1,611 research outputs found
Jet Breaks in Short Gamma-Ray Bursts. I: The Uncollimated Afterglow of GRB 050724
We report the results of the \chandra observations of the \swift-discovered
short Gamma-Ray Burst GRB 050724. \chandra observed this burst twice, about two
days after the burst and a second time three weeks later. The first \chandra
pointing occurred at the end of a strong late-time flare. About 150 photons
were detected during this 49.3 ks observation in the 0.4-10.0 keV range. The
spectral fit is in good agreement with spectral analysis of earlier \swift XRT
data. In the second \chandra pointing the afterglow was clearly detected with 8
background-subtracted photons in 44.6 ks. From the combined \swift XRT and
\chandra-ACIS-S light curve we find significant flaring superposed on an
underlying power-law decay slope of =0.98. There is
no evidence for a break between about 1 ks after the burst and the last
\chandra pointing about three weeks after the burst. The non-detection of a jet
break places a lower limit of 25 on the jet opening angle, indicating
that the outflow is less strongly collimated than most previously-reported long
GRBs. This implies that the beaming corrected energy of GRB 050724 is at least
ergs.Comment: 7 pages, ApJ acceped, scheduled for December 20, 2006, ApJ, 65
The Prelude to and Aftermath of the Giant Flare of 2004 December 27: Persistent and Pulsed X-ray Properties of SGR 1806-20 from 1993 to 2005
On 2004 December 27, a highly-energetic giant flare was recorded from the
magnetar candidate SGR 1806-20. In the months preceding this flare, the
persistent X-ray emission from this object began to undergo significant
changes. Here, we report on the evolution of key spectral and temporal
parameters prior to and following this giant flare. Using the Rossi X-ray
Timing Explorer, we track the pulse frequency of SGR 1806-20 and find that the
spin-down rate of this SGR varied erratically in the months before and after
the flare. Contrary to the giant flare in SGR 1900+14, we find no evidence for
a discrete jump in spin frequency at the time of the December 27th flare
(|dnu/nu| < 5 X 10^-6). In the months surrounding the flare, we find a strong
correlation between pulsed flux and torque consistent with the model for
magnetar magnetosphere electrodynamics proposed by Thompson, Lyutikov &
Kulkarni (2002). As with the flare in SGR 1900+14, the pulse morphology of SGR
1806-20 changes drastically following the flare. Using the Chandra X-ray
Observatory and other publicly available imaging X-ray detector observations,
we construct a spectral history of SGR 1806-20 from 1993 to 2005. The usual
magnetar persistent emission spectral model of a power-law plus a blackbody
provides an excellent fit to the data. We confirm the earlier finding by
Mereghetti et al. (2005) of increasing spectral hardness of SGR 1806-20 between
1993 and 2004. Contrary to the direct correlation between torque and spectral
hardness proposed by Mereghetti et al., we find evidence for a sudden torque
change that triggered a gradual hardening of the energy spectrum on a timescale
of years. Interestingly, the spectral hardness, spin-down rate, pulsed, and
phase-averaged of SGR 1806-20 all peak months before the flare epoch.Comment: 37 pages, 8 figures, 8 tables. Accepted for publication in ApJ. To
appear in the Oct 20 2006 editio
The Burst Cluster: Dark Matter in a Cluster Merger Associated with the Short Gamma Ray Burst, GRB 050509B
We have identified a merging galaxy cluster with evidence of two distinct
sub-clusters. The X-ray and optical data suggest that the subclusters are
moving away from each other after closest approach. This cluster merger was
discovered from observations of the well localized short-duration gamma-ray
burst (GRB), GRB 050509B. The Swift/Burst Alert Telescope (BAT) source position
is coincident with a cluster of galaxies ZwCl 1234.0+02916. The subsequent
Swift/X-Ray Telescope (XRT) localization of the X-ray afterglow found the GRB
coincident with 2MASX J12361286+2858580, a giant red elliptical galaxy in the
cluster. Deep multi-epoch optical images were obtained to constrain the
evolution of the GRB afterglow, including a 27480s exposure in the F814W band
with Hubble Space Telescope Advanced Camera for Surveys (ACS), among the
deepest imaging ever obtained towards a known galaxy cluster in a single
passband. We perform a weak gravitational lensing analysis, including mapping
the total mass distribution of the merger system. Combined with Chandra X-ray
Observatory and Swift/XRT observations, we investigate the dynamical state of
the merger to probe the nature of the dark matter component. Our weak
gravitational lensing measurements reveal a separation of the X-ray centroid of
the western subcluster from the center of the mass and galaxy light
distributions, similar to that of the famous "Bullet cluster". We conclude that
the "Burst cluster" is another candidate merger system for determining the
nature of dark matter and for studying the environment of short GRBs. We
discuss connections between the cluster dynamical state and/or matter
composition and compact object mergers, the leading model for the origin of
short GRBs. Finally, we present results from a weak lensing survey based on
archival Very Large Telescope (VLT) images in the areas of 5 other short GRBs.Comment: 17 pages, 7 figures, accepted by Ap
Chandra Observations of the Faintest Low-Mass X-ray Binaries
There exists a group of persistently faint galactic X-ray sources that, based
on their location in the galaxy, high L_x/L_opt, association with X-ray bursts,
and absence of low frequency X-ray pulsations, are thought to be low-mass X-ray
binaries (LMXBs). We present results from Chandra observations for eight of
these systems: 4U 1708-408, 2S 1711-339, KS 1739-304, SLX 1735-269, GRS
1736-297, SLX 1746-331, 1E 1746.7-3224, and 4U 1812-12. Locations for all
sources, excluding GRS 1736-297, SLX 1746-331, and KS 1739-304 (which were not
detected) were improved to 0.6" error circles (90% confidence). Our
observations support earlier findings of transient behavior of GRS 1736-297, KS
1739-304, SLX 1746-331, and 2S 1711-339 (which we detect in one of two
observations). Energy spectra for 4U 1708-408, 2S 1711-339, SLX 1735-269, 1E
1746.7-3224, and 4U 1812-12 are hard, with power law indices typically 1.4-2.1,
which are consistent with typical faint LMXB spectra.Comment: 15 pages, 3 figures. Accepted by Ap
Chandra Observations of the Anomalous X-ray Pulsar 4U 0142+61
We present X-ray imaging, timing, and phase resolved spectroscopy of the
anomalous X-ray pulsar 4U 0142+61 using the Chandra X-ray Observatory. The
spectrum is well described by a power law plus blackbody model with power law
index = 3.35(2), kT_BB=0.458(3) keV, and N_H=0.91(2) x 10^{22} cm^{-2}$; we
find no significant evidence for spectral features (0.5-7.0 keV). Time resolved
X-ray spectroscopy shows evidence for evolution in phase in either index, or
KT_BB, or some combination thereof as a function of pulse phase. We derive a
precise X-ray position for the source and determine its spin period,
P=8.68866(30) s. We have detected emission beyond 4 arcsec from the central
source and extending beyond 100 arcsec, likely due to dust scattering in the
interstellar medium.Comment: 15 pages, accepted for publication in Ap
Daksha: On Alert for High Energy Transients
We present Daksha, a proposed high energy transients mission for the study of
electromagnetic counterparts of gravitational wave sources, and gamma ray
bursts. Daksha will comprise of two satellites in low earth equatorial orbits,
on opposite sides of earth. Each satellite will carry three types of detectors
to cover the entire sky in an energy range from 1 keV to >1 MeV. Any transients
detected on-board will be announced publicly within minutes of discovery. All
photon data will be downloaded in ground station passes to obtain source
positions, spectra, and light curves. In addition, Daksha will address a wide
range of science cases including monitoring X-ray pulsars, studies of
magnetars, solar flares, searches for fast radio burst counterparts, routine
monitoring of bright persistent high energy sources, terrestrial gamma-ray
flashes, and probing primordial black hole abundances through lensing. In this
paper, we discuss the technical capabilities of Daksha, while the detailed
science case is discussed in a separate paper.Comment: 9 pages, 3 figures, 1 table. Additional information about the mission
is available at https://www.dakshasat.in
Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV
The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8 TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, with the photons being measured through conversions to e⁺e⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum
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