2,043 research outputs found
Identification of the infrared counterpart of SGR 1935+2154 with the Hubble Space Telescope
We present deep Hubble Space Telescope observations of a new magnetar source,
the soft gamma-repeater SGR 1935+2154, discovered by Swift. We obtained three
epochs of observations: while the source was active in March 2015, during a
quiescent period in August 2015, and during a further active phase in May 2016.
Close to the center of the X-ray error region identified by Chandra we find a
faint (F140W(AB)=25.3) source, which fades by a factor of ~2 over the course of
5 months between the first two epochs of observations, before rebrightening
during the second active period. If this source is indeed the counterpart to
SGR 1935+2154 then it is amongst the faintest yet located for a magnetar. Our
observations are spaced over 1.3 years and enable us to place limits on the
source velocity of km s kpc; observations on
timescales of a decade can hence probe proper motion limits smaller than the
velocities observed for the majority of pulsars. The comparison of the
optical/IR and X-ray lightcurves of the source suggests that emission in the
two regimes is associated but not directly correlated, offering support for a
magnetospheric versus a fallback disc origin.Comment: 7 pages, 3 figures, accepted for publication in Ap
Creep fatigue life prediction for engine hot section materials (isotropic)
A series of high temperature strain controlled fatigue tests have been completed to study the effects of thermomechanical fatigue, multiaxial loading, reactive environments, and imposed mean stresses. The baseline alloy used in these tests was cast B1900+Hf (with and without coatings); a small number of tests of wrought INCO 718 are also included. A strong path dependence was demonstrated during the thermomechanical fatigue testing, using in-phase, out-phase, and non-proportional (elliptical and 'dogleg') strain-temperature cycles. The multiaxial tests also demonstrated cycle path to be a significant variable, using both proportional and non-proportional tension-torsion loading. Environmental screening tests were conducted in moderate pressure oxygen and purified argon; the oxygen reduced the specimen lives by two, while the argon testing produced ambiguous data. Both NiCoCrAlY overlay and diffusion aluminide coatings were evaluated under isothermal and TMF conditions; in general, the lives of the coated specimens were higher that those of uncoated specimens. Controlled mean stress TMF tests showed that small mean stress changes could change initiation lives by orders of magnitude; these results are not conservatively predicted using traditional linear damage summation rules. Microstructures were evaluated using optical, SEM and TEM methods
Progenitors of Long Gamma-ray Bursts
Pinpointing the progenitors of long duration gamma-ray bursts (LGRBs) remains
an extremely important question, although it is now clear that at least a
fraction of LGRBs originate in the core collapse of massive stars in type Ic
supernovae, the pathways to the production of these stars, and their initial
masses, remain uncertain. Rotation is thought to be vital in the creation of
LGRBs, and it is likely that black hole creation is also necessary. We suggest
that these two constraints can be met if the GRB progenitors are very massive
stars (>20 solar masses) and are formed in tight binary systems. Using simple
models we compare the predictions of this scenario with observations and find
that the location of GRBs on their host galaxies are suggestive of
main-sequence masses in excess of 20 solar masses, while 50% of the known
compact binary systems may have been sufficiently close to have had the
necessary rotation rates for GRB creation. Thus, massive stars in compact
binaries are a likely channel for at least some fraction of LGRBs.Comment: To appear in "Gamma-ray bursts: Prospects for GLAST", AIP Conference
proceedings 906, Editors M. Axelsson and F Ryd
Short gamma-ray bursts from SGR giant flares and neutron star mergers: two populations are better than one
‘The definitive version is available at www.blackwell-synergy.com.’ Copyright Blackwell Publishing. DOI: 10.1111/j.1365-2966.2009.14610.xThere is increasing evidence of a local population of short duration gamma-ray bursts (sGRB), but it remains to be seen whether this is a separate population to higher redshift bursts. Here we choose plausible luminosity functions (LFs) for both neutron star binary mergers and giant flares from soft gamma repeaters (SGR), and combined with theoretical and observed Galactic intrinsic rates we examine whether a single progenitor model can reproduce both the overall Burst and Transient Source Experiment (BATSE) sGRB number counts and a local population, or whether a dual progenitor population is required. Though there are large uncertainties in the intrinsic rates, we find that at least a bimodal LF consisting of lower and higher luminosity populations is required to reproduce both the overall BATSE sGRB number counts and a local burst distribution. Furthermore, the best-fitting parameters of the lower luminosity population agree well with the known properties of SGR giant flares, and the predicted numbers are sufficient to account for previous estimates of the local sGRB population.Peer reviewe
Swift J1112.2-8238: A Candidate Relativistic Tidal Disruption Flare
We present observations of Swift J1112.2-8238, and identify it as a candidate
relativistic tidal disruption flare (rTDF). The outburst was first detected by
Swift/BAT in June 2011 as an unknown, long-lived (order of days) -ray
transient source. We show that its position is consistent with the nucleus of a
faint galaxy for which we establish a likely redshift of based on a
single emission line that we interpret as the blended [OII]
doublet. At this redshift, the peak X/-ray luminosity exceeded
ergs s, while a spatially coincident optical transient source
had (M at ) during early
observations, days after the Swift trigger. These properties place
Swift J1112.2-8238 in a very similar region of parameter space to the two
previously identified members of this class, Swift J1644+57 and Swift
J2058+0516. As with those events the high-energy emission shows evidence for
variability over the first few days, while late time observations, almost 3
years post-outburst, demonstrate that it has now switched off. Swift
J1112.2-8238 brings the total number of such events observed by Swift to three,
interestingly all detected by Swift over a 3 month period ( of its
total lifetime as of March 2015). While this suggests the possibility that
further examples may be uncovered by detailed searches of the BAT archives, the
lack of any prime candidates in the years since 2011 means these events are
undoubtedly rare.Comment: 11 pages, 5 figures, accepted for publication by MNRA
Discovery of the Very Red Near-Infrared and Optical Afterglow of the Short-Duration GRB 070724A
[Abridged] We report the discovery of the near-infrared and optical afterglow
of the short-duration gamma-ray burst GRB070724A. The afterglow is detected in
i,J,H,K observations starting 2.3 hr after the burst with K=19.59+/-0.16 mag
and i=23.79+/-0.07 mag, but is absent in images obtained 1.3 years later.
Fading is also detected in the K-band between 2.8 and 3.7 hr at a 4-sigma
significance level. The optical/near-IR spectral index, beta_{O,NIR}=-2, is
much redder than expected in the standard afterglow model, pointing to either
significant dust extinction, A_{V,host}~2 mag, or a non-afterglow origin for
the near-IR emission. The case for extinction is supported by a shallow optical
to X-ray spectral index, consistent with the definition for ``dark bursts'',
and a normal near-IR to X-ray spectral index. Moreover, a comparison to the
optical discovery magnitudes of all short GRBs with optical afterglows
indicates that the near-IR counterpart of GRB070724A is one of the brightest to
date, while its observed optical emission is one of the faintest. In the
context of a non-afterglow origin, the near-IR emission may be dominated by a
mini-supernova, leading to an estimated ejected mass of M~10^-4 Msun and a
radioactive energy release efficiency of f~0.005 (for v~0.3c). However, the
mini-SN model predicts a spectral peak in the UV rather than near-IR,
suggesting that this is either not the correct interpretation or that the
mini-SN models need to be revised. Finally, the afterglow coincides with a star
forming galaxy at z=0.457, previously identified as the host based on its
coincidence with the X-ray afterglow position (~2" radius). Our discovery of
the optical/near-IR afterglow makes this association secure.Comment: Submitted to ApJ; 10 pages, 5 figures, 1 tabl
Exploring the Cosmic Evolution of Habitability with Galaxy Merger Trees
We combine inferred galaxy properties from a semi-analytic galaxy evolution
model incorporating dark matter halo merger trees with new estimates of
supernova and gamma ray burst rates as a function of metallicity from stellar
population synthesis models incorporating binary interactions. We use these to
explore the stellar mass fraction of galaxies irradiated by energetic
astrophysical transients and its evolution over cosmic time, and thus the
fraction which is potentially habitable by life like our own. We find that 18
per cent of the stellar mass in the Universe is likely to have been irradiated
within the last 260 Myr, with GRBs dominating that fraction. We do not see a
strong dependence of irradiated stellar mass fraction on stellar mass or
richness of the galaxy environment. We consider a representative merger tree as
a Local Group analogue, and find that there are galaxies at all masses which
have retained a high habitable fraction (>40 per cent) over the last 6 Gyr, but
also that there are galaxies at all masses where the merger history and
associated star formation have rendered galaxies effectively uninhabitable.
This illustrates the need to consider detailed merger trees when evaluating the
cosmic evolution of habitability.Comment: 11 page, 10 figures. MNRAS accepted 13th Dec 2017. Updated to match
accepted version, with additional discussion of metallicity effect
Chandra Observations of the X-ray Environs of SN 1998bw/GRB 980425
(Abrigded) We report X-ray studies of the environs of SN 1998bw and GRB
980425 using the Chandra X-Ray Observatory 1281 days after the GRB. Combining
our observation of the supernova with others of the GRB afterglow, a smooth
X-ray light curve, spanning ~1300 days, is obtained by assuming the burst and
supernova were coincident at 35.6 Mpc. When this X-ray light curve is compared
with those of the X-ray ``afterglows'' of ordinary GRBs, X-ray Flashes, and
ordinary supernovae, evidence emerges for at least two classes of lightcurves,
perhaps bounding a continuum. By three to ten years, all these phenomena seem
to converge on a common X-ray luminosity, possibly indicative of the supernova
underlying them all. This convergence strengthens the conclusion that SN 1998bw
and GRB 980425 took place in the same object. One possible explanation for the
two classes is a (nearly) standard GRB observed at different angles, in which
case X-ray afterglows with intermediate luminosities should eventually be
discovered. Finally, we comment on the contribution of GRB afterglows to the
ULX source population.Comment: 26 pages, 5 figues, submitted to Ap
GRB 070714B - Discovery of the Highest Spectroscopically Confirmed Short Burst Redshift
Gemini Nod & Shuffle spectroscopy on the host of the short GRB 070714B shows
a single emission line at 7167 angstroms which, based on a grizJHK photometric
redshift, we conclude is the 3727 angstrom [O II] line. This places the host at
a redshift of z=.923 exceeding the previous record for the highest
spectroscopically confirmed short burst redshift of z=.546 held by GRB 051221.
This dramatically moves back the time at which we know short bursts were being
formed, and suggests that the present evidence for an old progenitor population
may be observationally biased.Comment: Conference procedings for Gamma Ray Bursts 2007 November 5-9, 2007
Santa Fe, New Mexico (4 pages, 2 figures
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