707 research outputs found
X-ray variability during the quiescent state of the neutron-star X-ray transient in the globular cluster NGC 6440
The globular cluster NGC 6440 is known to harbor a bright neutron-star X-ray
transient. We observed the globular cluster with Chandra on two occasions when
the bright transient was in its quiescent state in July 2000 and June 2003
(both observations were made nearly 2 years after the end of their preceding
outbursts). The quiescent spectrum during the first observation is well
represented by a two component model (a neutron-star atmosphere model plus a
power-law component which dominates at energies above 2 keV). During the second
observation (which was roughly of equal duration to the first observation) we
found that the power-law component could no longer be detected. Our spectral
fits indicate that the effective temperature of the neutron-star surface was
consistent between the two observations. We conclude that the effect of the
change in power-law component caused the 0.5-10 keV flux to be a factor of ~2
lower during the second observation compared to the first observation. We
discuss plausible explanations for the variations, including variable residual
accretion onto the neutron star magnetosphere or some variation in the
interaction of the pulsar wind with the matter still outflowing from the
companion star.Comment: 18 pages, 3 color figs, 1 b&w figures, 3 tables; discussion expanded;
accepted for publication in Ap
Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440
After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright
transient X-ray source turning on in August 2001, as noted with the RXTE
All-Sky Monitor. We carried out a short target of opportunity observation with
the Chandra X-ray Observatory and are able to associate the transient with the
brightest of 24 X-ray sources detected during quiescence in July 2000 with
Chandra. Furthermore, we securely identify the optical counterpart and
determine that the 1998 X-ray outburst in NGC 6440 was from the same object.
This is the first time that an optical counterpart to a transient in a globular
cluster is securely identified. Since the transient is a type I X-ray burster,
it is established that the compact accretor is a neutron star. Thus, this
transient provides an ideal case to study the quiescent emission in the optical
and X-ray of a transiently accreting neutron star while knowing the distance
and reddening accurately. One model that fits the quiescent spectrum is an
absorbed power law plus neutron star hydrogen atmosphere model. We find an
intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric
luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is
consistent with predictions for a cooling neutron star.Comment: Accepted for publication in ApJ Letter
X-ray spectral and timing properties of the 2001 superburst of 4U 1636-536
Preliminary results are reported on the spectral and timing properties of the
spectacular 2001 superburst of 4U 1636-536 as seen by the RXTE/PCA. The
(broad-band) power-spectral and hardness properties during the superburst are
compared to those just before and after the superburst. Not all of the
superburst emission can be fitted by pure black-body radiation. We also
gathered BeppoSAX/WFC and RXTE/ASM data, as well as other RXTE/PCA data,
obtained days to months before and after the superburst to investigate the
normal X-ray burst behavior around the time of the superburst. The first normal
X-ray burst after the 2001 superburst was detected 23 days later. During
inspection of all the RXTE/ASM data we found a third superburst. This
superburst took place on June 26, 1999, which is ~2.9 yrs after the 1996
superburst and ~1.75 yrs before the 2001 superburst. The above findings are the
strongest constraints observed so far on the duration of the cessation of
normal X-ray bursts after a superburst and the superburst recurrence times.Comment: 4 pages, 4 figures,to appear in the proceedings of "X-Ray Timing
2003: Rossi and Beyond", eds. P. Kaaret, F.K. Lamb, & J.H. Swank (Melville,
NY: AIP
A Soft X-Ray Spectral Episode for the Clocked Burster, GS 1826-24 as Measured by Swift and NuSTAR
We report on NuSTAR and Swift observations of a soft state of the neutron
star low-mass X-ray binary GS 1826-24, commonly known as the "clocked" burster.
The transition to the soft state was recorded in 2014 June through an increase
of the 2-20 keV source intensity measured by MAXI, simultaneous with a decrease
of the 15-50 keV intensity measured by Swift/BAT. The episode lasted
approximately two months, after which the source returned to its usual hard
state. We analyze the broad-band spectrum measured by Swift/XRT and NuSTAR, and
estimate the accretion rate during the soft episode to be about 13% of
Eddington, within the range of previous observations. However, the best fit
spectral model, adopting the double Comptonization used previously, exhibits
significantly softer components. We detect seven type-I X-ray bursts, all
significantly weaker (and with shorter rise and decay times) than observed
previously. The burst profiles and recurrence times vary significantly, ruling
out the regular bursts that are typical for this source. One burst exhibited
photospheric radius expansion, and we estimate the source distance at about
(5.7 / xi_b^1/2) kpc, where xi_b parameterizes the possible anisotropy of the
burst emission. Interpreting the soft state as a transition from an optically
thin inner flow to an optically thick flow passing through a boundary layer, as
is commonly observed in similar systems, is contradicted by the lower optical
depth measured for the double Comptonization model we find for this soft state.
The effect of a change in disk geometry on the burst behavior remains unclear.Comment: 40 pages (single-column, doubled spaced format), 9 figures, 3 tables;
submitted to Ap
Broad band X-ray spectral properties of Gamma-ray bursts with BeppoSAX
In about one year, five gamma-ray bursts were simultaneously observed with
the Wide Field Cameras and Gamma Ray Burst Monitor aboard the BeppoSAX
satellite. From some of them X-ray afterglow emission has been clearly detected
with the same satellite. In order to understand how GRB emission is related to
the X-ray afterglow, we are performing a systematic study of the spectral
properties of these events. We report here preliminary results of this study.Comment: 5 pages, 3 figures. To appear in the Proceedings of the 4th
Huntsville Gamma-ray Burst Symposiu
The prompt X-ray emission of GRB011211: possible evidence of a transient absorption feature
We report on observation results of the prompt X- and gamma-ray emission from
GRB011211. This event was detected with the Gamma-Ray Burst Monitor and one of
the Wide Field Cameras aboard the BeppoSAX satellite. The optical counterpart
to the GRB was soon identified and its redshift determined (z = 2.140), while
with the XMM-Newton satellite, the X-ray afterglow emission was detected.
Evidence of soft X-ray emission lines was reported by Reeves et al. (2002), but
not confirmed by other authors. In investigating the spectral evolution of the
prompt emission we find the possible evidence of a transient absorption feature
at 6.9^{+0.6}_{-0.5} keV during the rise of the primary event. The significance
of the feature is derived with non parametric tests and numerical simulations,
finding a chance probability which ranges from 3x10^{-3} down to 4x10^{-4}. The
feature shows a Gaussian profile and an equivalent width of 1.2^{+0.5}_{-0.6}
keV. We discuss our results and their possible interpretation.Comment: 23 pages, 3 Tables, 6 Figures. Accepted for publication in
Astrophysical Journa
Puzzling thermonuclear burst behaviour from the transient low-mass X-ray binary IGR J17473-2721
We investigate the thermonuclear bursting behaviour of IGR J17473-2721, an
X-ray transient that in 2008 underwent a six month long outburst, starting
(unusually) with an X-ray burst. We detected a total of 57 thermonuclear bursts
throughout the outburst with AGILE, Swift, RXTE, and INTEGRAL. The wide range
of inferred accretion rates (between <1% and about 20% of the Eddington
accretion rate m-dot_Edd) spanned during the outburst allows us to study
changes in the nuclear burning processes and to identify up to seven different
phases. The burst rate increased gradually with the accretion rate until it
dropped (at a persistent flux corresponding to about 15% of m-dot_Edd) a few
days before the outburst peak, after which bursts were not detected for a
month. As the persistent emission subsequently decreased, the bursting activity
resumed with a much lower rate than during the outburst rise. This hysteresis
may arise from the thermal effect of the accretion on the surface nuclear
burning processes, and the timescale is roughly consistent with that expected
for the neutron star crust thermal response. On the other hand, an undetected
superburst, occurring within a data gap near the outburst peak, could have
produced a similar quenching of burst activity.Comment: 18 pages, 12 figures, 1 table, accepted for publication in MNRA
Identification of the LMXB and Faint X-ray Sources in NGC 6652
We have detected three new x-ray point sources, in addition to the known
low-mass x-ray binary (LMXB) X1832-330, in the globular cluster NGC 6652 with a
Chandra 1.6 ksec HRC-I exposure. Star 49 (M_{V}~4.7), suggested by Deutsch et
al.(1998) as the optical candidate for the LMXB, is identified (<0.3") not with
the LMXB, but with another, newly detected source (B). Using archival HST
images, we identify (<0.3") the LMXB (A) and one of the remaining new sources
(C) with blue variable optical counterparts at M_{V}~3.7 and 5.3 respectively.
The other new source (D) remains unidentified in the crowded cluster core. In
the 0.5-2.5 keV range, assuming a 5 keV thermal bremsstrahlung spectrum and
N_{H}=5.5*10^{20}, source A has intrinsic luminosity L_{X}~5.3*10^{35} ergs/s.
Assuming a 1 keV thermal bremsstrahlung spectrum, B has L_{X}~4.1*10^{33}
ergs/s, while C and D have L_{X}~8*10^{32}$ ergs/s. Source B is probably a
quiescent LMXB, while source C may be either a luminous CV or quiescent LMXB.Comment: 14 pages, 3 figures, accepted by Astrophysical Journa
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