364 research outputs found
Understanding the spectral and timing behavior of a newly discovered transient X-ray pulsar Swift J0243.6+6124
We present the results obtained from timing and spectral studies of the newly
discovered accreting X-ray binary pulsar Swift J0243.6+6124 using a NuSTAR
observation in 2017 October at a flux level of ~280 mCrab. Pulsations at
9.85423(5) s were detected in the X-ray light curves of the pulsar. Pulse
profiles of the pulsar were found to be strongly energy dependent. A broad
profile at lower energies was found to evolve into a double peaked profile in
30keV. The 3-79 keV continuum spectrum of the pulsar was well described
with a negative and positive exponential cutoff or high energy cutoff power law
models modified with a hot blackbody at 3 keV. An iron emission line was
also detected at 6.4 keV in the source spectrum. We did not find any signature
of cyclotron absorption line in our study. Results obtained from phase-resolved
and time-resolved spectroscopy are discussed in the paper.Comment: 7 pages, 6 figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Society Journa
A curious case of the accretion-powered X-ray pulsar GX 1+4
We present detailed spectral and timing studies using a NuSTAR observation of
GX 1+4 in October 2015 during an intermediate intensity state. The measured
spin period of 176.778 s is found to be one of the highest values since its
discovery. In contrast to a broad sinusoidal-like pulse profile, a peculiar
sharp peak is observed in profiles below ~25 keV. The profiles at higher
energies are found to be significantly phase-shifted compared to the soft X-ray
profiles. Broadband energy spectra of GX 1+4, obtained from NuSTAR and Swift
observations, are described with various continuum models. Among these, a two
component model consisting of a bremsstrahlung and a blackbody component is
found to best-fit the phase-averaged and phase-resolved spectra. Physical
models are also used to investigate the emission mechanism in the pulsar, which
allows us to estimate the magnetic field strength to be in
(5-10)10 G range. Phase-resolved spectroscopy of NuSTAR
observation shows a strong blackbody emission component in a narrow pulse phase
range. This component is interpreted as the origin of the peculiar peak in the
pulse profiles below 25 keV. The size of emitting region is calculated to
be 400 m. The bremsstrahlung component is found to dominate in hard
X-rays and explains the nature of simple profiles at high energies.Comment: 13 Pages, 7 Figues; Accepted for publication in Monthly Notices of
the Royal Astronomical Society journa
INTEGRAL monitoring of unusually long X-ray bursts
X-ray bursts are thermonuclear explosions on the surface of accreting neutron
stars in low mass X-ray binaries. As most of the known X-ray bursters are
frequently observed by INTEGRAL, an international collaboration have been
taking advantage of its instrumentation to specifically monitor the occurrence
of exceptional burst events lasting more than ~10 minutes. Half of the
so-called intermediate long bursts registered so far have been observed by
INTEGRAL. The goal is to derive a comprehensive picture of the relationship
between the nuclear ignition processes and the accretion states of the system
leading up to such long bursts. Depending on the composition of the accreted
material, these bursts may be explained by either the unstable burning of a
large pile of mixed hydrogen and helium, or the ignition of a thick pure helium
layer. Intermediate long bursts are particularly expected to occur at very low
accretion rates and make possible to study the transition from a hydrogen-rich
bursting regime to a pure helium regime.Comment: Talk presented at the 7th INTEGRAL Workshop, An INTEGRAL View of
Compact Objects, Copenhagen, 8-11 September 2008. Submitted to Proceedings of
Science. 12 pages, 6 figures, 2 tables, 40 reference
Detection of Very Low-Frequency Quasi-Periodic Oscillations in the 2015 Outburst of V404 Cygni
In June 2015, the black hole X-ray binary (BHXRB) V404 Cygni went into
outburst for the first time since 1989. Here, we present a comprehensive search
for quasi-periodic oscillations (QPOs) of V404 Cygni during its recent
outburst, utilizing data from six instruments on board five different X-ray
missions: Swift/XRT, Fermi/GBM, Chandra/ACIS, INTEGRAL's IBIS/ISGRI and JEM-X,
and NuSTAR. We report the detection of a QPO at 18 mHz simultaneously with both
Fermi/GBM and Swift/XRT, another example of a rare but slowly growing new class
of mHz-QPOs in BHXRBs linked to sources with a high orbital inclination.
Additionally, we find a duo of QPOs in a Chandra/ACIS observation at 73 mHz and
1.03 Hz, as well as a QPO at 136 mHz in a single Swift/XRT observation that can
be interpreted as standard Type-C QPOs. Aside from the detected QPOs, there is
significant structure in the broadband power, with a strong feature observable
in the Chandra observations between 0.1 and 1 Hz. We discuss our results in the
context of current models for QPO formation.Comment: 17 pages, 9 figures, published in Ap
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
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