258 research outputs found
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
A BeppoSAX observation of MKN6
We have used the BeppoSAX satellite to study the broad band (0.5-100 keV)
X-ray spectrum of the Seyfert 1.5 galaxy MKN6. The source is characterized by a
power law of Gamma=1.7 [+0.08, -0.07] and there is no strong evidence for
either a reflection bump or a high energy cut-off. We have detected a narrow
iron line at 6.4 keV (rest frame) with an equivalent width of 98 [+33, -35] eV.
MKN6 also exhibits strong and complex absorption. At least two components (NH_1
=1.34 [+0.4,-0.4] x 10^(22) cm^(-2) and NH_2 = 4.18 [+2.2, -1.3] x 10^(22)
cm^(-2)) are present and they both partially cover the source with covering
fractions of ~90% and ~50% respectively. Comparison with a previous ASCA
observation indicates that in both absorbing columns the NH is variable over a
2 year timescale, while the covering fractions are constant over the same
amount of time. The state of each absorber is cold or mildly photoionized. The
Broad Line Region (BLR) is suggested as the possible location for this complex
absorption.Comment: 5 pages, 5 figures, to be published in A&
Testing the gamma-ray burst variability/peak luminosity correlation on a Swift homogeneous sample
We test the gamma-ray burst correlation between temporal variability and peak
luminosity of the -ray profile on a homogeneous sample of 36 Swift/BAT
GRBs with firm redshift determination. This is the first time that this
correlation can be tested on a homogeneous data sample. The correlation is
confirmed, as long as the 6 GRBs with low luminosity (<5x10^{50} erg s^{-1} in
the rest-frame 100-1000 keV energy band) are ignored. We confirm that the
considerable scatter of the correlation already known is not due to the
combination of data from different instruments with different energy bands, but
it is intrinsic to the correlation itself. Thanks to the unprecedented
sensitivity of Swift/BAT, the variability/peak luminosity correlation is tested
on low-luminosity GRBs. Our results show that these GRBs are definite outliers.Comment: Accepted for Publication in MNRAS. 10 pages, 5 figures, 3 table
The 26 year-long X-ray light curve and the X-ray spectrum of the BL Lac Object 1E 1207.9+3945 in its brightest state
We studied the temporal and spectral evolution of the synchrotron emission
from the high energy peaked BL Lac object 1E 1207.9+3945. Two recent
observations have been performed by the XMM-Newton and Swift satellites; we
carried out X-ray spectral analysis for both of them, and photometry in
optical-ultraviolet filters for the Swift one. Combining the results thus
obtained with archival data we built the long-term X-ray light curve, spanning
a time interval of 26 years, and the Spectral Energy Distribution (SED) of this
source. The light curve shows a large flux increasing, about a factor of six,
in a time interval of a few years. After reaching its maximum in coincidence
with the XMM-Newton pointing in December 2000 the flux decreased in later
years, as revealed by Swift. The very good statistics available in the 0.5-10
keV XMM-Newton X-ray spectrum points out a highly significant deviation from a
single power law. A log-parabolic model with a best fit curvature parameter of
0.25 and a peak energy at ~1 keV describes well the spectral shape of the
synchrotron emission. The simultaneous fit of Swift UVOT and XRT data provides
a milder curvature (b~0.1) and a peak at higher energies (~15 keV), suggesting
a different state of source activity. In both cases UVOT data support the
scenario of a single synchrotron emission component extending from the
optical/UV to the X-ray band. New X-ray observations are important to monitor
the temporal and spectral evolution of the source; new generation gamma-ray
telescopes like AGILE and GLAST could for the first time detect its inverse
Compton emission.Comment: 7 pages, 6 figures, accepted for publication in A&
The seven year Swift-XRT point source catalog (1SWXRT)
Swift is a multi-wavelength observatory specifically designed for gamma-ray
burst (GRB) astronomy that is operational since 2004. Swift is also a very
flexible multi-purpose facility that supports a wide range of scientific fields
such as active galactic nuclei, supernovae, cataclysmic variables, Galactic
transients, active stars and comets. The Swift X-ray Telescope (XRT) has
collected more than 150 Ms of observations in its first seven years of
operations. We present the list of all the X-ray point sources detected in XRT
imaging data taken in photon counting mode during the first seven years of
Swift operations. All these point-like sources, excluding the Gamma-Ray Bursts
(GRB), will be stored in a catalog publicly available (1SWXRT). We consider all
XRT observations with exposure time > 500 s taken in the period 2005-2011. Data
were reduced and analyzed with standard techniques and a list of detected
sources for each observation was produced. A careful visual inspection was
performed to remove extended, spurious and piled-up sources. Positions, count
rates, fluxes and the corresponding uncertainties were computed. We have
analyzed more than 35,000 XRT fields, with exposures ranging between 500 s and
100 ks, for a total exposure time of 140 Ms. The catalog includes ~ 89,000
entries, of which almost 85,000 are not affected by pile-up and are not GRBs.
Since many XRT fields were observed several times, we have a total of ~36,000
distinct celestial sources. We computed count rates in three energy bands:
0.3-10 keV (Full, or F), 0.3-3 keV (Soft, or S) and 2-10 keV (Hard, or H). Each
entry has a detection in at least one of these bands. In particular, we detect
~ 80,000, ~ 70,000 and ~ 25,500$ in the F, S and H band, respectively. Count
rates were converted into fluxes in the 0.5-10, 0.5-2 and 2-10 keV bands. Some
possible scientific uses of the catalog are also highlighted.Comment: 11 pages, 15 ps figures, accepted for publication in A&A. Abstract
shortened with respect to the original versio
Multifrequency Observations of the Blazar 3C 279 in January 2006
We report first results of a multifrequency campaign from radio to hard X-ray
energies of the prominent gamma-ray blazar 3C 279, which was organised around
an INTEGRAL ToO observation in January 2006, and triggered on its optical
state. The variable blazar was observed at an intermediate optical state, and a
well-covered multifrequency spectrum from radio to hard X-ray energies could be
derived. The SED shows the typical two-hump shape, the signature of non-thermal
synchrotron and inverse-Compton (IC) emission from a relativistic jet. By the
significant exposure times of INTEGRAL and Chandra, the IC spectrum (0.3 - 100
keV) was most accurately measured, showing - for the first time - a possible
bending. A comparison of this 2006 SED to the one observed in 2003, also
centered on an INTEGRAL observation, during an optical low-state, reveals the
surprising fact that - despite a significant change at the high-energy
synchrotron emission (near-IR/optical/UV) - the rest of the SED remains
unchanged. In particular, the low-energy IC emission (X- and hard X-ray
energies) remains the same as in 2003, proving that the two emission components
do not vary simultaneously, and provides strong constraints on the modelling of
the overall emission of 3C 279.Comment: 4 pages, 6 figures; to be published in the Proc. of the 6th INTEGRAL
workshop "The Obscured Universe" (Moscow, July 2-8, 2006), eds. S. Grebenev,
R. Sunyaev, C. Winkler, ESA SP 622 (2006
Discovery and evolution of the new black hole candidate Swift J1539.2-6227 during its 2008 outburst
We report on the discovery by the Swift Gamma-Ray Burst Explorer of the black
hole candidate Swift J1539.2-6227 and the subsequent course of an outburst
beginning in November 2008 and lasting at least seven months. The source was
discovered during normal observations with the Swift Burst Alert Telescope
(BAT) on 2008 November 25. An extended observing campaign with the Rossi X-Ray
Timing Explorer (RXTE) and Swift provided near-daily coverage over 176 days,
giving us a good opportunity to track the evolution of spectral and timing
parameters with fine temporal resolution through a series of spectral states.
The source was first detected in a hard state during which strong low-frequency
quasi-periodic oscillations (QPOs) were detected. The QPOs persisted for about
35 days and a signature of the transition from the hard to soft intermediate
states was seen in the timing data. The source entered a short-lived thermal
state about 40 days after the start of the outburst. There were variations in
spectral hardness as the source flux declined and returned to a hard state at
the end of the outburst. The progression of spectral states and the nature of
the timing features provide strong evidence that Swift J1539.2-6227 is a
candidate black hole in a low-mass X-ray binary system.Comment: Accepted by the Astrophysical Journa
The broad band spectrum and variability of NGC 4151 observed by BeppoSAX
We present an analysis of all SAX observations of NGC 4151. This source was observed 5 times from 1996 to 2001 with durations ranging from a day to four days. The intrinsic continuum, is absorbed at low energies by a complex system: a cold patchy absorber plus a warm uniform screen photoionized by the central continuum. We find that this dual absorber is the main driver of the observed variability, up to a factor of eight, at 3 keV. In particular the covering fraction of the cold absorber changes on time scales of the order of a day, supporting its association with the Broad Line Region. The column density of the warm gas varies on a longer time scale (months to year). Some of the small amplitude spectral variability above 10 keV can be explained with an intrinsic variation (\Delta\Gamma~0.2).The flux below 1 keV remains constant confirming an extended origin. Its spectrum is reproduced by a combination of a thermal component and a power law with the same slope as the intrinsic continuum but with an intensity a few per cent. A Compton reflection component is significantly detected in 1996 (averaged value of \Omega/2\pi ~0.4), with intensity decreasing on time scale of year, and it desappears in 2000 and 2001. The long time scale of variations argues for an association with an optically thick torus at a distance of few light years. An iron line was detected in all spectra. Its energy is consistent with fluorescence by cold iron. We find that the line is variable. Its behaviour is reproduced by a variable component proportional to the level of the reflection flux plus a constant component. The flux of the latter is consistent with the extended line emission observed by Chandra. We conclude that the first component is likely arising from the torus and the second is produced in the extended Narrow Line Region
A refined position catalog of the Swift XRT afterglows
We present a catalogue of refined positions of 68 gamma ray burst (GRB)
afterglows observed by the Swift X-ray Telescope (XRT) from the launch up to
2005 Oct 16. This is a result of the refinement of the XRT boresight
calibration. We tested this correction by means of a systematic study of a
large sample of X-ray sources observed by XRT with well established optical
counterparts. We found that we can reduce the systematic error radius of the
measurements by a factor of two, from 6.5" to 3.2" (90% of confidence). We
corrected all the positions of the afterglows observed by XRT in the first 11
months of the Swift mission. This is particularly important for the 37 X-ray
afterglows without optical counterpart. Optical follow-up of dark GRBs, in
fact, will be more efficient with the use of the more accurate XRT positions.Comment: 4 pages, 4 figures, 1 table ; accepted for publication in A&A
Letters. The revised version contains updated position
- …