228 research outputs found
Multiwavelength Observations of GX 339-4 in 1996. III. Keck Spectroscopy
As part of our multiwavelength campaign of observations of GX 339-4 in 1996
we present our Keck spectroscopy performed on May 12 UT. At this time, neither
the ASM on the RXTE nor BATSE on the CGRO detected the source. The optical
emission was still dominated by the accretion disk with V approximately 17 mag.
The dominant emission line is H alpha, and for the first time we are able to
resolve a double peaked profile. The peak separation Delta v = 370 +/- 40 km/s.
Double peaked H alpha emission lines have been seen in the quiescent optical
counterparts of many black hole X-ray novae. However, we find that the peak
separation is significantly smaller in GX 339-4, implying that the optical
emission comes from a larger radius than in the novae. The H alpha emission
line may be more akin to the one in Cygnus X-1, where it is very difficult to
determine if the line is intrinsically double peaked because absorption and
emission lines from the companion star dominate.Comment: Submitted to Astrophysical Journal. 10 pages. 2 figure
X-ray Spectroscopy of Candidate Ultracompact X-ray Binaries
We present high-resolution spectroscopy of the neutron star/low-mass X-ray
binaries (LMXBs) 4U 1850-087 and 4U 0513-40 as part of our continuing study of
known and candidate ultracompact binaries. The LMXB 4U 1850-087 is one of four
systems in which we had previously inferred an unusual Ne/O ratio in the
absorption along the line of sight, most likely from material local to the
binaries. However, our recent Chandra X-ray Observatory LETGS spectrum of 4U
1850-087 finds a Ne/O ratio by number of 0.22+/-0.05, smaller than previously
measured and consistent with the expected interstellar value. We propose that
variations in the Ne/O ratio due to source variability, as previously observed
in these sources, can explain the difference between the low- and
high-resolution spectral results for 4U 1850-087. Our XMM-Newton RGS
observation of 4U 0513-40 also shows no unusual abundance ratios in the
absorption along the line of sight. We also present spectral results from a
third candidate ultracompact binary, 4U 1822-000, whose spectrum is well fit by
an absorbed power-law + blackbody model with absorption consistent with the
expected interstellar value. Finally, we present the non-detection of a fourth
candidate ultracompact binary, 4U 1905+000, with an upper limit on the source
luminosity of < 1 x 10^{32} erg s^{-1}. Using archival data, we show that the
source has entered an extended quiescent state.Comment: 8 pages, 3 figures, accepted for publication to the Astrophysical
Journa
XMM-Newton spectral and timing analysis of the faint millisecond pulsars PSR J0751+1807 and PSR J1012+5307
We present XMM-Newton MOS imaging and PN timing data of the faint millisecond
pulsars PSR J0751+1807 and PSR J1012+5307. We find 46 sources in the MOS field
of view of PSR J0751+1807 searching down to an unabsorbed flux limit of 3 x
10^-15 ergs cm^-2 s^-1 (0.2-10.0 keV). We present, for the first time, the
X-ray spectra of these two faint millisecond pulsars. We find that a power law
model best fits the spectrum of PSR J0751+1807, Gamma=1.59+/-0.20, with an
unabsorbed flux of 4.4 x 10^-14 ergs cm^-2 s^-1 (0.2-10.0 keV). A power law is
also a good description of the spectrum of PSR J1012+5307, Gamma=1.78+/-0.36,
with an unabsorbed flux of 1.2 x 10^-13 ergs cm^-2 s^-1 (0.2-10.0 keV).
However, a blackbody model can not be excluded as the best fit to this data. We
present some evidence to suggest that both of these millisecond pulsars show
pulsations in this X-ray band. We find some evidence for a single broad X-ray
pulse for PSR J0751+1807 and we discuss the possibility that there are two
pulses per spin period for PSR J1012+5307.Comment: 8 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
A Catalog of Transient X-ray Sources in M31
From October 1999 to August 2002, 45 transient X-ray sources were detected in
M31 by Chandra and XMM-Newton. We have performed spectral analysis of all
XMM-Newton and Chandra ACIS detections of these sources, as well as flux
measurements of Chandra HRC detections. The result is absorption-corrected
X-ray lightcurves for these sources covering this 2.8 year period, along with
spectral parameters for several epochs of the outbursts of most of the
transient sources. We supply a catalog of the locations, outburst dates, peak
observed luminosities, decay time estimates, and spectral properties of the
transient sources, and we discuss similarities with Galactic X-ray novae. Duty
cycle estimates are possible for 8 of the transients and range from 40% to 2%;
upper limits to the duty cycles are estimated for an additional 15 transients
and cover a similar range. We find 5 transients which have rapid decay times
and may be ultra-compact X-ray binaries. Spectra of three of the transients
suggest they may be faint Galactic foreground sources. If even one is a
foreground source, this suggests a surface density of faint transient X-ray
sources of >~1 deg.Comment: 63 pages, 22 figures, 3 tables, accepted for publication in Ap
The intriguing nature of the high energy gamma ray source XSSJ12270-4859
The nature of the hard X-ray source XSSJ12270-4859 is still unclear though it
was claimed to be a magnetic Cataclysmic Variable. We here present a broad-band
X-ray and gamma ray study based on a recent XMM-Newton observation and archival
INTEGRAL and RXTE data. From the Fermi/LAT 1-year point source catalogue, we
tentatively associate XSSJ12270-4859 with 1FGLJ1227.9-4852, a source of high
energy gamma rays with emission up to 10GeV. We complement the study with UV
photometry from XMM-Newton and ground-based optical and near-IR photometry. The
X-ray emission is highly variable showing flares and intensity dips. The X-ray
flares consist of flare-dip pairs. Flares are also detected in the UV range but
not the dips. Aperiodic dipping behaviour is also observed during X-ray
quiescence but not in the UV. The 0.2-100keV spectrum is featureless and
described by a power law model with Gamma=1.7. The 100MeV-10GeV spectrum is
instead represented by a power law index of 2.45. The luminosity ratio between
0.1-100GeV and 0.2--100keV is ~0.8, hence the GeV emission is a significant
component of the total energy output. Furthermore, the X-ray spectrum does not
greatly change during flares, quiescence and the dips seen in quiescence but it
hardens during the post-flare dips. Optical photometry reveals a period of
4.32hr likely related to the binary orbit. Near-IR, possibly ellipsoidal,
variations are detected. Large amplitude variability on shorter (tens mins)
timescales are found to be non-periodic. The observed variability at all
wavelengths and the spectral characteristics strongly favour a low-mass
atypical low-luminosity X-ray binary and are against a Cataclysmic Variable
nature. The association with a Fermi/LAT high energy gamma ray source further
strengths this interpretation.Comment: 12 pages, 11 figures, 3 tables; Accepted for publication in Astronomy
& Astrophysics Main Journ
Generic tests of the existence of the gravitational dipole radiation and the variation of the gravitational constant
We present results from the high precision timing analysis of the
pulsar-white dwarf (WD) binary PSR J1012+5307 using 15 years of multi-telescope
data. Observations were performed regularly by the European Pulsar Timing Array
(EPTA) network, consisting of Effelsberg, Jodrell Bank, Westerbork and
Nan\c{c}ay. All the timing parameters have been improved from the previously
published values, most by an order of magnitude. In addition, a parallax
measurement of mas is obtained for the first time for PSR
J1012+5307, being consistent with the optical estimation from the WD companion.
Combining improved 3D velocity information and models for the Galactic
potential the complete evolutionary Galactic path of the system is obtained. A
new intrinsic eccentricity upper limit of is acquired,
one of the smallest calculated for a binary system and a measurement of the
variation of the projected semi-major axis also constrains the system's orbital
orientation for the first time. It is shown that PSR J1012+5307 is an ideal
laboratory for testing alternative theories of gravity. The measurement of the
change of the orbital period of the system of is used to set an upper limit on the dipole gravitational wave
emission that is valid for a wide class of alternative theories of gravity.
Moreover, it is shown that in combination with other binary pulsars PSR
J1012+5307 is an ideal system to provide self-consistent, generic limits, based
only on millisecond pulsar data, for the dipole radiation and the variation of
the gravitational constant .Comment: accepted for publication in MNRAS, 11 pages, 5 figures, 2 table
HST/ACS Imaging of Omega Centauri: Optical Counterpart for the Quiescent Low-Mass X-Ray Binary
We report the discovery of an optical counterpart to a quiescent neutron star
in the globular cluster Omega Centauri (NGC 5139). The star was found as part
of our wide-field imaging study of Omega Cen using the Advanced Camera for
Surveys (ACS) on Hubble Space Telescope. Its magnitude and color (R_625 = 25.2,
B_435 - R_625 = 1.5) place it more than 1.5 magnitudes to the blue side of the
main sequence. Through an H-alpha filter it is ~ 1.3 magnitudes brighter than
cluster stars of comparable M_625 magnitude. The blue color and H-alpha excess
suggest the presence of an accretion disk, implying that the neutron star is
accreting from a binary companion and is thus a quiescent low-mass X-ray
binary. If the companion is a main-sequence star, then the faint absolute
magnitude (M_625 ~ 11.6) constrains it to be of very low mass (M <~ 0.14
Msolar). The faintness of the disk (M_435 ~ 13) suggests a very low rate of
accretion onto the neutron star. We also detect 13 probable white dwarfs and
three possible BY Draconis stars in the 20" x 20" region analyzed here,
suggesting that a large number of white dwarfs and active binaries will be
observable in the full ACS study.Comment: Accepted for Publication in the Astrophysical Journal. 5 pages, 4
figures, 1 table, uses emulateapj.sty. Figures 1 and 3 at reduced resolution.
New version contains revised magnitude calibration
Two Different Long-term Behaviors in Black-Hole Candidates: Evidence for Two Accretion Flows?
We discuss the results of long-term hard x-ray monitoring of Galactic
black-hole candidates 1E 1740.7-2942, GRS 1758-258, Cyg X-1, GX 339-4, and Cyg
X-3 with the Rossi X-Ray Timing Explorer (RXTE). The objects divide into two
classes. In the first class, exemplified by Cyg X-1, luminosity and spectral
hardness evolve simultaneously. In the second class, the relation is more
complicated: the softest spectra occur while the count rate is dropping. Most
models of accretion, tailored to Cyg X-1, do not predict the second sort of
behavior. One interpretation is a simple model with two simultaneous,
independent accretion flows: a thin disk and a hot halo. A drop in the
accretion rate affecting both flows would propagate through the halo
immediately but might take up to several weeks to propagate through the disk.
While the inner halo is thus temporarily depleted compared to the disk, a
temporary soft state is expected. This picture is supported by the observation
that those sources which show delays (1E 1740.7-2942, GRS 1758-258, and GX
339-4) are expected to have low-mass companions, and those which do not (Cyg
X-1, Cyg X-3) are known or thought to have high-mass companions. Low-mass
companions imply accretion by Roche-lobe overflow, with a high specific angular
momentum in the accreting material, and therefore a large disk with a long
viscous timescale. Wind accretion from massive companions is expected to result
in a much smaller disk, and thus little viscous delay.Comment: 37 pages, 17 figures, 1 table. Accepted for publication in The
Astrophysical Journal. A MAJOR revision from the version first posted on
astro-ph and a major improvement, much longer than the original, and
including additional important dat
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