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

Long Gamma-Ray Burst Host Galaxies and their Environments

In this book-chapter we first briefly discuss some basic observational issues related to what a GRB host galaxy is (whether they are operationally well defined as a class) and sample completeness. We then describe some of the early studies of GRB hosts starting with statistical studies of upper limits done prior to the first detections, the first host detection after the BeppoSAX breakthrough and leading up to the current Swift era. Finally, we discuss the status of efforts to construct a more complete sample of GRBs based on Swift and end with an outlook. We only consider the host galaxies of long-duration GRBs.Comment: 31 pages, 14 figures; Chapter 13 in "Gamma-Ray Bursts", eds. C. Kouveliotou, R. A. M. J. Wijers, S. E. Woosley, Cambridge University Press, 201

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 $\chi^2$ of our best redshifts ($z_{best}$) compared with known spectroscopic redshifts ($z_{spec}$) is 0.86, and the average value of $log_{10}(z_{best}/z_{spec})$ is 0.01, with this indicating that our error bars are good and our estimates are not biased. The RMS scatter of $log_{10}(z_{best}/z_{spec})$ 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 $z_{best}$ 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

The Optically Unbiased GRB Host (TOUGH) survey. IV. Lyman-alpha emitters

We report the results of a spectroscopic search for Lyman-alpha emission from gamma-ray burst host galaxies. Based on the well-defined TOUGH sample of 69 X-ray selected Swift GRBs, we have targeted the hosts of a subsample of 20 GRBs known from afterglow spectroscopy to be in the redshift range 1.8-4.5. We detect Lya emission from 7 out of the 20 hosts, with the typical limiting 3sigma line flux being 8E-18 erg/cm2/s, corresponding to a Lya luminosity of 6E41 erg/s at z=3. The Lya luminosities for the 7 hosts in which we detect Lya emission are in the range (0.6-2.3)E42 erg/s corresponding to star-formation rates of 0.6-2.1 Msun/yr (not corrected for extinction). The rest-frame Lya equivalent widths (EWs) for the 7 hosts are in the range 9-40A. For 6 of the 13 hosts for which Lya is not detected we place fairly strong 3sigma upper limits on the EW (<20A), while for others the EW is either unconstrained or has a less constraining upper limit. We find that the distribution of Lya EWs is inconsistent with being drawn from the Lya EW distribution of bright Lyman break galaxies at the 98.3% level, in the sense that the TOUGH hosts on average have larger EWs than bright LBGs. We can exclude an early indication, based on a smaller, heterogeneous sample of pre-Swift GRB hosts, that all GRB hosts are Lya emitters. We find that the TOUGH hosts on average have lower EWs than the pre-Swift GRB hosts, but the two samples are only inconsistent at the 92% level. The velocity centroid of the Lya line is redshifted by 200-700 km/s with respect to the systemic velocity, similar to what is seen for LBGs, possibly indicating star-formation driven outflows from the host galaxies. There seems to be a trend between the Lya EW and the optical to X-ray spectral index of the afterglow (beta_OX), hinting that dust plays a role in the observed strength and even presence of Lya emission. [ABRIDGED]Comment: ApJ accepted (v2: minor changes in the Subject headings and reference list

Swift Identification of Dark Gamma-Ray Bursts

We present an optical flux vs. X-ray flux diagram for all known gamma-ray bursts (GRBs) for which an X-ray afterglow has been detected. We propose an operational definition of dark bursts as those bursts that are optically subluminous with respect to the fireball model, i.e., which have an optical-to-X-ray spectral index beta_OX < 0.5. Out of a sample of 52 GRBs we identify 5 dark bursts. The definition and diagram serve as a simple and quick diagnostic tool for identifying dark GRBs based on limited information, particularly useful for early and objective identification of dark GRBs observed with the Swift satellite.Comment: 4 pages, 1 figure. ApJ Letters, in pres

The galaxies in the field of the nearby GRB980425/SN1998bw

We present spectroscopic observations of ESO 184-G82, the host galaxy of GRB980425/SN1998bw, and six galaxies in its field. A host redshift of z=0.0087+/-0.0006 is derived, consistent with that measured by Tinney et al. (1998). Redshifts are obtained for the six surrounding galaxies observed. Three of these galaxies lie within 11 Mpc of each other, confirming the suggestion that some of these galaxies form a group. However, all of the field galaxies observed lie at significantly greater distances than ESO 184-G82 and are therefore not associated with it. The host galaxy of GRB980425/SN1998bw thus appears to be an isolated dwarf galaxy and interactions with other galaxies do not seem to be responsible for its star formation.Comment: 5 pages, 2 figures, accepted for publication in A&

The distribution of equivalent widths in long GRB afterglow spectra

The extreme brightness of gamma-ray burst (GRB) afterglows and their simple spectral shape make them ideal beacons to study the interstellar medium of their host galaxies through absorption line spectroscopy. Using 69 low-resolution GRB afterglow spectra, we conduct a study of the rest-frame equivalent width (EW) distribution of features with an average rest-frame EW larger than 0.5 A. To compare an individual GRB with the sample, we develop EW diagrams as a graphical tool, and we give a catalogue with diagrams for the 69 spectra. We introduce a line strength parameter (LSP) that allows us to quantify the strength of the absorption features as compared to the sample by a single number. Using the distributions of EWs of single-species features, we derive the distribution of column densities by a curve of growth (CoG) fit. We find correlations between the LSP and the extinction of the GRB, the UV brightness of the host galaxies and the neutral hydrogen column density. However, we see no significant evolution of the LSP with the redshift. There is a weak correlation between the ionisation of the absorbers and the energy of the GRB, indicating that, either the GRB event is responsible for part of the ionisation, or that galaxies with high-ionisation media produce more energetic GRBs. Spectral features in GRB spectra are, on average, 2.5 times stronger than those seen in QSO intervening damped Lyman-alpha (DLA) systems and slightly more ionised. In particular we find larger excess in the EW of CIV1549 relative to QSO DLAs, which could be related to an excess of Wolf-Rayet stars in the environments of GRBs. From the CoG fitting we obtain an average number of components in the absorption features of GRBs of 6.00(-1.25,+1.00). The most extreme ionisation ratios in our sample are found for GRBs with low neutral hydrogen column density, which could be related to ionisation by the GRB emission.Comment: 37 pages, 31 figures, 15 tables. Accepted for publication in Astonomy and Astrophysic

Gamma-ray burst host galaxies and the link to star-formation

We briefly review the current status of the study of long-duration gamma-ray burst (GRB) host galaxies. GRB host galaxies are mainly interesting to study for two reasons: 1) they may help us understand where and when massive stars were formed throughout cosmic history, and 2) the properties of host galaxies and the localisation within the hosts where GRBs are formed may give essential clues to the precise nature of the progenitors. The main current problem is to understand to what degree GRBs are biased tracers of star formation. If GRBs are only formed by low-metallicity stars, then their host galaxies will not give a representative view of where stars are formed in the Universe (at least not a low redshifts). On the other hand, if there is no dependency on metallicity then the nature of the host galaxies leads to the perhaps surprising conclusion that most stars are formed in dwarf galaxies. In order to resolve this issue and to fully exploit the potential of GRBs as probes of star-forming galaxies throughout the observable universe it is mandatory that a complete sample of bursts with redshifts and host galaxy detections is built.Comment: 9 pages, 3 figures. To appear in the proceedings of the Eleventh Marcel Grossmann Meeting on General Relativity, eds. H. Kleinert, R. T. Jantzen & R. Ruffini, World Scientific, Singapore, 200

Testing the E_p,i - L_p,iso - T_0.45 correlation on a BeppoSAX and Swift sample of gamma-ray bursts

Using a sample of 14 BeppoSAX and 74 Swift GRBs with measured redshift we tested the correlation between the intrinsic peak energy of the time-integrated spectrum, E_p,i, the isotropic-equivalent peak luminosity, L_p,iso, and the duration of the most intense parts of the GRB computed as T_0.45 ("Firmani correlation"). For 41 out of 88 GRBs we could estimate all of the three required properties. Apart from 980425, which appears to be a definite outlier and notoriously peculiar in many respects, we used 40 GRBs to fit the correlation with the maximum likelihood method discussed by D'Agostini, suitable to account for the extrinsic scatter in addition to the intrinsic uncertainties affecting every single GRB. We confirm the correlation. However, unlike the results by Firmani et al., we found that the correlation does have a logarithmic scatter comparable with that of the E_p,i-E_iso ("Amati") correlation. We also find that the slope of the product L_p,iso T_0.45 is equal to ~0.5, which is consistent with the hypothesis that the E_p,i-L_p,iso-T_0.45 correlation is equivalent to the E_p,i-E_iso correlation (slope ~0.5). We conclude that, based on presently available data, there is no clear evidence that the E_p,i-L_p,iso-T_0.45 correlation is different (both in terms of slope and dispersion) from the E_p,i-E_iso correlation.Comment: 9 pages, 4 figures, revised version submitted to MNRA