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Tanvir N.R.,University of Leicester | Levan A.J.,University of Warwick | Fruchter A.S.,US Space Telescope Science Institute | Fynbo J.P.U.,Copenhagen University | And 18 more authors.
Astrophysical Journal

We present late-time Hubble Space Telescope (HST) imaging of the fields of six Swift gamma-ray bursts (GRBs) lying at 5.0 ≲ z ≲ 9.5. Our data include very deep observations of the field of the most distant spectroscopically confirmed burst, GRB090423, at z = 8.2. Using the precise positions afforded by their afterglows, we can place stringent limits on the luminosities of their host galaxies. In one case, that of GRB060522 at z = 5.11, there is a marginal excess of flux close to the GRB position which may be a detection of a host at a magnitude J AB 28.5. None of the others are significantly detected, meaning that all the hosts lie below L* at their respective redshifts, with star formation rates (SFRs) ≲ 4 M yr -1 in all cases. Indeed, stacking the five fields with WFC3-IR data, we conclude a mean SFR <0.17 M yr-1 per galaxy. These results support the proposition that the bulk of star formation, and hence integrated UV luminosity, at high redshifts arises in galaxies below the detection limits of deep-field observations. Making the reasonable assumption that GRB rate is proportional to UV luminosity at early times allows us to compare our limits with expectations based on galaxy luminosity functions (LFs) derived from the Hubble Ultra-Deep Field and other deep fields. We infer that an LF, which is evolving rapidly toward steeper faint-end slope (α) and decreasing characteristic luminosity (L*), as suggested by some other studies, is consistent with our observations, whereas a non-evolving LF shape is ruled out at ≳ 90% confidence. Although it is not yet possible to make stronger statements, in the future, with larger samples and a fuller understanding of the conditions required for GRB production, studies like this hold great potential for probing the nature of star formation, the shape of the galaxy LF, and the supply of ionizing photons in the early universe. © 2012. The American Astronomical Society. All rights reserved.. Source

Kuin N.P.M.,University College London | Landsman W.,US Space Telescope Science Institute | Breeveld A.A.,University College London | Page M.J.,University College London | And 19 more authors.
Monthly Notices of the Royal Astronomical Society

We present the calibration of the Swift Ultraviolet and Optical Telescope (UVOT ) grisms, of which there are two, providing low-resolution field spectroscopy in the ultraviolet and optical bands, respectively. The UV grism covers the range ?1700-5000 Å with a spectral resolution (λ/Δλ) of 75 at λ2600 Å for source magnitudes of u=10-16 mag, while the visible grism covers the range λ2850-6600 Å with a spectral resolution of 100 at λ4000 Å for source magnitudes of b=12-17 mag. This calibration extends over all detector positions, for all modes used during operations. The wavelength accuracy (1σ) is 9 Å in the UV grism clocked mode, 17 Å in the UV grism nominal mode and 22 Å in the visible grism. The range below λ2740 Å in the UV grism and λ5200 Å in the visible grism never suffers from overlapping by higher spectral orders. The flux calibration of the grisms includes a correction we developed for coincidence loss in the detector. The error in the coincidence loss correction is less than 20 per cent. The position of the spectrum on the detector only affects the effective area (sensitivity) by a fewper cent in the nominal modes, but varies substantially in the clocked modes. The error in the effective area is from 9 per cent in the UV grism clocked mode to 15 per cent in the visible grism clocked mode. © 2015 The Authors. Source

Oates S.R.,University College London | Oates S.R.,Institute Astrofsica Of Andalucia Iaa Csic | Racusin J.L.,NASA | De Pasquale M.,University College London | And 10 more authors.
Monthly Notices of the Royal Astronomical Society

In this paper, we further investigate the relationship, reported by Oates et al., between the optical/UV afterglow luminosity (measured at restframe 200 s) and average afterglow decay rate (measured from restframe 200 s onwards) of long duration gamma-ray bursts (GRBs). We extend the analysis by examining the X-ray light curves, finding a consistent correlation. We therefore explore how the parameters of these correlations relate to the prompt emission phase and, using a Monte Carlo simulation, explore whether these correlations are consistent with predictions of the standard afterglow model. We find significant correlations between: log LO,200 s and log LX,200 s; αO,>200 s and αX,>200 s, consistent with simulations. The model also predicts relationships between log Eiso and log L200 s; however, while we find such relationships in the observed sample, the slope of the linear regression is shallower than that simulated and inconsistent at>3σ. Simulations also do not agree with correlations observed between log L200 s and α>200 s, or log Eiso and α>200 s. Overall, these observed correlations are consistent with a common underlying physical mechanism producing GRBs and their afterglows regardless of their detailed temporal behaviour. However, a basic afterglow model has difficulty explaining all the observed correlations. This leads us to briefly discuss alternative more complex models. © 2015 The Authors. Source

Perley D.A.,California Institute of Technology | Perley D.A.,Copenhagen University | Kruhler T.,Copenhagen University | Kruhler T.,European Southern Observatory | And 24 more authors.
Astrophysical Journal

We introduce the Swift Gamma-Ray Burst Host Galaxy Legacy Survey ("SHOALS"), a multi-observatory high-redshift galaxy survey targeting the largest unbiased sample of long-duration gamma-ray burst (GRB) hosts yet assembled (119 in total). We describe the motivations of the survey and the development of our selection criteria, including an assessment of the impact of various observability metrics on the success rate of afterglow-based redshift measurement. We briefly outline our host galaxy observational program, consisting of deep Spitzer/IRAC imaging of every field supplemented by similarly deep, multicolor optical/near-IR photometry, plus spectroscopy of events without preexisting redshifts. Our optimized selection cuts combined with host galaxy follow-up have so far enabled redshift measurements for 110 targets (92%) and placed upper limits on all but one of the remainder. About 20% of GRBs in the sample are heavily dust obscured, and at most 2% originate from Using this sample, we estimate the redshift-dependent GRB rate density, showing it to peak at and fall by at least an order of magnitude toward low (z = 0) redshift, while declining more gradually toward high () redshift. This behavior is consistent with a progenitor whose formation efficiency varies modestly over cosmic history. Our survey will permit the most detailed examination to date of the connection between the GRB host population and general star-forming galaxies, directly measure evolution in the host population over cosmic time and discern its causes, and provide new constraints on the fraction of cosmic star formation occurring in undetectable galaxies at all redshifts. © 2016. The American Astronomical Society. All rights reserved.. Source

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