765 research outputs found
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 Environments of the Most Energetic Gamma-Ray Bursts
We analyze the properties of a sample of long gamma-ray bursts (LGRBs)
detected by the Fermi satellite that have a spectroscopic redshift and good
follow-up coverage at both X-ray and optical/nIR wavelengths. The evolution of
LGRB afterglows depends on the density profile of the external medium, enabling
us to separate wind or ISM-like environments based on the observations. We do
this by identifying the environment that provides the best agreement between
estimates of , the index of the underlying power-law distribution of
electron energies, as determined by the behavior of the afterglow in different
spectral/temporal regimes. At 11 rest-frame hours after trigger, we find a
roughly even split between ISM-like and wind-like environments. We further find
a 2 separation in the prompt emission energy distributions of wind-like
and ISM-like bursts. We investigate the underlying physical parameters of the
shock, and calculate the (degenerate) product of density and magnetic field
energy (). We show that must be to avoid
implied densities comparable to the intergalactic medium. Finally, we find that
the most precisely constrained observations disagree on by more than would
be expected based on observational errors alone. This suggests additional
sources of error that are not incorporated in the standard afterglow theory.
For the first time, we provide a measurement of this intrinsic error which can
be represented as an error in the estimate of of magnitude .
When this error is included in the fits, the number of LGRBs with an identified
environment drops substantially, but the equal division between the two types
remains.Comment: 31 pages (+14 appendix), 9 figures, 6 tables. Accepted for
publication in Ap
Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228
We develop the ``simulated extinction method'' to measure average foreground
Galactic extinction from field galaxy number-counts and colors. The method
comprises simulating extinction in suitable reference fields by changing the
isophotal detection limit. This procedure takes into account selection effects,
in particular, the change in isophotal detection limit (and hence in isophotal
magnitude completeness limit) with extinction, and the galaxy color--magnitude
relation.
We present a first application of the method to the HST WFPC2 images of the
gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields,
including the HDF, are used as reference to measure the average extinction
towards the GRB in the F606W passband. From the counts, we derive an average
extinction of A_V = 0.5 mag, but the dispersion of 0.4 mag between the
estimates from the different reference fields is significantly larger than can
be accounted by Poisson plus clustering uncertainties. Although the counts
differ, the average colors of the field galaxies agree well. The extinction
implied by the average color difference between the GRB field and the reference
galaxies is A_V = 0.6 mag, with a dispersion in the estimated extinction from
the four reference fields of only 0.1 mag. All our estimates are in good
agreement with the value of 0.81\pm0.27 mag obtained by Burstein & Heiles, and
with the extinction of 0.78\pm0.12 measured by Schlegel et al. from maps of
dust IR emission. However, the discrepancy between the widely varying counts
and the very stable colors in these high-latitude fields is worth
investigating.Comment: 14 pages, 2 figures; submitted to the Astrophysical Journa
Evidence for Cosmic Acceleration is Robust to Observed Correlations Between Type Ia Supernova Luminosity and Stellar Age
Type Ia Supernovae (SNe Ia) are powerful standardizable candles for
constraining cosmological models and provided the first evidence of the
accelerated expansion of the universe. Their precision derives from empirical
correlations, now measured from SNe Ia, between their luminosities,
light-curve shapes, colors and most recently with the stellar mass of their
host galaxy. As mass correlates with other galaxy properties, alternative
parameters have been investigated to improve SN Ia standardization though none
have been shown to significantly alter the determination of cosmological
parameters. We re-examine a recent claim, based on 34 SN Ia in nearby passive
host galaxies, of a 0.05 mag/Gyr dependence of standardized SN Ia luminosity on
host age which if extrapolated to higher redshifts, would be a bias up to 0.25
mag, challenging the inference of dark energy. We reanalyze this sample of
hosts using both the original method and a Bayesian hierarchical model and find
after a fuller accounting of the uncertainties the significance of a dependence
on age to be and after the removal of a single
poorly-sampled SN Ia. To test the claim that a trend seen in old stellar
populations can be applied to younger ages, we extend our analysis to a larger
sample which includes young hosts. We find the residual dependence of host age
(after all standardization typically employed for cosmological measurements) to
be consistent with zero for 254 SNe Ia from the Pantheon sample, ruling out the
large but low significance trend seen in passive hosts.Comment: 9 pages, 3 figures, 3 tables. Accepted for publication in ApJ
Proper-Motion Measurements with the VLA. II. Observations of Twenty-eight Pulsars
Using the Very Large Array, we have measured the proper motions of
twenty-eight radio pulsars. On average, the pulsars studied are fainter and
more distant than those studied in earlier work, reducing the selection biases
inherent in surveys restricted to the Solar neighborhood. The typical
measurement precision achieved is a few milliarcseconds per year, corresponding
to a few tens of kilometers per second for a pulsar a kiloparsec away. While
our results compare well with higher-precision measurements done using
very-long baseline interferometry, we find that several earlier proper motion
surveys appear to have reported overly optimistic measurement uncertainties,
most likely because of a failure to fully account for ionospheric effects. We
discuss difficulties inherent in estimating pulsar velocities from proper
motions given poorly constrained pulsar distances. Our observations favor a
distribution with 20% of pulsars in a low velocity component (sigma_1D = 99
km/s) and 80% in a high velocity component (sigma_1D = 294 km/s). Furthermore,
our sample is consistent with a scale height of pulsar birthplaces comparable
to the scale height of the massive stars that are their presumed progenitors.
No evidence is found in our data for a significant population of young pulsars
born far from the plane. We find that estimates of pulsar ages based on
kinematics agree well with the canonical spin-down age estimate, but agreement
is improved if braking indexes are drawn from a Gaussian distribution centered
at n=3 with width 0.8.Comment: 20 pages. Accepted for publication in the Astronomical Journa
HST-NICMOS Observations of Terzan 5: Stellar Content and Structure of the Core
We report results from HST-NICMOS imaging of the extremely dense core of the
globular cluster Terzan 5. This highly obscured bulge cluster has been
estimated to have one of the highest collision rates of any galactic globular
cluster, making its core a particularly conducive environment for the
production of interacting binary systems. We have reconstructed high-resolution
images of the central 19"x19" region of Terzan 5 by application of the drizzle
algorithm to dithered NIC2 images in the F110W, F187W, and F187N near-infrared
filters. We have used a DAOPHOT/ALLSTAR analysis of these images to produce the
deepest color-magnitude diagram (CMD) yet obtained for the core of Terzan 5. We
have also analyzed the parallel 11"X11" NIC1 field, centered 30" from the
cluster center and imaged in F110W and F160W, and an additional NIC2 field that
is immediately adjacent to the central field. This imaging results in a clean
detection of the red-giant branch and horizontal branch in the central NIC2
field, and the detection of these plus the main-sequence turnoff and the upper
main sequence in the NIC1 field. We have constructed an H versus J-H CMD for
the NIC1 field. We obtain a new distance estimate of 8.7 kpc, which places
Terzan 5 within less than 1 kpc of the galactic center. We have also determined
a central surface-density profile which results in a maximum likelihood
estimate of 7.9" +/- 0.6" for the cluster core radius. We discuss the
implications of these results for the dynamical state of Terzan 5.Comment: 17 pages, 9 figures, accepted for publication in ApJ, for May 20,
200
Black Widow Pulsars: the Price of Promiscuity
The incidence of evaporating 'black widow' pulsars (BWPs) among all
millisecond pulsars (MSPs) is far higher in globular clusters than in the
field. This implies a special formation mechanism for them in clusters. Cluster
MSPs in wide binaries with WD companions exchange them for turnoff-mass stars.
These new companions eventually overflow their Roche lobes because of
encounters and tides. The millisecond pulsars eject the overflowing gas from
the binary, giving mass loss on the binary evolution timescale. The systems are
only observable as BWPs at epochs where this evolution is slow, making the mass
loss transparent and the lifetime long. This explains why observed BWPs have
low-mass companions. We suggest that at least some field BWPs were ejected from
globular clusters or entered the field population when the cluster itself was
disrupted.Comment: 6 pages, 2 figures, MNRAS in pres
On the nature of X-Ray Flashes in the SWIFT era
X-Ray Flashes (XRFs) are soft gamma-ray bursts whose nature is not clear.
Their soft spectrum can be due to cosmological effects (high redshift), an
off-axis view of the jet or can be intrinsic to the source. We use SWIFT
observations to investigate different scenarios proposed to explain their
origin. We have made a systematic analysis of the afterglows of XRFs with known
redshift observed by SWIFT. We derive their redshift and luminosity
distributions, and compare their properties with a sample of normal GRBs
observed by the same instrument. The high distance hypothesis is ruled out by
the redshift distribution of our sample of XRFs, indicating that, at least for
our sample, the off-axis and sub-energetic hypotheses are preferred. Of course,
this does not exclude that some XRFs without known redshift could be at high
distance. However we find that taking into account the sensitivity of the BAT
instrument, XRFs cannot be detected by SWIFT beyond ~ 3. The luminosity
distribution of XRF afterglows is similar to the GRB one. This would rule out
most off-axis models, but for the homogeneous jet model. However this model
predicts a GRB rate uncomfortably near the observed rate of supernovae. This
implies that XRFs, at least those of our sample, are intrinsically soft.Comment: 4 pages, 2 color figures. Astronomy and Astrophysics Letters,
accepte
Late-epoch optical and near-infrared observations of the GRB000911 afterglow and its host galaxy
We present the results of an optical and near-infrared (NIR) monitoring
campaign of the counterpart of Gamma-Ray Burst (GRB) 000911, located at
redshift z=1.06, from 5 days to more than 13 months after explosion. Our
extensive dataset is a factor of 2 larger and spans a time interval about 4
times longer than the ones considered previously for this GRB afterglow; this
allows a more thorough analysis of its light curve and of the GRB host galaxy
properties. The afterglow light curves show a single power-law temporal
decline, modified at late times by light from a host galaxy with moderate
intrinsic extinction, and possibly by an emerging supernova (SN). The afterglow
evolution is interpreted within the classical "fireball" scenario as a weakly
collimated adiabatic shock propagating in the interstellar medium. The presence
of a SN light curve superimposed on the non-thermal afterglow emission is
investigated: while in the optical bands no significant contribution to the
total light is found from a SN, the NIR J-band data show an excess which is
consistent with a SN as bright as the known hypernova SN1998bw. If the SN
interpretation is true, this would be the farthest GRB-associated SN, as well
as the farthest core-collapse SN, discovered to date. However, other possible
explanations of this NIR excess are also investigated. Finally, we studied the
photometric properties of the host, and found that it is likely to be a
slightly reddened, subluminous, extreme starburst compact galaxy, with
luminosity about 0.1 L*, an age of about 0.5 Gyr and a specific Star Formation
Rate (SFR) of approximately 30 Msol yr-1 (L/L*)-1. This is the highest specific
SFR value for a GRB host inferred from optical/NIR data.Comment: 13 pages, 6 figures, 3 tables. Accepted for publication in A&A, main
journa
Gamma-ray bursts and terrestrial planetary atmospheres
We describe results of modeling the effects on Earth-like planets of
long-duration gamma-ray bursts (GRBs) within a few kiloparsecs. A primary
effect is generation of nitrogen oxide compounds which deplete ozone. Ozone
depletion leads to an increase in solar UVB radiation at the surface, enhancing
DNA damage, particularly in marine microorganisms such as phytoplankton. In
addition, we expect increased atmospheric opacity due to buildup of nitrogen
dioxide produced by the burst and enhanced precipitation of nitric acid. We
review here previous work on this subject and discuss recent developments,
including further discussion of our estimates of the rates of impacting GRBs
and the possible role of short-duration bursts.Comment: 12 pages including 5 figures (4 in color). Added discussion of GRB
rates and biological effects. Accepted for publication in New Journal of
Physics, for special issue "Focus on Gamma-Ray Bursts
- âŠ