454 research outputs found
The X-ray Properties of Low-Frequency Quasi-Periodic Oscillations from GRS 1915+105 up to 120 keV
We present a study of the properties of strong 0.8-3.0 Hz quasi-periodic
oscillations (QPOs) that occurred during 1997 RXTE observations of the
microquasar GRS 1915+105 in the low-hard state. The high count rates allow us
to track individual QPO peaks, and we exploit this to develop a QPO folding
technique. In contrast to previous QPO studies with RXTE, we emphasize the high
energy QPO properties and report the detection of a QPO in the 60-124 keV
energy band. Our technique allows us, for the first time, to measure the phase
of the QPO harmonics relative to the fundamental. Variation in this phase
difference leads to changes in the shape of the QPO profile with energy and
over time. The strength of the QPO fundamental increases up to 19 keV, but the
data do not suggest that the strength continues to increase above this energy.
In some cases, the QPO amplitudes in the 30-60 keV and 60-124 keV energy bands
are significantly less than in the 13-19 keV and 19-29 keV energy bands. We
also use our technique to measure the phase lag of the QPO fundamental and
harmonics. In the case where negative phase lags are detected for the
fundamental, positive phase lags are detected for the first harmonic.Comment: Submitted to ApJ, Refereed, 9 page
Wind, jet, hybrid corona and hard X-ray flares: multiwavelength evolution of GRO J1655-40 during the 2005 outburst rise
We have investigated the complex multiwavelength evolution of GRO J1655-40
during the rise of its 2005 outburst. We detected two hard X-ray flares, the
first one during the transition from the soft state to the ultra-soft state,
and the second one in the ultra-soft state. The first X-ray flare coincided
with an optically thin radio flare. We also observed a hint of increased radio
emission during the second X-ray flare. To explain the hard flares without
invoking a secondary emission component, we fit the entire data set with the
eqpair model. This single, hybrid Comptonization model sufficiently fits the
data even during the hard X-ray flares if we allow reflection fractions greater
than unity. In this case, the hard X-ray flares correspond to a Comptonizing
corona dominated by non-thermal electrons. The fits also require absorption
features in the soft and ultra-soft state which are likely due to a wind. In
this work we show that the wind and the optically thin radio flare co-exist.
Finally, we have also investigated the radio to optical spectral energy
distribution, tracking the radio spectral evolution through the quenching of
the compact jet and rise of the optically thin flare, and interpreted all data
using state transition models.Comment: 16 pages, 11 figure
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
X-Ray Flares and Oscillations from the Black Hole Candidate X-Ray Transient XTE J1650-500 at Low Luminosity
We report on X-ray observations made with the Rossi X-ray Timing Explorer of
the black hole candidate (BHC) transient XTE J1650-500 at the end of its first,
and currently only, outburst. By monitoring the source at low luminosities over
several months, we found 6 bright ~100 second X-ray flares and long time scale
oscillations of the X-ray flux. The oscillations are aperiodic with a
characteristic time scale of 14.2 days and an order of magnitude variation in
the 2.8-20 keV flux. The oscillations may be related to optical
"mini-outbursts" that have been observed at the ends of outbursts for other
short orbital period BHC transients. The X-ray flares have durations between 62
and 215 seconds and peak fluxes that are 5-24 times higher than the persistent
flux. The flares have non-thermal energy spectra and occur when the persistent
luminosity is near 3E34 (d/4 kpc)^2 erg/s (2.8-20 keV). The rise time for the
brightest flare demonstrates that physical models for BHC systems must be able
to account for the situation where the X-ray flux increases by a factor of up
to 24 on a time scale of seconds. We discuss the flares in the context of
observations and theory of Galactic BHCs and compare the flares to those
detected from Sgr A*, the super-massive black hole at the Galactic center. We
also compare the flares to X-ray bursts that are seen in neutron star systems.
While some of the flare light curves are similar to those of neutron star
bursts, the flares have non-thermal energy spectra in contrast to the blackbody
spectra exhibited in bursts. This indicates that X-ray bursts should not be
taken as evidence that a given system contains a neutron star unless the
presence of a blackbody component in the burst spectrum can be demonstrated.Comment: 9 pages, accepted by Ap
IGR J17448-3232 point source: A blazar candidate viewed through the galactic centre?
The error region of the INTEGRAL source, IGRJ17448-3232, contains an X-ray point source at the edge of a ~ 3' radius extended X-ray source. It has been suggested that the extended emission is a young supernovae remnant (SNR) while the point source may be an isolated neutron star, associated with the SNR, that received a kick when the supernova occurred. We identify the infrared counterpart of the X-ray point source, visible from 2.2 ”m to 24 ”m, and place limits on the flux at longer wavelengths by comparison with radio catalogues. Multi-wavelength spectral modeling shows that the data are consistent with a reddened and absorbed single power law over five orders of magnitude in frequency. This implies non-thermal, possibly synchrotron emission that renders the previous identification of this source as a possible pulsar, and its association to the SNR, unlikely; we instead propose that the emission may be due to a blazar viewed through the plane of the Galaxy
Chandra observation of the fast X-ray transient IGR J17544-2619: evidence for a neutron star?
IGR J17544-2619 belongs to a distinct group of at least seven fast X-ray
transients that cannot readily be associated with nearby flare stars or
pre-main sequence stars and most probably are X-ray binaries with wind
accretion. Sofar, the nature of the accretor has been determined in only one
case (SAX J1819.3-2525/V4641 Sgr). We carried out a 20 ks Chandra ACIS-S
observation of IGR J17544-2619 which shows the source in quiescence going into
outburst. The Chandra position confirms the previous tentative identification
of the optical counterpart, a blue O9Ib supergiant at 3 to 4 kpc (Pellizza,
Chaty & Negueruela, in prep.). This is the first detection of a fast X-ray
transient in quiescence. The quiescent spectrum is very soft. The photon index
of 5.9+/-1.2 (90% confidence error margin) is much softer than 6 quiescent
black hole candidates that were observed with Chandra ACIS-S (Kong et al. 2002;
Tomsick et al. 2003). Assuming that a significant fraction of the quiescent
photons comes from the accretor and not the donor star, we infer that the
accretor probably is a neutron star. A fit to the quiescent spectrum of the
neutron star atmosphere model developed by Pavlov et al. (1992) and Zavlin et
al. (1996) implies an unabsorbed quiescent 0.5--10 keV luminosity of
(5.2+/-1.3) x 10^32 erg/s. We speculate on the nature of the brief outbursts.Comment: accepted for publication in Astronomy & Astrophysic
Suzaku Observations of Four Heavily Absorbed HMXBs
We report on Suzaku observations of four unidentified sources from the
INTEGRAL and Swift BAT Galactic plane surveys. All the sources have a large
neutral hydrogen column density and are likely members of an emerging class of
heavily absorbed high mass X-ray binary (HMXB) first identified in INTEGRAL
observations. Two of the sources in our sample are approximately constant flux
sources, one source shows periodic variation and one source exhibited a short,
bright X-ray outburst. The periodicity is transient, suggesting it is produced
by a neutron star in an elliptical orbit around a stellar wind source. We
analyze the flaring source in several segments to look for spectral variation
and discuss the implications of the findings for the nature of the source. We
conclude that all four sources in our sample can be identified with the
emerging class of highly absorbed HMXBs, that one is a newly identified
transient X-ray pulsar and that at least one is a newly identified supergiant
fast X-ray transient (SFXT).Comment: 22 pages, 5 figures, submitted to Ap
Study of the X-ray properties of the neutron-star binary 4U 172834 from the soft to hard state
We studied five XMM-Newton observations of the neutron-star binary 4U
172834 covering the hard, intermediate and soft spectral states. By jointly
fitting the spectra with several reflection models, we obtained an inclination
angle of 2553 and an iron abundance up to 10 times the solar. From the
fits with reflection models, we found that the fluxes of the reflection and the
Comptonised components vary inconsistently; since the latter is assumed to be
the illuminating source, this result possibly indicates the contribution of the
neutron star surface/boundary layer to the disc reflection. As the source
evolved from the relatively soft to the intermediate state, the disc inner
radius decreased, opposite to the prediction of the standard accretion disc
model. We also explore the possible reasons why the supersolar iron abundance
is required by the data and found that this high value is probably caused by
the absence of the hard photons in the XMM-Newton data.Comment: 14 pages, 6 figure
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