Michael I. Andersen

A very energetic supernova associated with the |[gamma]|-ray burst of 29 March 2003

Over the past five years evidence has mounted that long-duration (>2 s) γ-ray bursts (GRBs)—the most luminous of all astronomical explosions—signal the collapse of massive stars in our Universe. This evidence was originally based on the probable association of one unusual GRB with a supernova, but now includes the association of GRBs with regions of massive star formation in distant galaxies, the appearance of supernova-like ‘bumps’ in the optical afterglow light curves of several bursts and lines of freshly synthesized elements in the spectra of a few X-ray afterglows. These observations support, but do not yet conclusively demonstrate, the idea that long-duration GRBs are associated with the deaths of massive stars, presumably arising from core collapse. Here we report evidence that a very energetic supernova (a hypernova) was temporally and spatially coincident with a GRB at redshift z = 0.1685. The timing of the supernova indicates that it exploded within a few days of the GRB, strongly suggesting that core-collapse events can give rise to GRBs, thereby favouring the ‘collapsar’ model.

On the Lyalpha emission from gamma-ray burst host galaxies: Evidence for low metallicities

We report on the results of a search for Ly emission from the host galaxy of the z= 2:140 GRB 011211 and other galaxies in its surrounding field. We detect Ly emission from the host as well as from six other galaxies in the field. The restframe equivalent width of the Ly line from the GRB 011211 host is about 21 A. This is the fifth detection of Ly emission out of five possible detections from GRB host galaxies, strongly indicating that GRB hosts, at least at high redshifts, are Ly emitters. This is intriguing as only 25% of the Lyman-Break selected galaxies at similar redshifts have Ly emission lines with restframe equivalent width larger than 20 A. Possible explanations are i) a preference for GRB progenitors to be metal-poor as expected in the collapsar model, ii) an optical afterglow selection bias against dusty hosts, and iii) a higher fraction of Ly emitters at the faint end of the luminosity function for high- z galaxies. Of these, the current evidence seems to favour i).

Very high column density and small reddening toward GRB 020124 at z = 3.20

We present optical and near-infrared observations of the dim afterglow of GRB 020124, obtained between 2 and 68 hr after the gamma-ray burst. The burst occurred in a very faint (R 29.5) damped Lyα absorber (DLA) at a redshift of z = 3.198 ± 0.004. The derived column density of neutral hydrogen is log(N) = 21.7 ± 0.2, and the rest-frame reddening is constrained to be E(B-V) < 0.065, i.e., AV < 0.20 for standard extinction laws with RV ≈ 3. The resulting dust-to-gas ratio is less than 11% of that found in the Milky Way but consistent with the SMC and high-redshift QSO DLAs, indicating a low metallicity and/or a low dust-to-metal ratio in the burst environment. A gray extinction law (large RV), produced through preferential destruction of small dust grains by the gamma-ray burst, could increase the derived AV and dust-to-gas ratio. The dimness of the afterglow is, however, fully accounted for by the high redshift: if GRB 020124 had been at z = 1, it would have been approximately 1.8 mag brighter—in the range of typical bright afterglows.

The Bright Gamma-Ray Burst of 2000 February 10: A Case Study of an Optically Dark Gamma-Ray Burst

The gamma-ray burst GRB 000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet, but it did not have a detected optical afterglow, despite prompt and deep searches down to Rlim � 23:5. It is therefore one of the events recently classified as dark GRBs, whose origin is still unclear. Chandra observations allowed us to localize the X-ray afterglow of GRB 000210 to within � 1 00 , and a radio transient was detected with the Very Large Array. The precise X-ray and radio positions allowed us to identify the likely host galaxy of this burst and to measure its redshift, z ¼ 0:846. The probability that this galaxy is a field object is � 1:6 � 10 � 2 . The X-ray spectrum of the afterglow shows significant absorption in excess of the Galactic one corresponding, at the redshift of the galaxy, to NH ¼ð 5 � 1 Þ� 10 21 cm � 2 . The amount of dust needed to absorb the optical flux of this object is consistent with the above H i column density, given a dust-to-gas ratio similar to that of our Galaxy. We do not find evidence for a partially ionized absorber expected if the absorption takes place in a giant molecular cloud. We therefore conclude that either the gas is local to the GRB but is condensed in small-scale high-density (ne10 9 cm � 3 ) clouds, or the GRB is located in a dusty, gas-rich region of the Galaxy. Finally, we examine the hypothesis that GRB 000210 lies at ze5 (and

GRB 050509B: Constraints on Short Gamma-Ray Burst Models*

We have obtained deep optical images with the Very Large Telescope at ESO of the first well-localized short-duration gamma-ray burst, GRB 050509b. We observed in the V and R bands at epochs starting at {approx}2 days after the GRB trigger and lasting up to three weeks. We detect no variable objects inside the small Swift/XRT X-ray error circle down to 5{sigma} limiting magnitudes of V = 26.5 and R = 25.2. The X-ray error circle includes a giant elliptical galaxy at z = 0.225, which has been proposed as the likely host of this GRB. Our limits indicate that if the GRB originated at z = 0.225, any supernova-like event accompanying the GRB would have to be over 100 times fainter than normal Type Ia SNe or Type Ic hypernovae, 5 times fainter than the faintest known Ia or Ic SNe, and fainter than the faintest known Type II SNe. Moreover, we use the optical limits to constrain the energetics of the GRB outflow, and conclude that there was very little radioactive material produced during the GRB explosion. These limits strongly constrain progenitor models for this short GRB.We have obtained deep optical images with the Very Large Telescope at ESO of the first well-localized short-duration gamma-ray burst, GRB 050509B. From V and R imaging, initiated ~2 days after the GRB trigger and lasting up to three weeks, we detect no variable object inside the small Swift XRT X-ray error circle down to 2 σ limits of V = 26.5 and R = 25.1. The X-ray error circle includes a giant elliptical galaxy at z = 0.225, which has been proposed as the likely host of this GRB. Our limits indicate that if the GRB originated at z = 0.225, any supernova-like event accompanying the GRB would have to be over 100 times fainter than normal Type Ia SNe or Type Ic hypernovae, 5 times fainter than the faintest known Ia or Ic SNe, and fainter than the faintest known Type II SNe. Moreover, we use the optical limits to constrain the energetics of the GRB outflow. Simple models indicate that unless the intrinsic energy in the outflow from GRB 050509B was 1051 ergs, there was very little radioactive material with efficient decay timescales for generating a large luminosity. These limits strongly constrain progenitor models for this short GRB.

Evidence for diverse optical emission from gamma-ray burst sources

Optical Transients from gamma-ray burst sources, in addition to offering a distance determination, convey important information about the physics of the emission mechanism, and perhaps also about the underlying energy source. As the gamma-ray phenomenon is extremely diverse, with timescales spanning several orders of magnitude, some diversity in optical counterpart signatures appears plausible. We have studied the optical transient that accompanied the gamma-ray burst of 1997 May 8, GRB 970508. Observations conducted at the 2.5 m Nordic Optical Telescope (NOT) and the 2.2 m telescope at the German-Spanish Calar Alto observatory (CAHA) cover the time interval starting 3 hr 5 minutes to 96 days after the high-energy event. This brackets all other published observations, including radio. When analyzed in conjunction with optical data from other observatories, evidence emerges for a composite light curve. The first interval, from 3 to 8 hr after the event, was characterized by a constant or slowly declining brightness. At a later moment, the brightness started increasing rapidly, and reached a maximum approximately 40 hr after the GRB. From that moment, the GRB brightness decayed approximately as a power law of index -1.21. The last observation, after 96 days, mR = 24.28 ± 0.10, is brighter than the extrapolated power law, and hints that a constant component, mR = 25.50 ± 0.40, is present. The optical transient is unresolved (FWHM 083) at the faintest magnitude level. The brightness of the optical transient, its duration, and the general shape of the light curve set this source apart from the single other optical transient known, that of the 1997 February 28 event.

Strömgren Photometry of Globular Clusters: The Distance and Age of M13, Evidence for Two Populations of Horizontal-Branch Stars

We present deep CCD photometry of the globular cluster M13 (NGC 6205) in the Stromgren uvbyβ system and determine a foreground reddening of E(b-y)=0.015±0.01 mag. From a fit to the main sequence of metal-poor subdwarfs with Hipparcos parallaxes, we derive (m-M) -->0=14.38±0.10, which implies an age near 12 Gyr assuming [Fe/H] = -1.61 and [α/Fe] = 0.3. The distance-independent [(b-y) -->0, c -->0] diagram indicates that M13 and metal-poor field subdwarfs of similar metallicity must be coeval to within ±1 Gyr. In addition, we find that, at any given (b-y) -->0 color, there is a large spread in the c -->0 index for M13 red giant branch (RGB) stars. We suspect that this scatter, which extends at least as faint as the base of the RGB, is most likely due to star-to-star variations in the atmospheric abundances of the CNO elements. We also note the existence of what appears to be two separate stellar populations on the horizontal branch of M13. Among other possibilities, it could arise as the result of differences in the extent to which deep mixing occurs in the precursor red giants.

On the afterglow of the x-ray flash of 2003 July 23: photometric evidence for an off-axis gamma-ray burst with an associated supernova?

We present optical and near-infrared follow-up observations of the X-ray flash (XRF) of 2003 July 23. Our observations in the R band cover the temporal range from 4.2 hr to 64 days after the high-energy event. We also present the results of multicolor imaging extending to the K band on three epochs. The light curve of the R-band afterglow the first week after the burst is similar to the light curve for long-duration gamma-ray bursts (GRBs), i.e., a broken power law with a late time slope of � � 2: 0( F� / t � � ). Furthermore, the spectral energy distribution (SED) has a power-law (F� / � � � ) shape with slope � � 1:0. However, the decay slope at t < 1d ay is shallow, consistent with zero. This is in qualitative agreement with the prediction that XRFs are off-axis classical GRBs. After the first week there is a strong bump in the light curve, which peaks at around 16 days. The SED after the peak becomes significantly redder. We discuss the possible interpretations of this bump and conclude that an underlying supernova is the most likely explanation since no other model appears consistent with the evolution of the SED. Finally, we present deep spectroscopy of the burst both in the afterglow and in the bump

Detection of GRB 060927 at z = 5.47: Implications for the Use of Gamma-Ray Bursts as Probes of the End of the Dark Ages

We report on follow-up observations of the gamma-ray burst GRB 060927 using the robotic ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hr after the trigger shows a continuum break at lambda~8070 A, produced by neutral hydrogen absorption at z~5.6. We also detect an absorption line at 8158 A, which we interpret as Si II lambda1260 at z=5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lyalpha profile with a column density with log(NH/cm-2)=22.50+/-0.15. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: (1) GRB afterglows originating from z>~6 should be relatively easy to detect from the ground, but rapid near-infrared monitoring is necessary to ensure that they are found; (2) the presence of large H I column densities in some GRB host galaxies at z>5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; and (3) GRBs appear crucial to locate typical star-forming galaxies at z>5, and therefore the type of galaxies responsible for the reionization of the universe. Partly based on observations carried out with the ESO telescopes under programs 077.D-0661, 077.A-0667, 078.D-0416, and the large program 177.A-f0591.

The optical afterglow and host galaxy of GRB 000926

We present the discovery of the Optical Transient (OT) of the long{duration gamma-ray burst GRB 000926. The optical transient was detected independently with the Nordic Optical Telescope and at Calar Alto 22.2 hours after the burst. At this time the magnitude of the transient was R =1 9:36. The transient faded with ad ecay slope of about 1:7 during the rst two days after which the slope increased abruptly (within a few hours) to about 2:4. The light-curve started to flatten o after about a week indicating the presence of an underlying extended object. This object was detected in a deep image obtained one month after the GRB at R =2 3:87 0:15 and consists of several compact knots within about 5 arcsec. One of the knots is spatially coin- cident with the position of the OT and hence most likely belongs to the host galaxy. Higher resolution imaging is needed to resolve whether all the compact knots belong to the host galaxy or to several independent objects. In a separate paper we present a discussion of the optical spectrum of the OT, and its inferred redshift (Mller et al., in prep.).