126 research outputs found
A novel image reconstruction method applied to deep Hubble Space Telescope Images
We have developed a method for the linear reconstruction of an image from
undersampled, dithered data, which has been used to create the distributed,
combined Hubble Deep Field images -- the deepest optical images yet taken of
the universe. The algorithm, known as Variable-Pixel Linear Reconstruction (or
informally as "drizzling"), preserves photometry and resolution, can weight
input images according to the statistical significance of each pixel, and
removes the effects of geometric distortion both on image shape and photometry.
In this paper, the algorithm and its implementation are described, and
measurements of the photometric accuracy and image fidelity are presented. In
addition, we describe the use of drizzling to combine dithered images in the
presence of cosmic rays.Comment: Invited paper, to appear in Applications of Digital Image Processing
XX, ed. A. Tescher, Proc. S.P.I.E. vol. 3164, in press; 6 pages, 4 included
figures, SPIE LaTex style file include
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
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
The faint-galaxy hosts of gamma-ray bursts
The observed redshifts and magnitudes of the host galaxies of gamma-ray
bursts (GRBs) are compared with the predictions of three basic GRB models, in
which the comoving rate density of GRBs is (1) proportional to the cosmic star
formation rate density, (2) proportional to the total integrated stellar
density and (3) constant. All three models make the assumption that at every
epoch the probability of a GRB occuring in a galaxy is proportional to that
galaxy's broad-band luminosity. No assumption is made that GRBs are standard
candles or even that their luminosity function is narrow. All three rate
density models are consistent with the observed GRB host galaxies to date,
although model (2) is slightly disfavored relative to the others. Models (1)
and (3) make very similar predictions for host galaxy magnitude and redshift
distributions; these models will be probably not be distinguished without
measurements of host-galaxy star-formation rates. The fraction of host galaxies
fainter than 28 mag may constrain the faint end of the galaxy luminosity
function at high redshift, or, if the fraction is observed to be low, may
suggest that the bursters are expelled from low-luminosity hosts. In all
models, the probability of finding a z<0.008 GRB among a sample of 11 GRBs is
less than 10^(-4), strongly suggesting that GRB 980425, if associated with
supernova 1998bw, represents a distinct class of GRBs.Comment: 7 pages, ApJ in press, revised to incorporate yet more new and
revised observational result
The Near Infrared and Multiwavelength Afterglow of GRB 000301c
We present near-infrared observations of the counterpart of GRB 000301c. The
K' filter (2.1 micron) light curve shows a well-sampled break in the decay
slope at t=3.5 days post-burst. The early time slope is very shallow (~ -0.1),
while the late time slope is steep (-2.2). Comparison with the optical (R band)
light curve shows marginally significant differences, especially in the early
time decay slope (which is steeper in the optical) and the break time (which
occurs later in the optical). This is contrary to the general expectation that
light curve breaks should either be achromatic (e.g., for breaks due to
collimation effects) or should occur later at longer wavelengths (for most
other breaks). The observed color variations might be intrinsic to the
afterglow, or might indicate systematic errors of > 0.08 magnitude in all
fluxes. Even if the break is achromatic, we argue that its sharpness poses
difficulties for explanations that depend on collimated ejecta. The R light
curve shows further signs of fairly rapid variability (a bump, steep drop, and
plateau) that are not apparent in the K' light curve. In addition, by combining
the IR-optical-UV data with millimeter and radio fluxes, we are able to
constrain the locations of the self-absorption break and cooling break and to
infer the location of the spectral peak at t=3 days: f_nu = 3.4 mJy at nu=1e12
Hz. Using the multiwavelength spectral energy distribution, we are able to
constrain the blast wave energy, which was E > 3e53 erg if the explosion was
isotropic. This implies a maximum gamma ray production efficiency of ~ 0.15 for
GRB 000301C.Comment: Accepted to The Astrophysical Journal. 24 pages, 4 figures, 3 tables;
uses AASTeX 5 macros. This version includes a new figure (R-K' color vs.
time), a better sampled R band light curve, and more extensive discussion of
the optical data and error analysi
HST Images of the Eclipsing Pulsar B1957+20
We have obtained images of the eclipsing pulsar binary PSR~B1957+20 using the
Planetary Camera of the Hubble Space Telescope. The high spatial resolution of
this instrument has allowed us to separate the pulsar system from a nearby
background star which has confounded ground-based observations of this system
near optical minimum. Our images limit the temperature of the backside of the
companion to T \simlt 2800~K, about a factor of two less than the average
temperature of the side of the companion facing the pulsar, and provide a
marginal detection of the companion at optical minimum. The magnitude of this
detection is consistent with previous work which suggests that the companion
nearly fills its Roche lobe and is supported through tidal dissipation.Comment: uuencoded gzip-compressed postscript: 10 pages of text plus 2
postscript figures. This preprint is available in various formats from
http://archer.stsci.edu:1024/~fruchter/HST_1957/1957.html or
http://electra.stsci.edu:1024/~fruchter/HST_1957/1957.htm
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