402 research outputs found
Extended Emission from Cygnus X-3 Detected with Chandra
We have discovered extended X-ray emission from the microquasar Cyg X-3 in
archival Chandra X-ray Observatory observations. A 5" wide structure lies
approximately 16" to the NE from the core point source and may be extended in
that direction. This angular scale corresponds to a physical extent of roughly
0.8 lyr, at a distance of 2.5 lyr from Cyg X-3 (assuming a 10 kpc distance).
The flux varied by a factor of 2.5 during the four months separating two of the
observations, indicating significant substructure. The peak 2-10 keV luminosity
was about 5e34 ergs/s. There may also be weaker, extended emission of similar
scale oppositely directed from the core, suggesting a bipolar outflow. This
structure is not part of the dust scattering halo, nor is it caused by the
Chandra point spread function. In this Letter we describe the observations and
discuss possible origins of the extension.Comment: Submitted to ApJ Letters. 5 pages, 2 figures (1 color). Uses
emulateap
Very hard states in neutron star low-mass X-ray binaries
We report on unusually very hard spectral states in three confirmed
neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and
IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting
the Swift X-ray spectra (0.5 - 10 keV) in those states with an absorbed
power-law model, we found photon indices of \Gamma ~ 1, significantly lower
than the \Gamma = 1.5 - 2.0 typically seen when such systems are in their so
called hard state. For individual sources very hard spectra were already
previously identified but here we show for the first time that likely our
sources were in a distinct spectral state (i.e., different from the hard state)
when they exhibited such very hard spectra. It is unclear how such very hard
spectra can be formed; if the emission mechanism is similar to that operating
in their hard states (i.e., up-scattering of soft photons due to hot electrons)
then the electrons should have higher temperatures or a higher optical depth in
the very hard state compared to those observed in the hard state. By using our
obtained \Gamma as a tracer for the spectral evolution with luminosity, we have
compared our results with those obtained by Wijnands et al. (2015). We confirm
their general results in that also our sample of sources follow the same track
as the other neutron star systems, although we do not find that the accreting
millisecond pulsars are systematically harder than the non-pulsating systems.Comment: Accepted for publication in MNRA
Chandra/HETGS Spectroscopy of the Galactic Black Hole GX 339-4: A Relativistic Iron Line and Evidence for a Seyfert-like Warm Absorber
We observed the Galactic black hole GX 339-4 with the Chandra High Energy
Transmission Grating Spectrometer (HETGS) for 75 ksec during the decline of its
2002-2003 outburst. The sensitivity of this observation provides an
unprecedented glimpse of a Galactic black hole at about a tenth of the
luminosity of the outburst peak. The continuum spectrum is well described by a
model consisting of multicolor disk blackbody (kT = 0.6 keV) and power-law
(Gamma = 2.5) components. X-ray reflection models yield improved fits. A
strong, relativistic Fe K-alpha emission line is revealed, indicating that the
inner disk extends to the innermost stable circular orbit. The breadth of the
line is sufficient to suggest that GX 339-4 may harbor a black hole with
significant angular momentum. Absorption lines from H-like and He-like O, and
He-like Ne and Mg are detected, as well as lines which are likely due to Ne II
and Ne III. The measured line properties make it difficult to associate the
absorption with the coronal phase of the interstellar medium. A scenario
wherein the absorption lines are due to an intrinsic AGN-like warm-absorber
geometry -- perhaps produced by a disk wind in an extended disk-dominated state
-- may be more viable. We compare our results to Chandra observations of the
Galactic black hole candidate XTE J1650-500, and discuss our findings in terms
of prominent models for Galactic black hole accretion flows and connections to
supermassive black holes.Comment: 20 pages, 11 postscript figure files (many in color), uses
emulateapj.sty and apjfonts.sty, slightly expanded, accepted for publication
in Ap
A Long, Hard Look at the Low-Hard State in Accreting Black Holes
We present the first results of coordinated multi-wavelength observations of
the Galactic black hole GX 339-4 in a canonical low-hard state, obtained during
its 2004 outburst. XMM-Newton observed the source for 2 revolutions, or
approximately 280 ksec; RXTE monitored the source throughout this long stare.
The resulting data offer the best view yet obtained of the inner accretion flow
geometry in the low-hard state, which is thought to be analogous to the
geometry in low-luminosity active galactic nuclei. The XMM-Newton spectra
clearly reveal the presence of a cool accretion disk component, and a
relativistic Fe K emission line. The results of fits made to both components
strongly suggest that a standard thin disk remains at or near to the innermost
stable circular orbit, at least in bright phases of the low-hard state. These
findings indicate that potential links between the inner disk radius and the
onset of a steady compact jet, and the paradigm of a radially-recessed disk in
the low-hard state, do not hold universally. The results of our observations
can best be explained if a standard thin accretion disk fuels a corona which is
closely related to, or consistent with, the base of a compact jet. In a brief
examination of archival data, we show that Cygnus X-1 supports this picture of
the low/hard state. We discuss our results within the context of disk-jet
connections and prevailing models for accretion onto black holes.Comment: 13 pages, 10 figures (6 in color), ApJ, in pres
XMM-Newton Spectroscopy of the Accretion-Driven Millisecond X-ray Pulsar XTE J1751-305 in Outburst
We present an analysis of the first high-resolution spectra measured from an
accretion-driven millisecond X-ray pulsar in outburst. We observed XTE
J1751-305 with XMM-Newton on 2002 April 7 for approximately 35 ksec. Using a
simple absorbed blackbody plus power-law model, we measure an unabsorbed flux
of (6.6 +/- 0.1) * 10^(-10) erg/cm^2/s (0.5--10.0 keV). A hard power-law
component (Gamma = 1.44 +/- 0.01) contributes 83% of the unabsorbed flux in the
0.5-10.0 keV band, but a blackbody component (kT = 1.05 +/- 0.01 keV) is
required. We find no clear evidence for narrow or broad emission or absorption
lines in the time-averaged spectra, and the sensitivity of this observation has
allowed us to set constraining upper-limits on the strength of important
features. The lack of line features is at odds with spectra measured from some
other X-ray binaries which share some similarities with XTE J1751-305. We
discuss the implications of these findings on the accretion flow geometry in
XTE J1751-305.Comment: 5 pages, 3 figures (2 color). ApJ Letters, accepted. Uses
emulateapj.st
Binary evolution with LOFT
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of very faint X-ray binaries,
orbital period distribution of black hole X-ray binaries and neutron star spin
up. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timing. (v2 few typos corrected
QPOs in Cataclysmic Variables and in X-ray Binaries
Recent observations, reported by Warner and Woudt, of Dwarf Nova Oscillations
(DNOs) exhibiting frequency drift, period doubling, and 1:2:3 harmonic
structure, can be understood as disc oscillations that are excited by
perturbations at the spin frequency of the white dwarf or of its equatorial
layers. Similar quasi-periodic disc oscillations in black hole low-mass X-ray
binary (LMXB) transients in a 2:3 frequency ratio show no evidence of frequency
drift and correspond to two separate modes of disc oscillation excited by an
internal resonance. Just as no effects of general relativity play a role in
white dwarf DNOs, no stellar surface or magnetic field effects need be invoked
to explain the black hole QPOs.Comment: Revised version. Astronomy & Astrophysics (Letters), in pres
The nature of very-faint X-ray binaries: Near-infrared spectroscopy of 1RXH J173523.7354013 reveals a giant companion
Very-faint X-ray binaries (VFXBs) are a sub-class of black holes and neutron
stars in binaries that appear to be accreting at a very low rate. In addition
to providing interesting constraints on poorly understood forms of accretion,
elucidating the nature of VFXBs is particularly interesting for binary
evolution and population modeling. Through near-infrared (nIR) spectroscopy, we
here investigate the nature of the bursting neutron star and VFXB 1RXH
J173523.7354013 (J1735), which persistently accretes at an X-ray luminosity
of . Our analysis shows that the nIR
emission is dominated by that of the companion star, which we find to be a late
G or early K-type giant, making this the second neutron star identified as a
VFXB found to have a giant companion. We discuss how several of the system
properties are difficult to reconcile with a wind-fed symbiotic X-ray binary.
We therefore also propose an alternative scenario wherein J1735 is a wide
binary system (supported by the discovery of a 7.5 d modulation in the nIR
light curves) with a quiescent luminosity of , in which the donor star is overflowing its Roche lobe. This
raises the possibility that J1735 may, every century or more, exhibit very long
and very bright outbursts during which it reaches accretion rates around the
Eddington limit like the neutron star Z sources.Comment: 12 pages, 6 figures, accepted for publication in MNRA
Simultaneous Chandra and RXTE Spectroscopy of the Microquasar H~1743-322: Clues to Disk Wind and Jet Formation from a Variable Ionized Outflow
We observed the bright phase of the 2003 outburst of the Galactic black hole
candidate H 1743-322 in X-rays simultaneously with Chandra and RXTE on four
occasions. The Chandra/HETGS spectra reveal narrow, variable (He-like) Fe XXV
and (H-like) Fe XXVI resonance absorption lines. In the first observation, the
Fe XXVI line has a FWHM of 1800 +/- 400 km/s and a blue-shift of 700 +/- 200
km/s, suggesting that the highly ionized medium is an outflow. Moreover, the Fe
XXV line is observed to vary significantly on a timescale of a few hundred
seconds in the first observation, which corresponds to the Keplerian orbital
period at approximately 1 E+4 gravitational radii. Our models for the
absorption geometry suggest that a combination of geometric effects and
changing ionizing flux are required to account for the large changes in line
flux observed between observations, and that the absorption likely occurs at a
radius less than 1 E+4 radii for a 10 Msun black hole. Viable models for the
absorption geometry include cyclic absorption due to an accretion disk
structure, absorption in a clumpy outflowing disk wind, or possibly a
combination of these two. If the wind in H 1743-322 has unity filling factor,
the highest implied mass outflow rate is 20 percent of the Eddington mass
accretion rate. This wind may be a hot precursor to the Seyfert-like,
outflowing "warm absorber" geometries recently found in the Galactic black
holes GX 339-4 and XTE J1650-500. We discuss these findings in the context of
ionized Fe absorption lines found in the spectra of other Galactic sources, and
connections to warm absorbers, winds, and jets in other accreting systems.Comment: 18 pages, 7 figures, 5 in color, subm. to ApJ. Uses emulateapj.sty
and apjfonts.st
Chandra Detections of Two Quiescent Black Hole X-Ray Transients
Using the Chandra X-ray Observatory, we have detected the black hole
transients V4641 Sgr and XTE J1859+226 in their low luminosity, quiescent
states. The 0.3-8 keV luminosities are (4.0^(+3.3)_(-2.4))E31 (d/7 kpc)^2 erg/s
and (4.2^(+4.8)_(-2.2))E31 (d/11 kpc)^2 erg/s for V4641 Sgr and XTE J1859+226,
respectively. With the addition of these 2 systems, 14 out of the 15 transients
with confirmed black holes (via compact object mass measurements) now have
measured quiescent luminosities or sensitive upper limits. The only exception
is GRS 1915+105, which has not been in quiescence since its discovery in 1992.
The luminosities for V4641 Sgr and XTE J1859+226 are consistent with the median
luminosity of 2E31 erg/s for the systems with previous detections. Our analysis
suggests that the quiescent X-ray spectrum of V4641 Sgr is harder than for the
other systems in this group, but, due to the low statistical quality of the
spectrum, it is not clear if V4641 Sgr is intrinsically hard or if the column
density is higher than the interstellar value. Focusing on V4641 Sgr, we
compare our results to theoretical models for X-ray emission from black holes
in quiescence. Also, we obtain precise X-ray positions for V4641 Sgr and XTE
J1859+226 via cross-correlation of the X-ray sources detected near our targets
with IR sources in the 2 Micron All-Sky Survey catalog.Comment: 4 pages, Accepted by ApJ Letter
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