its flatter radio
spectral index (alpha ~ -0.3), we suggest that it is a young pulsar wind
nebula, a factor of 10 more radio luminous than the Crab.Comment: 12 pages, 11 figures. Accepted by MNRAS. High-resolution colour
images of NGC 7424 from the Gemini observations are available at
http://www.gemini.edu/2001igp
A variable absorption feature in the X-ray spectrum of a magnetar
Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly
rotating, isolated neutron stars that sporadically undergo episodes of
long-term flux enhancement (outbursts) generally accompanied by the emission of
short bursts of hard X-rays. This behaviour can be understood in the magnetar
model, according to which these sources are mainly powered by their own
magnetic energy. This is supported by the fact that the magnetic fields
inferred from several observed properties of AXPs and SGRs are greater than -
or at the high end of the range of - those of radio pulsars. In the peculiar
case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing
parameters, whereas a strong field has been proposed to reside in the stellar
interior and in multipole components on the surface. Here we show that the
X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which
depend strongly on the star's rotational phase. This line is interpreted as a
proton cyclotron feature and its energy implies a magnetic field ranging from
2E14 gauss to more than 1E15 gauss.Comment: Nature, 500, 312 (including Supplementary Information
Optical Afterglow Observations of the Unusual Short-Duration Gamma-Ray Burst 040924
The 1-m telescope at Lulin Observatory and the 0.76-m Katzman Automatic
Imaging Telescope at Lick Observatory were used to observe the optical
afterglow of the short-duration (1.2--1.5 s) gamma-ray burst (GRB) 040924. This
object has a soft high-energy spectrum, thus making it an exceptional case,
perhaps actually belonging to the short-duration tail of the long-duration
GRBs. Our data, combined with other reported measurements, show that the early
R-band light curve can be described by two power laws with index alpha = -0.7
(at t = 16-50 min) and alpha = -1.06 (at later times). The rather small
difference in the spectral indices can be more easily explained by an afterglow
model invoking a cooling break rather than a jet break.Comment: 13 pages, 3 figures, accepted for publication in ApJ
On the formation and evolution of black-hole binaries
We present the results of a systematic study of the formation and evolution
of binaries containing black holes and normal-star companions with a wide range
of masses. We first reexamine the standard formation scenario for close
black-hole binaries, where the spiral-in of the companion in the envelope of a
massive star causes the ejection of the envelope. We estimate the formation
rates for different companion masses and different assumptions about the
common-envelope structure and other model parameters. We find that black-hole
binaries with intermediate- and high-mass secondaries can form for a wide range
of assumptions, while black-hole binaries with low-mass secondaries can only
form with apparently unrealistic assumptions (in agreement with previous
studies). We then present detailed binary evolution sequences for black-hole
binaries with secondaries of 2 to 17 Msun and demonstrate that in these systems
the black hole can accrete appreciably even if accretion is Eddington limited
(up to 7 Msun for an initial black-hole mass of 10 Msun) and that the black
holes can be spun up significantly in the process. We discuss the implications
of these calculations for well-studied black-hole binaries (in particular GRS
1915+105), ultra-luminous X-ray sources and Cygnus X-1. Finally, we discuss how
some of the assumptions in the standard model could be relaxed to allow the
formation of low-mass, short-period black-hole binaries which appear to be very
abundant in Nature. (Abstract abridged)Comment: 21 pages, 9 figures, accepted by MNRAS, Figs. 2a/2b and 5 in very
reduced forma
Formation and Evolution of Supermassive Black Holes
The correlation between the mass of supermassive black holes in galaxy nuclei
and the mass of the galaxy spheroids or bulges (or more precisely their central
velocity dispersion), suggests a common formation scenario for galaxies and
their central black holes. The growth of bulges and black holes can commonly
proceed through external gas accretion or hierarchical mergers, and are both
related to starbursts. Internal dynamical processes control and regulate the
rate of mass accretion. Self-regulation and feedback are the key of the
correlation. It is possible that the growth of one component, either BH or
bulge, takes over, breaking the correlation, as in Narrow Line Seyfert 1
objects. The formation of supermassive black holes can begin early in the
universe, from the collapse of Population III, and then through gas accretion.
The active black holes can then play a significant role in the re-ionization of
the universe. The nuclear activity is now frequently invoked as a feedback to
star formation in galaxies, and even more spectacularly in cooling flows. The
growth of SMBH is certainly there self-regulated. SMBHs perturb their local
environment, and the mergers of binary SMBHs help to heat and destroy central
stellar cusps. The interpretation of the X-ray background yields important
constraints on the history of AGN activity and obscuration, and the census of
AGN at low and at high redshifts reveals the downsizing effect, already
observed for star formation. History appears quite different for bright QSO and
low-luminosity AGN: the first grow rapidly at high z, and their number density
decreases then sharply, while the density of low-luminosity objects peaks more
recently, and then decreases smoothly.Comment: 31 pages, 13 figures, review paper for Astrophysics Update
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