Optical/Multiwavelength Observations of GRB Afterglows

I review gamma-ray burst optical/multiwavelength afterglow observations since 1997, when the first counterparts to GRBs were discovered. I discuss what we have learned from multiwavelength observations of GRB afterglows in relation to the `standard' fireball plus relativistic blast-wave models. To first order the `standard' model describes the afterglow observations well, but a wealth of information can be gathered from the deviations of GRB afterglow observations from this `standard' model. These deviations provide information on the nature of the progenitor and on the physics of GRB production. In particular I focus on the possible connection of GRBs to supernovae, on jet and circumstellar wind models, on the early-time afterglow, and on the emission from the reverse shock.Comment: 10 pages, 4 figures; invited review for the 5th Huntsville Gamma-Ray Burst Workshop, eds. M. Kippen, R. Mallozzi, and G. Fishma

Discovery of a tight correlation between pulse lag/luminosity and jet-break times: a connection between gamma-ray burst and afterglow properties

A correlation is presented between the pulse lag and the jet-break time for seven BATSE gamma-ray bursts with known redshifts. This is, to our best knowledge, the first known direct tight correlation between a property of the gamma-ray burst phase (the pulse lag) and the afterglow phase (the jet-break time). As pulse lag and luminosity have been found to be correlated this also represents a correlation between peak luminosity and jet-break time. Observed timescales (variability or spectral lags) as well as peak luminosity naturally have a strong dependence on the Lorentz factor of the outflow and so we propose that much of the variety among GRBs has a purely kinematic origin (the speed or direction of the outflow). We explore a model in which the variation among GRBs is due to a variation in jet-opening angles, and find that the narrowest jets have the fastest outflows. We also explore models in which the jets have similar morphology and size, and the variation among bursts is caused by variation in viewing angle and/or due to a velocity profile. The relations between luminosity, variability, spectral lag and jet-break time can be qualitatively understood from models in which the Lorentz factor decreases as a function of angle from the jet axis. One expects to see high luminosities, short pulse lags and high variability as well as an early jet-break time for bursts viewed on axis, while higher viewing inclinations will yield lower luminosities, longer pulse lags, smoother bursts and later jet-break times.Comment: 10 pages, 3 figures, accepted to ApJ (new version contains minor changes

Grossman’s Missing Health Threshold

We present a generalized solution to Grossman’s model of health capital (1972), relaxing the widely used assumption that individuals can adjust their health stock instantaneously to an “optimal” level without adjustment costs. The Grossman model then predicts the existence of a health threshold above which individuals do not demand medical care. Our generalized solution addresses a significant criticism: the model’s prediction that health and medical care are positively related is consistently rejected by the data. We suggest structural and reduced form equations to test our generalized solution and contrast the predictions of the model with the empirical literature.health, demand for health, health capital, medical care, labor

The optical counterpart to gamma-ray burst GRB970228 observed using the Hubble Space Telescope

Although more than 2,000 astronomical gamma-ray bursts (GRBs) have been detected, and numerous models proposed to explain their occurrence, they have remained enigmatic owing to the lack of an obvious counterpart at other wavelengths. The recent ground-based detection of a transient source in the vicinity of GRB 970228 may therefore have provided a breakthrough. The optical counterpart appears to be embedded in an extended source which, if a galaxy as has been suggested, would lend weight to those models that place GRBs at cosmological distances. Here we report the observations using the Hubble Space Telescope of the transient counterpart and extended source 26 and 39 days after the initial gamma-ray outburst. We find that the counterpart has faded since the initial detection (and continues to fade), but the extended source exhibits no significant change in brightness between the two dates of observations reported here. The size and apparent constancy between the two epochs of HST observations imply that it is extragalactic, but its faintness makes a definitive statement about its nature difficult. Nevertheless, the decay profile of the transient source is consistent with a popular impulsive-fireball model, which assumes a merger between two neutron stars in a distant galaxy.Comment: 11 pages + 2 figures. To appear in Nature (29 May 1997 issue

Overcoming attenuation bias in regressions using polygenic indices

Measurement error in polygenic indices (PGIs) attenuates the estimation of their effects in regression models. We analyze and compare two approaches addressing this attenuation bias: Obviously Related Instrumental Variables (ORIV) and the PGI Repository Correction (PGI-RC). Through simulations, we show that the PGI-RC performs slightly better than ORIV, unless the prediction sample is very small (N &lt; 1000) or when there is considerable assortative mating. Within families, ORIV is the best choice since the PGI-RC correction factor is generally not available. We verify the empirical validity of the simulations by predicting educational attainment and height in a sample of siblings from the UK Biobank. We show that applying ORIV between families increases the standardized effect of the PGI by 12% (height) and by 22% (educational attainment) compared to a meta-analysis-based PGI, yet estimates remain slightly below the PGI-RC estimates. Furthermore, within-family ORIV regression provides the tightest lower bound for the direct genetic effect, increasing the lower bound for the standardized direct genetic effect on educational attainment from 0.14 to 0.18 (+29%), and for height from 0.54 to 0.61 (+13%) compared to a meta-analysis-based PGI.</p

Observations of GRB 970228 and GRB 970508, and the Neutron-Star Merger Model

We present the discovery observations for the optical counterpart of the $\gamma$-ray burster GRB 970508 and discuss its light curve in the context of the fireball model. We analyze the HST data for this object, and conclude that any underlying galaxy must be fainter than $R$ = 25.5. We also present a detailed analysis of the HST images of GRB 970228 claimed to show a proper motion of the optical counterpart and conclude that, within the uncertainties, there is no proper motion. We examine several aspects of the neutron-star merger model for $\gamma$-ray bursts. In particular, we use this model to predict the redshift distribution of $\gamma$-ray bursters, and adopting a recent determination of the cosmic star-formation history, we show that the predicted distribution of $\log N - \log P$ relation is consistent with that observed for GRBs.Comment: (6 figures). Accepted for publication in Astrophysical Journa

The Fading Optical Counterpart of GRB~970228, Six Months and One Year Later

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made with the Hubble Space Telescope STIS CCD approximately six months after outburst and with the HST/NICMOS and Keck/NIRC approximately one year after outburst. The unresolved counterpart is detected by STIS at V=28.0 +/- 0.25, consistent with a continued power-law decline with exponent -1.14 +/- 0.05. The counterpart is located within, but near the edge of, a faint extended source with diameter ~0."8 and integrated magnitude V=25.8 +/- 0.25. A reanalysis of HST and NTT observations performed shortly after the burst shows no evidence of proper motion of the point source or fading of the extended emission. Only the extended source is visible in the NICMOS images with a magnitude of H=23.3 +/- 0.1. The Keck observations find K = 22.8 +/- 0.3. Several distinct and independent means of deriving the foreground extinction in the direction of GRB 970228 all agree with A_V = 0.75 +/- 0.2. After adjusting for Galactic extinction, we find that the size of the observed extended emission is consistent with that of galaxies of comparable magnitude found in the Hubble Deep Field (HDF) and other deep HST images. Only 2% of the sky is covered by galaxies of similar or greater surface brightness; therefore the extended source is almost certainly the host galaxy. Additionally, we find that the extinction-corrected V - H and V - K colors of the host are as blue as any galaxy of comparable or brighter magnitude in the HDF. Taken in concert with recent observations of GRB 970508, GRB 971214, and GRB 980703 our work suggests that all four GRBs with spectroscopic identification or deep multicolor broad-band imaging of the host lie in rapidly star-forming galaxies.Comment: 24 pages, Latex, 4 PostScript figures, to appear in the May 10 issue of The Astrophysical Journal (Note: displayed abstract is abridged

The metamorphosis of SN1998bw

We present and discuss the photometric and spectroscopic evolution of the peculiar SN1998bw, associated with GRB980425, through an analysis of optical and near IR data collected at ESO-La Silla. The spectroscopic data, spanning the period from day -9 to day +376 (relative to B maximum), have shown that this SN was unprecedented, although somewhat similar to SN1997ef. Maximum expansion velocities as high as 3x10^4 km/s to some extent mask its resemblance to other Type Ic SNe. At intermediate phases, between photospheric and fully nebular, the expansion velocities (~10^4 km/s) remained exceptionally high compared to those of other recorded core-collapse SNe at a similar phase. The mild linear polarization detected at early epochs suggests the presence of asymmetry in the emitting material. The degree of asymmetry, however, cannot be decoded from these measurements alone. The HeI 1.083 mu and 2.058 mu lines are identified and He is suggested to lie in an outer region of the envelope. The temporal behavior of the fluxes and profiles of emission lines of MgI]4571A, [OI]6300,6364A and a feature ascribed to Fe are traced to stimulate future modeling work.Comment: 32 pages, 19 figures; ps file including figures at http://www.eso.org/~fpatat/sn98b