290 research outputs found
GRB optical afterglow and redshift selection effects: The learning curve effect at work
We show how the observed gamma ray burst (GRB) optical afterglow (OA) and
redshift distributions are changing in time from selection effects. For a
subset of {\it Swift} triggered long duration bursts, we show that the mean
time taken to acquire spectroscopic redshifts for a GRB OA has evolved to
shorter times. We identify a strong correlation between the mean time taken to
acquire a spectroscopic redshift and the measured redshift. This correlation
reveals that shorter response times favour smaller redshift bursts. This is
compelling evidence for a selection effect that biases longer response times
with relatively brighter high redshift bursts. Conversely, for shorter response
times, optically fainter bursts that are relatively closer are bright enough
for spectroscopic redshifts to be acquired. This selection effect could explain
why the average redshift, measured in 2005, has evolved to
, by mid 2008. Understanding these selection effects provides an
important tool for separating the contributions of intrinsically faint bursts,
those obscured by host galaxy dust and bursts not seen in the optical because
their OAs are observed at late times. The study highlights the importance of
rapid response telescopes capable of spectroscopy, and identifies a new
redshift selection effect that has not been considered previously, namely the
response time to measure the redshift.Comment: 5 pages, 4 figures, MNRAS Letter (accepted
GRBs as Probes of Massive Stars Near and Far
Long-duration gamma-ray bursts are the manifestations of massive stellar
death. Due to the immense energy release they are detectable from most of the
observable universe. In this way they allow us to study the deaths of single
(or binary) massive stars possibly throughout the full timespan massive stars
have existed in the Universe. GRBs provide a means to infer information about
the environments and typical galaxies in which massive stars are formed. Two
main obstacles remain to be crossed before the full potential of GRBs as probes
of massive stars can be harvested: i) we need to build more complete and well
understood samples in order not to be fooled by biases, and ii) we need to
understand to which extent GRBs may be intrinsically biased in the sense that
they are only formed by a limited subset of massive stars defined by most
likely a restricted metallicity interval. We describe the status of an ongoing
effort to build a more complete sample of long-duration GRBs with measured
redshifts. Already now we can conclude that the environments of GRB progenitors
are very diverse with metallicities ranging from solar to a hundredth solar and
extinction ranging from none to A_V>5 mag. We have also identified a sightline
with significant escape of Lyman continuum photons and another with a clear
2175AA extinction bump.Comment: Invited review - in "Massive Stars as Cosmic Engines", IAU Symp. 250
(Kauai), ed. F. Bresolin, P. A. Crowther, and J. Puls (Cambridge University
Press), p. 443-456. Typos and refs correcte
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
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
On the nature of the short duration GRB 050906
The definitive version is available at www.blackwell-synergy.com. Copyright Blackwell Publishing DOI : 10.1111/j.1365-2966.2007.11953.xPeer reviewe
Where are the missing gamma ray burst redshifts?
In the redshift range z = 0-1, the gamma ray burst (GRB) redshift
distribution should increase rapidly because of increasing differential volume
sizes and strong evolution in the star formation rate. This feature is not
observed in the Swift redshift distribution and to account for this
discrepancy, a dominant bias, independent of the Swift sensitivity, is
required. Furthermore, despite rapid localization, about 40-50% of Swift and
pre-Swift GRBs do not have a measured redshift. We employ a heuristic technique
to extract this redshift bias using 66 GRBs localized by Swift with redshifts
determined from absorption or emission spectroscopy. For the Swift and
HETE+BeppoSAX redshift distributions, the best model fit to the bias in z < 1
implies that if GRB rate evolution follows the SFR, the bias cancels this rate
increase. We find that the same bias is affecting both Swift and HETE+BeppoSAX
measurements similarly in z < 1. Using a bias model constrained at a 98% KS
probability, we find that 72% of GRBs in z < 2 will not have measurable
redshifts and about 55% in z > 2. To achieve this high KS probability requires
increasing the GRB rate density in small z compared to the high-z rate. This
provides further evidence for a low-luminosity population of GRBs that are
observed in only a small volume because of their faintness.Comment: 5 pages, submitted to MNRA
The dark GRB080207 in an extremely red host and the implications for GRBs in highly obscured environments
[Abridged] We present comprehensive X-ray, optical, near- and mid-infrared,
and sub-mm observations of GRB 080207 and its host galaxy. The afterglow was
undetected in the optical and near-IR, implying an optical to X-ray index <0.3,
identifying GRB 080207 as a dark burst. Swift X-ray observations show extreme
absorption in the host, which is confirmed by the unusually large optical
extinction found by modelling the X-ray to nIR afterglow spectral energy
distribution. Our Chandra observations obtained 8 days post-burst allow us to
place the afterglow on the sky to sub-arcsec accuracy, enabling us to pinpoint
an extremely red galaxy (ERO). Follow-up host observations with HST, Spitzer,
Gemini, Keck and the James Clerk Maxwell Telescope (JCMT) provide a photometric
redshift solution of z ~1.74 (+0.05,-0.06) (1 sigma), 1.56 < z < 2.08 at 2
sigma) for the ERO host, and suggest that it is a massive and morphologically
disturbed ultra-luminous infrared galaxy (ULIRG) system, with L_FIR ~ 2.4 x
10^12 L_solar. These results add to the growing evidence that GRBs originating
in very red hosts always show some evidence of dust extinction in their
afterglows (though the converse is not true -- some extinguished afterglows are
found in blue hosts). This indicates that a poorly constrained fraction of GRBs
occur in very dusty environments. By comparing the inferred stellar masses, and
estimates of the gas phase metallicity in both GRB hosts and sub-mm galaxies we
suggest that many GRB hosts, even at z>2 are at lower metallicity than the
sub-mm galaxy population, offering a likely explanation for the dearth of
sub-mm detected GRB hosts. However, we also show that the dark GRB hosts are
systematically more massive than those hosting optically bright events, perhaps
implying that previous host samples are severely biased by the exclusion of
dark events.Comment: 13 pages, 6 figures, accepted for publication in MNRA
Estimating Redshifts for Long Gamma-Ray Bursts
We are constructing a program to estimate the redshifts for GRBs from the
original Swift light curves and spectra, aiming to get redshifts for the Swift
bursts \textit{without} spectroscopic or photometric redshifts. We derive the
luminosity indicators from the light curves and spectra of each burst,
including the lag time between low and high photon energy light curves, the
variability of the light curve, the peak energy of the spectrum, the number of
peaks in the light curve, and the minimum rise time of the peaks. These
luminosity indicators can each be related directly to the luminosity, and we
combine their independent luminosities into one weighted average. Then with our
combined luminosity value, the observed burst peak brightness, and the
concordance redshift-distance relation, we can derive the redshift for each
burst. In this paper, we test the accuracy of our method on 107 bursts with
known spectroscopic redshift. The reduced of our best redshifts
() compared with known spectroscopic redshifts () is 0.86,
and the average value of is 0.01, with this
indicating that our error bars are good and our estimates are not biased. The
RMS scatter of is 0.26. For Swift bursts measured
over a relatively narrow energy band, the uncertainty in determining the peak
energy is one of the main restrictions on our accuracy. Although the accuracy
of our values are not as good as that of spectroscopic redshifts, it
is very useful for demographic studies, as our sample is nearly complete and
the redshifts do not have the severe selection effects associated with optical
spectroscopy.Comment: The Astrophysical Journal accepte
Low-Luminosity Gamma-Ray Bursts as a Distinct GRB Population:A Firmer Case from Multiple Criteria Constraints
The intriguing observations of Swift/BAT X-ray flash XRF 060218 and the
BATSE-BeppoSAX gamma-ray burst GRB 980425, both with much lower luminosity and
redshift compared to other observed bursts, naturally lead to the question of
how these low-luminosity (LL) bursts are related to high-luminosity (HL)
bursts. Incorporating the constraints from both the flux-limited samples
observed with CGRO/BATSE and Swift/BAT and the redshift-known GRB sample, we
investigate the luminosity function for both LL- and HL-GRBs through
simulations. Our multiple criteria, including the log N - log P distributions
from the flux-limited GRB sample, the redshift and luminosity distributions of
the redshift-known sample, and the detection ratio of HL- and LL- GRBs with
Swift/BAT, provide a set of stringent constraints to the luminosity function.
Assuming that the GRB rate follows the star formation rate, our simulations
show that a simple power law or a broken power law model of luminosity function
fail to reproduce the observations, and a new component is required. This
component can be modeled with a broken power, which is characterized by a sharp
increase of the burst number at around L < 10^47 erg s^-1}. The lack of
detection of moderate-luminosity GRBs at redshift ~0.3 indicates that this
feature is not due to observational biases. The inferred local rate, rho_0, of
LL-GRBs from our model is ~ 200 Gpc^-3 yr^-1 at ~ 10^47 erg s^-1, much larger
than that of HL-GRBs. These results imply that LL-GRBs could be a separate GRB
population from HL-GRBs. The recent discovery of a local X-ray transient
080109/SN 2008D would strengthen our conclusion, if the observed non-thermal
emission has a similar origin as the prompt emission of most GRBs and XRFs.Comment: 22 pages, 9 figures, 3 tables; MNRAS, in press; Updated analysis and
figure
- …