Holger Pedersen

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.

The Fading Optical Counterpart of GRB 970228, 6 Months and 1 Year Later

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made with the Hubble Space Telescope (HST) and the Keck I telescope. The gamma-ray burst (GRB) was observed approximately 6 months after outburst, on 1997 September 4, using the HST/STIS CCD, and approximately 1 year after outburst, on 1998 February 24, using HST/NICMOS, and on 1998 April 4 using the NIRC on Keck. The unresolved counterpart is detected by STIS at V=28.0 ± 0.25, consistent with a continued power-law decline with exponent -1.10 ± 0.05. The counterpart is located within, but near the edge of, a faint extended source with diameter ~08 and integrated magnitude V=25.8 ± 0.25. A reanalysis of HST and New Technology Telescope observations performed shortly after the burst shows no evidence of proper motion of the point source or fading of the extended emission. Although the optical transient is not detected in the NICMOS images (H≥25.3), the extended source is visible and has a total magnitude H=23.3 ± 0.1. The Keck observations find K=22.8 ± 0.3. Comparison with observations obtained shortly after outburst suggests that the nebular luminosity has also been stable in the infrared. We find that several distinct and independent means of deriving the foreground extinction in the direction of GRB 970228 all agree with AV=0.75 ± 0.2. After adjusting for this 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. We therefore conclude that the extended source observed about GRB 970228 is almost certainly its host galaxy. Additionally, we find that independent of assumed redshift, the host is significantly bluer than typical nearby blue dwarf irregulars. With the caveat that the presently available infrared observations of the HDF are only fully complete to a limit about one-half magnitude brighter than the host, 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 broadband imaging of the host lie in rapidly star-forming galaxies.

Evolution of the polarization of the optical afterglow of the γ-ray burst GRB030329

The association of a supernova with GRB030329 strongly supports the ‘collapsar’ model of γ-ray bursts, where a relativistic jet forms after the progenitor star collapses. Such jets cannot be spatially resolved because γ-ray bursts lie at cosmological distances; their existence is instead inferred from ‘breaks’ in the light curves of the afterglows, and from the theoretical desire to reduce the estimated total energy of the burst by proposing that most of it comes out in narrow beams. Temporal evolution of the polarization of the afterglows may provide independent evidence for the jet structure of the relativistic outflow. Small-level polarization (∼1–3 per cent) has been reported for a few bursts, but its temporal evolution has yet to be established. Here we report polarimetric observations of the afterglow of GRB030329. We establish the polarization light curve, detect sustained polarization at the per cent level, and find significant variability. The data imply that the afterglow magnetic field has a small coherence length and is mostly random, probably generated by turbulence, in contrast with the picture arising from the high polarization detected in the prompt γ-rays from GRB021206 (ref. 18).

Optical Photometry of GRB 021004: The First Month*

We present UBVRCIC photometry of the optical afterglow of the gamma-ray burst GRB 021004 taken at the Nordic Optical Telescope between approximately 8 hours and 30 days after the burst. These data are combined with an analysis of the 87 ks Chandra X-ray observations of GRB 021004 taken at a mean epoch of 33 hr after the burst to investigate the nature of this GRB. We find an intrinsic spectral slope at optical wavelengths of ?UH = 0.39 ? 0.12 and an X-ray slope of ?X = 0.94 ? 0.03. There is no evidence for color evolution between 8.5 hr and 5.5 days after the burst. The optical decay becomes steeper approximately 5 days after the burst. This appears to be a gradual break due to the onset of sideways expansion in a collimated outflow. Our data suggest that the extragalactic extinction along the line of sight to the burst is between AV ? 0.3 and 0.5 and has an extinction law similar to that of the Small Magellanic Cloud. The optical and X-ray data are consistent with a relativistic fireball with the shocked electrons being in the slow cooling regime and having an electron index of p = 1.9 ? 0.1. The burst occurred in an ambient medium that is homogeneous on scales larger than approximately 1018 cm but inhomogeneous on smaller scales. The mean particle density is similar to what is seen for other bursts (0.1 cm-3 n 100 cm-3). Our results support the idea that the brightening seen at approximately 0.1 days was due to interaction with a clumpy ambient medium within 1017?1018 cm of the progenitor. The agreement between the predicted optical decay and that observed approximately 10 minutes after the burst suggests that the physical mechanism controlling the observed flux at t ? 10 minutes is the same as the one operating at t > 0.5 days.

Hubble Space Telescope Imaging of the Optical Transient Associated with GRB 970508

We report on Hubble Space Telescope (HST) observations of the optical transient (OT) discovered in the error box of the gamma-ray burst GRB970508. The object was imaged on 1997 June 2 with the Space Telescope Imaging Spectrograph (STIS) and Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). The observations reveal a point-like source with R = 23.1 ± 0.2 and H = 20.6 ± 0.3, in agreement with the power-law temporal decay seen in ground-based monitoring. Unlike the case of GRB970228, no nebulosity is detected surrounding the OT of GRB970508. We set very conservative upper limits of R ∼ 24.5 and H ∼ 22.2 on the brightness of any underlying extended source. If this subtends a substantial fraction of an arcsecond, then the R band limit is ∼25.5. In combination with Keck spectra that show Mg I absorption and [O II] emission at a redshift of z = 0.835, our observations suggest that the OT

The optical light curve of the low-mass X-ray binary XB 1254-690

The discovery of periodic optical variations from the counterpart of the low-mass X-ray binary XB 1254-690 is reported. The period is 0.163890 + or - 0.000009 days, identical to the recurrence period of X-ray dips. The full amplitude of the light curve is 0.4 mag and the mean V-magnitude is 19.1. From spectrophotometric observations the authors derive a B-V color index of 0.31 + or - 0.05 and an interstellar reddening of E(B-V) = 0.4 + or - 0.1. The optical minimum occurs 0.15 in phase after X-ray dips. Most of the optical light curve is probably due to varying aspect of the X-ray-heated secondary atmosphere. The contribution of the X-ray-heated bulge does not dominate the optical variability. The X-ray-heated secondary explanation supports the interpretation that the X-ray dips are due to eclipses by a bulge located on the edge of the accretion disk and indicates that at least in the case of XB 1254-690 the accretion disk does not shadow completely the companion star. 37 references.

Small-scale variations in the radiating surface of the GRB 011211 jet

We report the discovery of the afterglow of the X-ray rich, long-duration gamma-ray burst GRB 011211 and present evidence for oscillatory behaviour in its early optical light curve. The time-scale of the fluctuations, ∼1 hour, is much smaller than the time of the observations, ∼12 hours from the onset of the gamma-ray burst. The character and strength of the fluctuations are unprecedented and are inconsistent with causally connected variations in the emission of a symmetric, relativistic blast wave, i.e. flux variations which are produced uniformly throughout the shell surface are ruled out. Therefore, the wiggles are the result of spherically asymmetric density or energy variations. Additionally, there is evidence for fluctuations in the X-ray afterglow light curve. If real, the resulting difference in the observed time of the peaks of the short-term variations at X-ray and optical frequencies, would demonstrate that the energy content across the jet-emitting surface is not uniform.

Rapid searches for counterparts of GRB 930131

A fading counterpart to a gamma-ray burst (GRB) would appear as a point source inside a GRB error region soon after the burst which dims on a timescale from minutes to days. The favorable circumstances of the burst GRB 930131 allowed for an international campaign to search for fading counterparts starting 6.8 hr after the burst. We report observations from many optical sites, two radio telescopes, and archival ROSAT data, including deep Schmidt exposures 35, 44, and 64 hr after the burst. No fading counterparts were detected with our observations.

Hubble space telescope STIS observations of GRB 000301C: CCD imaging and near-ultraviolet MAMA spectroscopy

We present Space Telescope Imaging Spectrograph observations of the optical transient (OT) counterpart of the gamma-ray burster GRB 000301C obtained 5 days after the burst, on 2000 March 6. CCD clear-aperture imaging reveals a R~=21.50+/-0.15 source with no apparent host galaxy. An 8000 s, 1150 A 18 on the line of sight to the OT. This measured redshift is conservatively a lower limit to the GRB redshift. However, as all other GRBs that have deep Hubble Space Telescope images appear to lie on the stellar field of a host galaxy, and as the large H I column density measured here and in later ground-based observations is unlikely on a random line of sight, we believe we are probably seeing absorption from H I in the host galaxy. In any case, this represents the largest direct redshift determination of a gamma-ray burster to date. Our data are compatible with an OT spectrum represented by a power law with an intrinsic index alpha=1.2 (fnu~nu-alpha) and no extinction in the host galaxy, or with alpha=0.5 and extinction by SMC-like dust in the OT rest frame with AV=0.15. The large NHI and the lack of a detected host are similar to the situation for damped Lyalpha absorbers at z>2. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.We present HST/STIS observations of the optical counterpart (OT) of the gamma-ray burster GRB 000301C obtained on 2000 March 6, five days after the burst. CCD clear aperture imaging reveals a R ~ 21.50+/-0.15 source with no apparent host galaxy. An 8000 s, 1150 18 on the line-of-sight to the OT. This value is conservatively a lower limit to the GRB redshift. However, the facts that large N(HI) system are usually considered as progenitors of present day galaxies and that other OTs are found associated with star forming galaxies strongly suggest that it is the GRB redshift. In any case, this represents the largest direct redshift determination of a gamma-ray burster to date. Our data are compatible with an OT spectrum represented by a power-law with an intrinsic index \alpha = 1.2((f_nu \propto nu^-alpha) and no extinction in the host galaxy or with alpha = 0.5 and extinction by a SMC-like dust in the OT rest-frame with A_V = 0.15. The large N(HI) and the lack of detected host is similar to the situation for damped Ly-alpha absorbers at z > 2.

Early human studies of a new carbonic anhydrase inhibitor (NSD 3004) with anticonvulsant properties

NSD 3004 is a nondiuretic carbonic anhydrase inhibitor which has shown anticonvulsant properties in animal experiments. The studies reported here with volunteers and patients confirmed the strong binding to carbonic anhydrase in human erythrocytes and an extremely long half‐life in these. The half‐life in plasma from 8 volunteers was 28 ± 4 hours (mean ± standard deviation) and the steady‐state concentration in plasma was 5.7 ± 1.3 µg per milliliter (mean ± standard deviation) on 200 mg. daily. About 10 per cent of the dose was excreted unchanged in the urine and about 20 per cent ay a conjugate. Low values of PCO2 and standard bicarbonate in arterial blood and of total CO2 in venous blood were seen during a two week medication. No other deviations were found in the clinical parameters. Electroencephalography including automatic period analysis revealed no changes related to NSD 3004 medication. Expected effects such as light paresthesias and slight dyspnea on exertion were observed in some volunteers and patients. NSD 3004 seemed to exhibit antiepileptic properties in human subjects; an extensive clinical evaluation is needed.