641 research outputs found
Automatic analysis of Swift-XRT data
The Swift spacecraft detects and autonomously observes ~100 Gamma Ray Bursts
(GRBs) per year, ~96% of which are detected by the X-ray telescope (XRT). GRBs
are accompanied by optical transients and the field of ground-based follow-up
of GRBs has expanded significantly over the last few years, with rapid response
instruments capable of responding to Swift triggers on timescales of minutes.
To make the most efficient use of limited telescope time, follow-up astronomers
need accurate positions of GRBs as soon as possible after the trigger.
Additionally, information such as the X-ray light curve, is of interest when
considering observing strategy. The Swift team at Leicester University have
developed techniques to improve the accuracy of the GRB positions available
from the XRT, and to produce science-grade X-ray light curves of GRBs. These
techniques are fully automated, and are executed as soon as data are available.Comment: 4 pages, 2 figures, to appear in the proceedings of ADASS XVII (ASP
Conference Series
A search for thermal X-ray signatures in Gamma-Ray Bursts I: Swift bursts with optical supernovae
The X-ray spectra of Gamma-Ray Bursts can generally be described by an
absorbed power law. The landmark discovery of thermal X-ray emission in
addition to the power law in the unusual GRB 060218, followed by a similar
discovery in GRB 100316D, showed that during the first thousand seconds after
trigger the soft X-ray spectra can be complex. Both the origin and prevalence
of such spectral components still evade understanding, particularly after the
discovery of thermal X-ray emission in the classical GRB 090618. Possibly most
importantly, these three objects are all associated with optical supernovae,
begging the question of whether the thermal X-ray components could be a result
of the GRB-SN connection, possibly in the shock breakout. We therefore
performed a search for blackbody components in the early Swift X-ray spectra of
11 GRBs that have or may have associated optical supernovae, accurately
recovering the thermal components reported in the literature for GRBs 060218,
090618 and 100316D. We present the discovery of a cooling blackbody in GRB
101219B/SN2010ma, and in four further GRB-SNe we find an improvement in the fit
with a blackbody which we deem possible blackbody candidates due to
case-specific caveats. All the possible new blackbody components we report lie
at the high end of the luminosity and radius distribution. GRB 101219B appears
to bridge the gap between the low-luminosity and the classical GRB-SNe with
thermal emission, and following the blackbody evolution we derive an expansion
velocity for this source of order 0.4c. We discuss potential origins for the
thermal X-ray emission in our sample, including a cocoon model which we find
can accommodate the more extreme physical parameters implied by many of our
model fits.Comment: 16 pages, 6 figures, accepted for MNRA
X-ray and UV observations of V751 Cyg in an optical high state
Aims: The VY Scl system (anti-dwarf nova) V751 Cyg is examined following a
claim of a super-soft spectrum in the optical low state. Methods: A
serendipitous XMM-Newton X-ray observation and, 21 months later, Swift X-ray
and UV observations, have provided the best such data on this source so far.
These optical high-state datasets are used to study the flux and spectral
variability of V751 Cyg. Results: Both the XMM-Newton and Swift data show
evidence for modulation of the X-rays for the first time at the known 3.467 hr
orbital period of V751 Cyg. In two Swift observations, taken ten days apart,
the mean X-ray flux remained unchanged, while the UV source brightened by half
a magnitude. The X-ray spectrum was not super-soft during the optical high
state, but rather due to multi-temperature optically thin emission, with
significant (10^{21-22} cm^-2) absorption, which was higher in the observation
by Swift than that of XMM-Newton. The X-ray flux is harder at orbital minimum,
suggesting that the modulation is related to absorption, perhaps linked to the
azimuthally asymmetric wind absorption seen previously in H-alpha.Comment: 6 pages, 9 figures, accepted for publication in A&
On the X-ray variability of magnetar 1RXS J170849.0-400910
We present a long-term X-ray flux and spectral analysis for 1RXS
J170849.0-400910 using Swift/XRT spanning over 8 years from 2005-2013. We also
analyze two observations from Chandra and XMM in the period from 2003-2004. In
this 10-yr period, 1RXS J170849.0-400910 displayed several rotational glitches.
Previous studies have claimed variations in the X-ray emission associated with
some of the glitches. From our analysis we find no evidence for significant
X-ray flux variations and evidence for only low-level spectral variations. We
also present an updated timing solution for 1RXS J170849.0-400910, from RXTE
and Swift observations, which includes a previously unreported glitch at MJD
56019. We discuss the frequency and implications of radiatively quiet glitches
in magnetars.Comment: 9 pages, 2 figures, accepted for publication in Ap
Short-period X-ray oscillations in super-soft novae and persistent SSS
Transient short-period <100s oscillations have been found in the X-ray light
curves of three novae during their SSS phase and in one persistent SSS. We
pursue an observational approach to determine possible driving mechanisms and
relations to fundamental system parameters such as the white dwarf mass.
We performed a systematic search for short-period oscillations in all
available XMM-Newton and Chandra X-ray light curves of persistent SSS and novae
during their SSS phase. To study time evolution, we divided each light curve
into short time segments and computed power spectra. We then constructed
dynamic power spectra from which we identified transient periodic signals even
when only present for a short time. From all time segments of each system, we
computed fractions of time when periodic signals were detected.
In addition to the previously known systems with short-period oscillations,
RS Oph (35s), KT Eri (35s), V339 Del (54s), and Cal 83 (67s), we found one
additional system, LMC 2009a (33s), and also confirm the 35s period from
Chandra data of KT Eri. The amplitudes of oscillations are of order <15% of the
respective count rates and vary without any clear dependence on the X-ray count
rate. The fractions of the time when the respective periods were detected at
2-sigma significance (duty cycle) are 11.3%, 38.8%, 16.9%, 49.2%, and 18.7% for
LMC 2009a, RS Oph, KT Eri, V339 Del, and Cal 83, respectively. The respective
highest duty cycles found in a single observation are 38.1%, 74.5%, 61.4%,
67.8%, and 61.8%.Comment: accepted for publication in A&
The spectroscopic evolution of the symbiotic-like recurrent nova V407 Cygni during its 2010 outburst. I. The shock and its evolution
On 2010 Mar 10, V407 Cyg was discovered in outburst, eventually reaching V< 8
and detected by Fermi. Using medium and high resolution ground-based optical
spectra, visual and Swift UV photometry, and Swift X-ray spectrophotometry, we
describe the behavior of the high-velocity profile evolution for this nova
during its first three months. The peak of the X-ray emission occurred at about
day 40 with a broad maximum and decline after day 50. The main changes in the
optical spectrum began at around that time. The He II 4686A line first appeared
between days 7 and 14 and initially displayed a broad, symmetric profile that
is characteristic of all species before day 60. Low-excitation lines remained
comparatively narrow, with v(rad,max) of order 200-400 km/s. They were
systematically more symmetric than lines such as [Ca V], [Fe VII], [Fe X], and
He II, all of which showed a sequence of profile changes going from symmetric
to a blue wing similar to that of the low ionization species but with a red
wing extended to as high as 600 km/s . The Na I D doublet developed a broad
component with similar velocity width to the other low-ionization species. The
O VI Raman features were not detected. We interpret these variations as
aspherical expansion of the ejecta within the Mira wind. The blue side is from
the shock penetrating into the wind while the red wing is from the low-density
periphery. The maximum radial velocities obey power laws, v(rad,max) t^{-n}
with n ~ 1/3 for red wing and ~0.8 for the blue. (truncated)Comment: Accepted for publication, A&A (submitted: 9 Oct 2010; accepted: 1 Dec
2010) in press; based on data obtained with Swift, Nordic Optical Telescope,
Ondrejov Observatory. Corrected typo, Fermi?LAT detection was at energies
above 100 MeV (with thanks to C. C. Cheung
Timing accuracy of the Swift X-Ray Telescope in WT mode
The X-Ray Telescope (XRT) on board Swift was mainly designed to provide
detailed position, timing and spectroscopic information on Gamma-Ray Burst
(GRB) afterglows. During the mission lifetime the fraction of observing time
allocated to other types of source has been steadily increased. In this paper,
we report on the results of the in-flight calibration of the timing
capabilities of the XRT in Windowed Timing read-out mode. We use observations
of the Crab pulsar to evaluate the accuracy of the pulse period determination
by comparing the values obtained by the XRT timing analysis with the values
derived from radio monitoring. We also check the absolute time reconstruction
measuring the phase position of the main peak in the Crab profile and comparing
it both with the value reported in literature and with the result that we
obtain from a simultaneous Rossi X-Ray Timing Explorer (RXTE) observation. We
find that the accuracy in period determination for the Crab pulsar is of the
order of a few picoseconds for the observation with the largest data time span.
The absolute time reconstruction, measured using the position of the Crab main
peak, shows that the main peak anticipates the phase of the position reported
in literature for RXTE by ~270 microseconds on average (~150 microseconds when
data are reduced with the attitude file corrected with the UVOT data). The
analysis of the simultaneous Swift-XRT and RXTE Proportional Counter Array
(PCA) observations confirms that the XRT Crab profile leads the PCA profile by
~200 microseconds. The analysis of XRT Photodiode mode data and BAT event data
shows a main peak position in good agreement with the RXTE, suggesting the
discrepancy observed in XRT data in Windowed Timing mode is likely due to a
systematic offset in the time assignment for this XRT read out mode.Comment: 6 pages, 4 figures. Accepted for publication on
Astronomy&Astrophysic
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