552 research outputs found
The X-ray flaring properties of Sgr A* during six years of monitoring with Swift
Starting in 2006, Swift has been targeting a region of ~21'X21' around
Sagittarius A* (Sgr A*) with the onboard X-ray telescope. The short,
quasi-daily observations offer an unique view of the long-term X-ray behavior
of the supermassive black hole. We report on the data obtained between 2006
February and 2011 October, which encompasses 715 observations with a total
accumulated exposure time of ~0.8 Ms. A total of six X-ray flares were detected
with Swift, which all had an average 2-10 keV luminosity of Lx (1-4)E35 erg/s
(assuming a distance of 8 kpc). This more than doubles the number of such
bright X-ray flares observed from Sgr A*. One of the Swift-detected flares may
have been softer than the other five, which would indicate that flares of
similar intensity can have different spectral properties. The Swift campaign
allows us to constrain the occurrence rate of bright (Lx > 1E35 erg/s) X-ray
flares to be ~0.1-0.2 per day, which is in line with previous estimates. This
analysis of the occurrence rate and properties of the X-ray flares seen with
Swift offers an important calibration point to asses whether the flaring
behavior of Sgr A* changes as a result of its interaction with the gas cloud
that is projected to make a close passage in 2013.Comment: 8 pages, 5 figures, 3 tables. Shortened, accepted to Ap
The Swift X-ray monitoring campaign of the center of the Milky Way
In 2006 February, shortly after its launch, Swift began monitoring the center
of the Milky Way with the onboard X-Ray Telescope using short 1-ks exposures
performed every 1-4 days. Between 2006 and 2014, over 1200 observations have
been obtained, amounting to ~1.2 Ms of exposure time. This has yielded a wealth
of information about the long-term X-ray behavior of the supermassive black
hole Sgr A*, and numerous transient X-ray binaries that are located within the
25'x25' region covered by the campaign. In this review we highlight the
discoveries made during these first nine years, which includes 1) the detection
of seven bright X-ray flares from Sgr A*, 2) the discovery of the magnetar SGR
J1745-29, 3) the first systematic analysis of the outburst light curves and
energetics of the peculiar class of very-faint X-ray binaries, 4) the discovery
of three new transient X-ray sources, 5) exposing low-level accretion in
otherwise bright X-ray binaries, and 6) the identification of a candidate X-ray
binary/millisecond radio pulsar transitional object. We also reflect on future
science to be done by continuing this Swift's legacy campaign of the Galactic
center, which includes high-cadence monitoring of how the interaction between
the gaseous object `G2' and Sgr A* plays out in the future.Comment: 13 pages, 6 figures, 4 tables. Invited review to appear in Elsevier's
Journal of High Energy Astrophysics dedicated issue "Swift: 10 years of
discovery
The Galactic center X-ray transients AX J1745.6-2901 and GRS 1741-2853
AX J1745.6-2901 and GRS 1741-2853 are two transient neutron star low-mass
X-ray binaries that are located within ~10' from the Galactic center.
Multi-year monitoring observations with the Swift/XRT has exposed several
accretion outbursts from these objects. We report on their updated X-ray light
curves and renewed activity that occurred in 2010-2013.Comment: 2 pages, 1 figure, 1 table. To appear in conference proceedings of
IAU symposium 303 "The Galactic Center: Feeding and Feedback in a Normal
Galactic Nucleus
Monitoring Supergiant Fast X-ray Transients with Swift. Rise to the outburst in IGR J16479-4514
IGR J16479-4514 is a Supergiant Fast X-ray Transient (SFXT), a new class of
High Mass X-ray Binaries, whose number is rapidly growing thanks to the
observations of the Galactic plane performed with the INTEGRAL satellite. IGR
J16479-4514 has been regularly monitored with Swift/XRT since November 2007, to
study the quiescent emission, the outburst properties and their recurrence. A
new bright outburst, reaching fluxes above 10 erg cm s,
was caught by the Swift/BAT.
Swift immediately re-pointed at the target with the narrow-field instruments
so that, for the first time, an outburst from a SFXT where a periodicity in the
outburst recurrence is unknown could be observed simultaneously in the 0.2--150
keV energy band. The X-ray emission is highly variable and spans almost four
orders of magnitude in count rate during the Swift/XRT observations covering a
few days before and after the bright peak. The X-ray spectrum in outburst is
hard and highly absorbed. The power-law fit resulted in a photon index of
0.98, and in an absorbing column density of
cm. These observations demonstrate that in this source (similarly to
what was observed during the 2007 outburst from the periodic SFXT IGR
J11215-5952), the accretion phase lasts much longer than a few hours.Comment: Accepted for publication on Astrophysical Journal Letters. 5 pages, 4
figure
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
Optimisation of the Swift X-ray follow-up of Advanced LIGO and Virgo gravitational wave triggers in 2015--16
One of the most exciting near-term prospects in physics is the potential
discovery of gravitational waves by the advanced LIGO and Virgo detectors. To
maximise both the confidence of the detection and the science return, it is
essential to identify an electromagnetic counterpart. This is not trivial, as
the events are expected to be poorly localised, particularly in the near-term,
with error regions covering hundreds or even thousands of square degrees. In
this paper we discuss the prospects for finding an X-ray counterpart to a
gravitational wave trigger with the Swift X-ray Telescope, using the assumption
that the trigger is caused by a binary neutron star merger which also produces
a short gamma-ray burst. We show that it is beneficial to target galaxies
within the GW error region, highlighting the need for substantially complete
galaxy catalogues out to distances of 300 Mpc. We also show that nearby,
on-axis short GRBs are either extremely rare, or are systematically less
luminous than those detected to date. We consider the prospects for detecting
afterglow emission from an an off-axis GRB which triggered the GW facilities,
finding that the detectability, and the best time to look, are strongly
dependent on the characteristics of the burst such as circumburst density and
our viewing angle.Comment: 17 pages, 14 figures. Accepted for publication in MNRA
Swift/XRT detection of an active X-ray transient near the Galactic center
Daily monitoring observations of the Galactic center performed with the Swift/XRT (Atel #5006; see link below) reveal that in addition to the new magnetar SGR J1745-29 (Atels #5009,#5011, #5020, #5032, #5037, #5046, #5053; Kennea et al. 2013; Mori et al. 2013) a transient X-ray source located ~20" north of Sgr A* is currently active. This object is clearly detected during ~1.0 ks PC mode observations obtained on 2013 May 15, 16, 18 and 19, at count rates of ~(1.5-5.0)E-2 counts/s
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