arXiv subjects

J. Hjorth

Publications and source records attributed to J. Hjorth.

At least 271 records · Page 15Linked to original sources

arXiv2001

The optical afterglow and host galaxy of GRB 000926

In this paper we illustrate with the case of GRB 000926 how Gamma Ray Bursts (GRBs) can be used as cosmological lighthouses to identify and study star forming galaxies at high redshifts. The optical afterglow of the burst was located with optical imaging at the Nordic Optical Telescope 20.7 hours after the burst. Rapid follow-up spectroscopy allowed the determination of the redshift of the burst and a measurement of the host galaxy HI-column density in front of the burst. With late-time narrow band Ly-alpha as well as broad band imaging, we have studied the emission from the host galaxy and found that it is a strong Ly-alpha emitter in a state of active star formation.

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arXiv2001

Constraints on the optical afterglow emission of the short/hard burst GRB 010119

We report optical observations of the short/hard burst GRB 010119 error box, one of the smallest error boxes reported to date for short/hard GRBs. Limits of R > 22.3 and I > 21.2 are imposed by observations carried out 20.31 and 20.58 hours after the gamma-ray event, respectively. They represent the most constraining limits imposed to date on the optical emission from a short/hard gamma-ray burst afterglow.

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arXiv2001

GRB 000301C: a possible short/intermediate duration burst connected to a DLA system

We discuss two main aspects of the GRB 000301C afterglow (Fynbo et al. 2000, Jensen et al. 2000); its short duration and its possible connection with a Damped Ly-alpha Absorber (DLA). GRB 000301C falls in the short class of bursts, though it is consistent with belonging to the proposed intermediate class or the extreme short end of the distribution of long-duration GRBs. Based on two VLT spectra we estimate the HI column density to be Log(N(HI))=21.2+/-0.5. This is the first direct indication of a connection between GRB host galaxies and Damped Ly-alpha Absorbers.

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arXiv2001

The optical afterglow and host galaxy of GRB000926

We present the discovery of the Optical Transient (OT) of the long-duration gamma-ray burst GRB000926. 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 = 19.36. The transient faded with a decay slope of about 1.7 during the first two days after which the slope increased abruptly (within a few hours) to about 2.4. The light-curve started to flatten off 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=23.87+-0.15 and consists of several compact knots within about 5 arcsec. One of the knots is spatially coincident 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 (Moller et al. in prep.).

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arXiv2001

The Jet and the Supernova in GRB990712

The optical light curve of the afterglow following the gamma-ray burst GRB990712 is re-examined. Recently published polarization measurements of that source require a collimated outflow geometry that in turn predicts a break in the light curve. We show that the V-band light curve is consistent with such a break and that the post-break light curve evolution is dominated by a supernova contribution.

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arXiv2001

HST/STIS observations of GRB000301C: CCD imaging and NUV MAMA spectroscopy

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 < lambda/A < 3300 NUV-MAMA prism spectrum shows a relatively flat continuum (in f_lambda) between 2800 and 3300 A, with a mean flux 8.7 (+0.8,-1.6)+/- 2.6 10^(-18) ergs/s/cm^2/A, and a sharp break centered at 2797+/-25 A. We interpret it as HI Lyman break at z = 2.067+/-0.025 indicating the presence of a cloud with a HI column density log(HI) > 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 α= 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.

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arXiv2001

The Broken Light Curves of Gamma-Ray Bursts GRB 990123 and GRB 990510

We have collected all of the published photometry for GRB 990123 and GRB 990510, the first two gamma-ray bursts where breaks were seen in the light curves of their optical afterglows, and determined the shapes of their light curves and the break times. These parameters were used to investigate the physical mechanisms responsible for the breaks and the nature of the ambient medium that the bursts occurred in. The light curve for GRB 990123 is best fit by a broken power law with a break 1.68 +/- 0.19 days after the burst, a slope of alpha1 = -1.12 +/- 0.08 before the break, and a slope of alpha2 = -1.69 +/- 0.06 after the break. This is consistent with a collimated outflow with a fixed opening angle (theta0) of approximately five degrees. In this case the break in the light curve is due to the relativistic fireball slowing to a gamma factor of approximately 1 / theta0. The light curve for GRB 990510 is best fit by a continuous function with an early-time slope of alpha1 = -0.54 +/- 0.14, a late-time slope of alpha2 = -1.98 +/- 0.19, and a slow transition between the two regimes approximately one day after the burst. This is consistent with a collimated outflow with an opening angle of approximately five degrees that is initially radiative, but undergoes a sideways expansion that begins approximately one day after the burst. This sideways expansion is responsible for the slow break in the light curve.

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arXiv2001

The Host Galaxy and Optical Light Curve of the Gamma-Ray Burst GRB 980703

We present deep HST/STIS and ground-based photometry of the host galaxy of the gamma-ray burst GRB 980703 taken 17, 551, 710, and 716 days after the burst. We find that the host is a blue, slightly over-luminous galaxy with V_gal = 23.00 +/- 0.10, (V-R)_gal = 0.43 +/- 0.13, and a centre that is approximately 0.2 mag bluer than the outer regions of the galaxy. The galaxy has a star-formation rate of 8-13 M_sun/yr, assuming no extinction in the host. We find that the galaxy is best fit by a Sersic R^(1/n) profile with n ~= 1.0 and a half-light radius of 0.13 arcsec (= 0.72/h_100 proper kpc). This corresponds to an exponential disk with a scale radius of 0.22 arcsec (= 1.21/h_100 proper kpc). Subtracting a fit with elliptical isophotes leaves large residuals, which suggests that the host galaxy has a somewhat irregular morphology, but we are unable to connect the location of GRB 980703 with any special features in the host. The host galaxy appears to be a typical example of a compact star forming galaxy similar to those found in the Hubble Deep Field North. The R-band light curve of the optical afterglow associated with this gamma-ray burst is consistent with a single power-law decay having a slope of alpha = -1.37 +/- 0.14. Due to the bright underlying host galaxy the late time properties of the light-curve are very poorly constrained. The decay of the optical light curve is consistent with a contribution from an underlying Type Ic supernova like SN1998bw, or a dust echo, but such contributions cannot be securely established.

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arXiv2001

The afterglow of the short/intermediate-duration gamma-ray burst GRB 000301C: A jet at z=2.04

We present Ulysses and NEAR data from the detection of the short or intermediate duration (2 s) gamma-ray burst GRB000301C (2000 March 1.41 UT). The gamma-ray burst (GRB) was localised by the Inter Planetary Network (IPN) and RXTE to an area of 50 arcmin^2. A fading optical counterpart was subsequently discovered with the Nordic Optical Telescope (NOT) about 42h after the burst. The GRB lies at the border between the long-soft and the short-hard classes of GRBs. If GRB000301C belongs to the latter class, this would be the first detection of an afterglow to a short-hard burst. We present UBRI and JHK photometry from the time of the discovery until 11 days after the burst. Finally, we present spectroscopic observations of the optical afterglow obtained with the ESO VLT Antu telescope 4 and 5 days after the burst. The optical light curve is consistent with being achromatic from 2 to 11 days after the burst and exhibits a break. A broken power-law fit yields a shallow pre-break decay power-law slope of a_1=-0.72+-0.06, a break time of t_b=4.39+-0.26 days after the burst, and a post-break slope of a_2=-2.29+-0.17, which is best explained by a sideways expanding jet in an ambient medium of constant mean density. In the optical spectrum we find absorption features that are consistent with FeII, CIV, CII, SiII and Ly-a at a redshift of 2.0404+-0.0008. We find evidence for a curved shape of the spectral energy distribution of the observed afterglow. It is best fitted with a power-law spectral distribution with index b ~ -0.7 reddened by an SMC-like extinction law with A_V~0.1 mag. Based on the Ly-a absorption line we estimate the HI column density to be log(N(HI))=21.2+-0.5. This is the first direct indication of a connection between GRB host galaxies and Damped Ly-a Absorbers.

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arXiv2001

The extraordinarily bright optical afterglow of GRB 991208 and its host galaxy

Observations of the extraordinarily bright optical afterglow (OA) of GRB 991208 started 2.1 d after the event. The flux decay constant of the OA in the R-band is -2.30 +/- 0.07 up to 5 d, which is very likely due to the jet effect, and after that it is followed by a much steeper decay with constant -3.2 +/- 0.2, the fastest one ever seen in a GRB OA. A negative detection in several all-sky films taken simultaneously to the event implies either a previous additional break prior to 2 d after the occurrence of the GRB (as expected from the jet effect). The existence of a second break might indicate a steepening in the electron spectrum or the superposition of two events. Once the afterglow emission vanished, contribution of a bright underlying SN is found, but the light curve is not sufficiently well sampled to rule out a dust echo explanation. Our determination of z = 0.706 indicates that GRB 991208 is at 3.7 Gpc, implying an isotropic energy release of 1.15 x 10E53 erg which may be relaxed by beaming by a factor > 100. Precise astrometry indicates that the GRB coincides within 0.2" with the host galaxy, thus given support to a massive star origin. The absolute magnitude is M_B = -18.2, well below the knee of the galaxy luminosity function and we derive a star-forming rate of 11.5 +/- 7.1 Mo/yr. The quasi-simultaneous broad-band photometric spectral energy distribution of the afterglow is determined 3.5 day after the burst (Dec 12.0) implying a cooling frequency below the optical band, i.e. supporting a jet model with p = -2.30 as the index of the power-law electron distribution.

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arXiv2001

Detection of the optical afterglow of GRB 000630: Implications for dark bursts

We present the discovery of the optical transient of the long-duration gamma-ray burst GRB000630. The optical transient was detected with the Nordic Optical Telescope 21.1 hours after the burst. At the time of discovery the magnitude of the transient was R = 23.04+-0.08. The transient displayed a power-law decline characterized by a decay slope of alpha = -1.035+-0.097. A deep image obtained 25 days after the burst shows no indication of a contribution from a supernova or a host galaxy at the position of the transient. The closest detected galaxy is a R=24.68+-0.15 galaxy 2.0 arcsec north of the transient. The magnitudes of the optical afterglows of GRB980329, GRB980613 and GRB000630 were all R>=23 less than 24 hours from the burst epoch. We discuss the implications of this for our understanding of GRBs without detected optical transients. We conclude that i) based on the gamma-ray properties of the current sample we cannot conclude that GRBs with no detected OTs belong to another class of GRBs than GRBs with detected OTs and ii) the majority (>75%) of GRBs for which searches for optical afterglow have been unsuccessful are consistent with no detection if they were similar to bursts like GRB000630 at optical wavelengths.

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arXiv2000

Spectroscopic confirmation of a cluster of galaxies at z=1 in the field of the gravitational lens MG2016+112

We present new optical data on the cluster AX J2019+1127 identified by the X-ray satellite ASCA at z\sim 1 (Hattori et al. 1997). The data suggest the presence of a high-redshift cluster of galaxies responsible for the large separation triple quasar MG2016+112. Our deep photometry reveals an excess of z\sim 1 galaxy candidates, as already suspected by Benitez et al. (1999). Our spectroscopic survey of 44 objects in the field shows an excess of 6 red galaxies securely identified at z \sim 1, with a mean redshift of z =1.005 +/- 0.002. We estimate a velocity dispersion of σ= 771 (+430/-160) km s(-1) based on these 6 galaxies and a V-band mass-to-light ratio of 215 (+308/-77) h_50 M/L_sol. Our observations thus confirm the existence of a massive structure acting as the lens, which explains the unusual configuration of the triple quasar. Hence, there is no more need to invoke the existence of a ``dark cluster'' to understand this lens system.

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arXiv2000

VLT identification of the optical afterglow of the gamma-ray burst GRB 000131 at z=4.50

We report the discovery of the gamma-ray burst GRB 000131 and its optical afterglow. The optical identification was made with the VLT 84 hours after the burst following a BATSE detection and an Inter Planetary Network localization. GRB 000131 was a bright, long-duration GRB, with an apparent precursor signal 62 s prior to trigger. The afterglow was detected in ESO VLT, NTT, and DK1.54m follow-up observations. Broad-band and spectroscopic observations of the spectral energy distribution reveals a sharp break at optical wavelengths which is interpreted as a Ly-alpha absorption edge at 6700 A. This places GRB 000131 at a redshift of 4.500 +/- 0.015. The inferred isotropic energy release in gamma rays alone was approximately 10^54 erg (depending on the assumed cosmology). The rapid power-law decay of the afterglow (index alpha=2.25, similar to bursts with a prior break in the lightcurve), however, indicates collimated outflow, which relaxes the energy requirements by a factor of < 200. The afterglow of GRB 000131 is the first to be identified with an 8-m class telescope.

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arXiv2000

Depletion of background galaxies due to the cluster lens CL0024+1654: U and R band observations

We have obtained U and R band observations of the depletion of background galaxies due to the gravitational lensing of the galaxy cluster CL0024+1654 (z=0.39). The radial depletion curves show a significant depletion in both bands within a radius of 40''-70'' from the cluster center. This is the first time depletion is detected in the U band. This gives independent evidence for a break in the slope of the U band luminosity function at faint magnitudes. The radially averaged R band depletion curve is broader and deeper than in the U band. The differences can be attributed to the wavelength dependence of the slope of the luminosity function and to the different redshift distribution of the objects probed in the two bands. We estimate the Einstein radius of a singular isothermal sphere lens model using maximum likelihood analysis. Adopting a slope of the number counts of 0.2 and using the background density found beyond 150'' from the cluster center we find an Einstein radius of 17''+/-3'' and 25''+/-3'' in the U and R band, respectively. When combined with the redshift of the single background galaxy at z=1.675 seen as four giant arcs around 30'' from the cluster center, these values indicate a median redshift in the range 0.7 to 1.1 for the U_AB > 24 mag and R_AB > 24 mag populations.

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arXiv2000

HST/STIS Imaging of the Host Galaxy of GRB980425/SN1998bw

We present HST/STIS observations of ESO 184-G82, the host galaxy of the gamma-ray burst GRB 980425 associated with the peculiar Type Ic supernova SN1998bw. ESO 184-G82 is found to be an actively star forming SBc sub-luminous galaxy. We detect an object consistent with being a point source within the astrometric uncertainty of 0.018 arcseconds of the position of the supernova. The object is located inside a star-forming region and is at least one magnitude brighter than expected for the supernova based on a simple radioactive decay model. This implies either a significant flattening of the light curve or a contribution from an underlying star cluster.

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arXiv2000

The jet and circumburst stellar wind of GRB 980519

We present extensive multi-colour (UBVR_CI_C) photometry of the optical afterglow of GRB 980519. Upon discovery, 8.3 hours after the burst, the source was decaying as a power law, (t-t_GRB)^alpha, with a rapid decay rate alpha_1 = - 1.73+-0.04. About 13 hours after the burst a steepening of the light-curve to alpha_2 = -2.22+-0.04 was observed. Within the framework of current afterglow models, we argue that the rapid initial decline, the `break' in the light curve, and the spectral properties of the afterglow are best interpreted as being due to a collimated ultra-relativistic jet of fixed opening angle expanding into an inhomogeneous medium. In this scenario, we find that the circumburst medium has a density structure that goes as r^(-2.05+-0.22). This is characteristic of a preexisting wind expelled from a massive star. A possible physical scenario is that the progenitor star collapsed to form a black hole (i.e., a `collapsar'), producing the observed burst and afterglow. However, the supernova signature expected in the light curve in such a scenario is not detected. This either implies that the redshift of GRB980519 is greater than 1.5 or that supernovae accompanying GRBs are not standard candles.

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arXiv2000

An optical time-delay estimate for the double gravitational lens system B1600+434

We present optical I-band light curves of the gravitationally lensed double QSO B1600+434 from observations obtained at the Nordic Optical Telescope (NOT) between April 1998 and November 1999. The photometry has been performed by simultaneous deconvolution of all the data frames, involving a numerical lens galaxy model. Four methods have been applied to determine the time delay between the two QSO components, giving a mean estimate of Δ_t = 51+/-4 days (95% confidence level). This is the fourth optical time delay ever measured. Adopting a Omega=0.3, Lambda=0 Universe and using the mass model of Maller et al. (2000), this time-delay estimate yields a Hubble parameter of H_0=52 (+14, -8) km s^-1 Mpc^-1 (95% confidence level) where the errors include time-delay as well as model uncertainties. There are time-dependent offsets between the two (appropriately shifted) light curves that indicate the presence of external variations due to microlensing.

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arXiv1999

Ballerina - Pirouettes in Search of Gamma Bursts

The cosmological origin of gamma ray bursts has now been established with reasonable certainty. Many more bursts will need to be studied to establish the typical distance scale, and to map out the large diversity in properties which have been indicated by the first handful of events. We are proposing Ballerina, a small satellite to provide accurate positions and new data on the gamma-ray bursts. We anticipate a detection rate an order of magnitude larger than obtained from Beppo-SAX.

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