As you may expect from the geographical position of Iceland it took considerable time for Coperni... more As you may expect from the geographical position of Iceland it took considerable time for Copernican heliocentrism to become known and to gain support on this remote North–Atlantic island. Probably the first news of this new theory broke on Iceland in the 16 th century but we had to wait until the late 18 th century for the completion of the Copernican Revolution in Denmark and Iceland 1. In this paper we will first briefly discuss the Copernican Revolution as a whole and its reception in Europe. Because of the Danish connection some emphasis will be put on Tycho Brahe's role. We will also briefly describe the development in Denmark as a background for the transmission of ideas to Iceland, especially through the University of Copenhagen, which at this time was practically the only university attended by Icelandic students. The bulk of the paper discusses the cosmological ideas appearing in writings of learned Icelanders of the 17 th and the 18 th century. The most important sources for this story are disputations in Latin, written during the student years of the authors at the University of Copenhagen and printed there, albeit in small numbers. The subject has not been systematically studied until now, mainly due to the scarcity of combined knowledge of Latin and astronomical cosmology.
GRB 021004 is one of the best sampled gamma-ray bursts (GRB) to date, although the nature of its ... more GRB 021004 is one of the best sampled gamma-ray bursts (GRB) to date, although the nature of its light curve is still being debated. Here we present a large amount (107) of new optical, near-infrared (NIR) and millimetre observations, ranging from 2 hours to more than a year after the burst. Fitting the multiband data to a model based on multiple energy injections suggests that at least 7 refreshed shocks took place during the evolution of the afterglow, implying a total energy release (collimated within an angle of 1.8 deg) of ~ 8x10^51 erg. Analysis of the late photometry reveals that the GRB 021004 host is a low extinction (Av ~ 0.1) starburst galaxy with M_B ~ -22.0.
We report the discovery of the optical afterglow (OA) of the long-duration gamma-ray burst GRB 03... more We report the discovery of the optical afterglow (OA) of the long-duration gamma-ray burst GRB 030429, and present a comprehensive optical/near-infrared dataset used to probe the matter at different distance scales, i.e. in the burst environment, in the host galaxy and in an intervening absorber. A break in the afterglow light curve is seen approximately 1 day from the onset of the burst. The light curve displays a significant deviation from a simple broken power-law with a bright 1.5 magnitude bump with a duration of 2-3 days. The optical/near-infrared spectral energy distribution is best fit with a power-law with index beta = -0.36 +/- 0.12 reddened by an SMC-like extinction law with (a modest) A_V = 0.34 +/- 0.04. In addition, we present deep spectroscopic observations obtained with the Very Large Telescope. The redshift measured via metal absorption lines in the OA is z = 2.658 +/- 0.004. Based on the damped Ly-alpha absorption line in the OA spectrum we measure the HI column density to be log N(HI) = 21.6 +/- 0.2. This confirms the trend that GRBs tend to be located behind very large HI column densities. The resulting dust-to-gas ratio is consistent with that found in the SMC, indicating a low metallicity and/or a low dust-to-metal ratio in the burst environment. We find that a neighbouring galaxy, at a separation of only 1.2", has z = 0.841 +/- 0.001, ruling it out as the host of GRB 030429. The small impact parameter of this nearby galaxy, which is responsible for MgII absorption in the OA spectrum, is in contrast to previous identifications of most QSO absorption-selected galaxy counterparts. Finally, we demonstrate that the OA was not affected by strong gravitational lensing via the nearby galaxy.
We extend the standard fireball model, widely used to interpret gamma-ray burst (GRB) afterglow l... more We extend the standard fireball model, widely used to interpret gamma-ray burst (GRB) afterglow light curves, to include energy injections, and apply the model to the afterglow light curves of GRB 990510, GRB 000301C and GRB 010222. We show that discrete energy injections can cause temporal variations in the optical light curves and present fits to the light curves of GRB 000301C as an example. A continuous injection may be required to interpret other bursts such as GRB 010222. The extended model accounts reasonably well for the observations in all bands ranging from X-rays to radio wavelengths. In some cases, the radio light curves indicate that additional model ingredients may be needed.
Monthly Notices of The Royal Astronomical Society, 2011
We present high time resolution (1.09 s) photometry of GRB 080210 obtained with ULTRASPEC mounted... more We present high time resolution (1.09 s) photometry of GRB 080210 obtained with ULTRASPEC mounted on the ESO/3.6-m telescope, starting 68.22 min after the burst and lasting for 26.45 min. The light curve is smooth on both short (down to 2.18 s) and long time scales, confirmed by a featureless power spectrum. On top of the fireball power-law decay, bumps and wiggles at different time scales can, in principle, be produced by density fluctuations in the circumburst medium, substructures in the jet or by refreshed shocks. Comparing our constraints with variability limits derived from kinematic arguments, we exclude under-density fluctuations producing flux dips larger than 1 per cent with time scales \Deltat > 9.2 min (2 per cent on \Deltat > 2.3 min for many fluctuating regions). In addition, we study the afterglow VLT/FORS2 spectrum, the optical-to-X-ray spectral energy distribution (SED) and the time decay. The SED is best fit with a broken power law with slopes {\beta}opt = 0.71 \pm 0.01 and {\beta}X = 1.59 \pm 0.07, in disagreement with the fireball model, suggesting a non-standard afterglow for GRB 080210. We find AV = 0.18 \pm 0.03 mag optical extinction due to SMC-like dust and an excess X-ray absorption of log (NH/cm-2) = 21.58 +0.18 -0.26 assuming Solar abundances. The spectral analysis reveals a damped Ly{\alpha} absorber (log (NH I /cm-2) = 21.90 \pm 0.10) with a low metallicity ([X/H] = -1.21 \pm 0.16), likely associated with the interstellar medium of the GRB host galaxy (z = 2.641).
Monthly Notices of The Royal Astronomical Society, 1995
We investigate how a closed Friedmann-Robertson-Walker universe would appear to astronomers livin... more We investigate how a closed Friedmann-Robertson-Walker universe would appear to astronomers living at different cosmic epochs. For this purpose we apply the three observational tests of classical cosmology to closed universe models without a cosmological constant, both for the expanding and for the contracting phases of their evolution. In particular, we investigate how the Hubble diagram and other plots of observational quantities, such as angular size and number density of galaxies, change with cosmic time. Once recollapse has started, these diagrams become multivalued for certain ranges of the observational variables. Light from nearby galaxies is blueshifted whereas galaxies further away have redshifted spectra, these two regions of the observable universe being separated by a `surface of zero redshift'. In a contracting universe two different images of the same galaxy may appear in opposite directions of the sky since observers can, in principle, see beyond their own antipoles. Because of this and the fact that the cosmological diagrams are multivalued, interpretation of cosmological data could be considerably more difficult in the contracting phase than during the expanding phase of the evolution.
Monthly Notices of The Royal Astronomical Society, 2005
We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts ... more We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts (GRBs) 030226 (z= 1.986), 021004 (z= 2.335) and 020124 (z= 3.198). We find that the host galaxy of GRB 021004 is an extended (around 8 kpc) strong Lyα emitter with a rest-frame equivalent width (EW) of 68+12−11Å, and a star formation rate of 10.6 ± 2.0 M⊙ yr−1. We do not detect the hosts of GRB 030226 and GRB 020124, but the upper limits on their Lyα fluxes do not rule out large rest-frame EWs. In the fields of GRB 021004 and GRB 030226 we find seven and five other galaxies, respectively, with excess emission in the narrow-band filter. These galaxies are candidate Lyα-emitting galaxies in the environment of the host galaxies. We have also compiled a list of all z≳ 2 GRB hosts, and demonstrate that a scenario where they trace star formation in an unbiased way is compatible with current observational constraints. Fitting the z= 3 luminosity function (LF) under this assumption results in a characteristic luminosity of R*= 24.6 and a faint-end slope of α=−1.55, consistent with the LF measured for Lyman-break galaxies.
In this project we addresses the nature of host galaxies of gammaray bursts (GRBs) through numeri... more In this project we addresses the nature of host galaxies of gammaray bursts (GRBs) through numerical simulations of galaxy formation. GRBs are the most energetic events in the universe and those longer than about 2 seconds are thought to result from the core collapse and explosion of massive stars [Pir05]. Because of the cosmologically short lifetime of the massive progenitors, GRBs are generally considered to be powerful tracers of the massive star formation history of the universe. They can therefore also be expected to provide useful insights into the understanding of the galaxy formation process. The current interest in GRB research started in 1997 with the discovery of their optical afterglow emission. To date over 40 optical detections have been made, but the number of GRB host galaxy observations is still rather limited with only about 20 positive detections.
Several GRB afterglow light curves deviate strongly from the power law decay observed in most bur... more Several GRB afterglow light curves deviate strongly from the power law decay observed in most bursts. We show that these variations can be accounted for by including refreshed shocks in the standard fireball model previously used to interpret the overall afterglow behavior. As an example we consider GRB 021004 that exhibited strong light curve variations and has a reasonably well time-resolved polarimetry. We show that the light curves in the R-band, X-rays and in the radio can be accounted for by four energy injection episodes in addition to the initial event. The polarization variations are shown to be a consequence of the injections.
We have obtained U and R band observations of the depletion of background galaxies due to the gra... more 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.
GRB 021004 is one of the best sampled gamma-ray bursts (GRB) to date, however the nature of its l... more GRB 021004 is one of the best sampled gamma-ray bursts (GRB) to date, however the nature of its light curve is still being debated. A compilation of multiwavelength (from radio to X-rays) observations, including unpublished optical/near-infrared and millimetre observations, is used to fit a model based on 7 refreshed shocks that took place during the evolution of the afterglow. They imply a total energy release of ˜ 8×1051 erg. Analysis of the late photometry reveals that the GRB 021004 host is a low extinction ( A V ˜ 0.1) starburst galaxy with M B ≃ - 22.0.
When presenting numerical results we use the FRW world model with $\Omega_{m0} = 0.30$ and $\Omeg... more When presenting numerical results we use the FRW world model with $\Omega_{m0} = 0.30$ and $\Omega_{\Lambda0} = 0.70$ as our standard cosmological model. In this model the $\Lambda$-sphere is at a redshift of 0.67, and within a few Hubble times the event horizon will be stationary at a fixed proper distance of 5.1 Gpc (assuming $h_0 = 0.7$). All cosmological sources with present redshift larger than 1.7 have by now crossed the event horizon and are therefore completely out of causal contact.
Monthly Notices of The Royal Astronomical Society, 2005
We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts ... more We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts (GRBs) 030226 (z= 1.986), 021004 (z= 2.335) and 020124 (z= 3.198). We find that the host galaxy of GRB 021004 is an extended (around 8 kpc) strong Lyα emitter with a rest-frame equivalent width (EW) of 68+12-11Å, and a star formation rate of 10.6 +/- 2.0 Msolar yr-1. We do not detect the hosts of GRB 030226 and GRB 020124, but the upper limits on their Lyα fluxes do not rule out large rest-frame EWs. In the fields of GRB 021004 and GRB 030226 we find seven and five other galaxies, respectively, with excess emission in the narrow-band filter. These galaxies are candidate Lyα-emitting galaxies in the environment of the host galaxies. We have also compiled a list of all z>~ 2 GRB hosts, and demonstrate that a scenario where they trace star formation in an unbiased way is compatible with current observational constraints. Fitting the z= 3 luminosity function (LF) under this assumption results in a characteristic luminosity of R*= 24.6 and a faint-end slope of α=-1.55, consistent with the LF measured for Lyman-break galaxies.
ABSTRACT Using the standard fireball model, we create virtual populations of gamma-ray burst afte... more ABSTRACT Using the standard fireball model, we create virtual populations of gamma-ray burst afterglows and study their luminosity distributions. We find that the luminosity distributions can be described by a function similar to a log normal distribution with an exponential cutoff. The function parameters are frequency dependent but the choice of the model parameter distributions used to create the virtual populations has small effects on the results. Our results are compared to observations and indicate that a number of afterglows with low optical luminosities go unobserved.
Monthly Notices of The Royal Astronomical Society, 2007
We explore galaxy properties in general and properties of host galaxies of gamma-ray bursts (GRBs... more We explore galaxy properties in general and properties of host galaxies of gamma-ray bursts (GRBs) in particular, using N-body/Eulerian hydrodynamic simulations and the stellar population synthesis model, Starburst99, to infer observable properties. We identify simulated galaxies that have optical star formation rate (SFR) and SFR-to-luminosity ratio similar to those observed in a well-defined sample of ten host galaxies. Each of the numerical counterparts are found in catalogs at the same redshifts as the observed hosts. The counterparts are found to be low-mass galaxies, with low mass-to-light ratio, recent epoch of formation, and high ratio between the SFR and the average of the SFR. When compared to the overall galaxy population, they have colors much bluer than the high-mass star-forming galaxy population. Although their SFRs span a range of values, the specific rates of the numerical counterparts are equal to or higher than the median values estimated at the different redshifts. We also emphasize the strong relationships between the specific star formation rate (SFR) and quantities known to reflect the star formation history of galaxies, i.e. color and mass-to-light ratio: At intermediate redshift, the faintest and bluest galaxies are also the objects with the highest specific rates. These results suggest that GRB host galaxies are likely to be drawn from the high specific SFR sub-population of galaxies, rather than the high SFR galaxy population. Finally, as indicated by our catalogs, in an extended sample, the majority of GRB host galaxies is expected to have specific SFRs higher than found in the magnitude-limited sample studied here.
Results: We show that the luminosity functions can be described by a function similar to a log no... more Results: We show that the luminosity functions can be described by a function similar to a log normal distribution with an exponential cutoff. The function parameters are frequency dependent but not very dependent on the model parameter distributions used to create the virtual populations. Comparison with observations shows that while there is good general agreement with the data, it is difficult to explain simultaneously the X-ray and optical data. Possible reasons for this are discussed and the most likely one is that the standard fireball model is incomplete and that decoupling of the X-ray and optical emission mechanism may be needed.
We extend the standard fireball model, widely used to interpret gamma-ray burst (GRB) afterglow l... more We extend the standard fireball model, widely used to interpret gamma-ray burst (GRB) afterglow light curves, to include energy injections and apply the model to the afterglow light curves of GRB 990510, GRB 000301C, and GRB 010222. We show that discrete energy injections can cause temporal variations in the optical light curves, and we present fits to the light curves of GRB 000301C as an example. A continuous injection may be required to interpret other bursts, such as GRB 010222. The extended model accounts reasonably well for the observations in all bands ranging from X-rays to radio wavelengths. In some cases the radio light curves indicate that additional model ingredients may be needed.
Monthly Notices of The Royal Astronomical Society, 2004
We use numerical simulations of large scale structure formation to explore the cosmological prope... more We use numerical simulations of large scale structure formation to explore the cosmological properties of Gamma-Ray Burst (GRB) host galaxies. Among the different sub-populations found in the simulations, we identify the host galaxies as the most efficient star-forming objects, i.e. galaxies with high specific star formation rates. We find that the host candidates are low-mass, young galaxies with low to moderate star formation rate. These properties are consistent with those observed in GRB hosts, most of which are sub-luminous, blue galaxies. Assuming that host candidates are galaxies with high star formation rates would have given conclusions inconsistent with the observations. The specific star formation rate, given a galaxy mass, is shown to increase as the redshift increases. The low mass of the putative hosts makes them difficult to detect with present day telescopes and the probability density function of the specific star formation rate is predicted to change depending on whether or not these galaxies are observed.
As you may expect from the geographical position of Iceland it took considerable time for Coperni... more As you may expect from the geographical position of Iceland it took considerable time for Copernican heliocentrism to become known and to gain support on this remote North–Atlantic island. Probably the first news of this new theory broke on Iceland in the 16 th century but we had to wait until the late 18 th century for the completion of the Copernican Revolution in Denmark and Iceland 1. In this paper we will first briefly discuss the Copernican Revolution as a whole and its reception in Europe. Because of the Danish connection some emphasis will be put on Tycho Brahe's role. We will also briefly describe the development in Denmark as a background for the transmission of ideas to Iceland, especially through the University of Copenhagen, which at this time was practically the only university attended by Icelandic students. The bulk of the paper discusses the cosmological ideas appearing in writings of learned Icelanders of the 17 th and the 18 th century. The most important sources for this story are disputations in Latin, written during the student years of the authors at the University of Copenhagen and printed there, albeit in small numbers. The subject has not been systematically studied until now, mainly due to the scarcity of combined knowledge of Latin and astronomical cosmology.
GRB 021004 is one of the best sampled gamma-ray bursts (GRB) to date, although the nature of its ... more GRB 021004 is one of the best sampled gamma-ray bursts (GRB) to date, although the nature of its light curve is still being debated. Here we present a large amount (107) of new optical, near-infrared (NIR) and millimetre observations, ranging from 2 hours to more than a year after the burst. Fitting the multiband data to a model based on multiple energy injections suggests that at least 7 refreshed shocks took place during the evolution of the afterglow, implying a total energy release (collimated within an angle of 1.8 deg) of ~ 8x10^51 erg. Analysis of the late photometry reveals that the GRB 021004 host is a low extinction (Av ~ 0.1) starburst galaxy with M_B ~ -22.0.
We report the discovery of the optical afterglow (OA) of the long-duration gamma-ray burst GRB 03... more We report the discovery of the optical afterglow (OA) of the long-duration gamma-ray burst GRB 030429, and present a comprehensive optical/near-infrared dataset used to probe the matter at different distance scales, i.e. in the burst environment, in the host galaxy and in an intervening absorber. A break in the afterglow light curve is seen approximately 1 day from the onset of the burst. The light curve displays a significant deviation from a simple broken power-law with a bright 1.5 magnitude bump with a duration of 2-3 days. The optical/near-infrared spectral energy distribution is best fit with a power-law with index beta = -0.36 +/- 0.12 reddened by an SMC-like extinction law with (a modest) A_V = 0.34 +/- 0.04. In addition, we present deep spectroscopic observations obtained with the Very Large Telescope. The redshift measured via metal absorption lines in the OA is z = 2.658 +/- 0.004. Based on the damped Ly-alpha absorption line in the OA spectrum we measure the HI column density to be log N(HI) = 21.6 +/- 0.2. This confirms the trend that GRBs tend to be located behind very large HI column densities. The resulting dust-to-gas ratio is consistent with that found in the SMC, indicating a low metallicity and/or a low dust-to-metal ratio in the burst environment. We find that a neighbouring galaxy, at a separation of only 1.2", has z = 0.841 +/- 0.001, ruling it out as the host of GRB 030429. The small impact parameter of this nearby galaxy, which is responsible for MgII absorption in the OA spectrum, is in contrast to previous identifications of most QSO absorption-selected galaxy counterparts. Finally, we demonstrate that the OA was not affected by strong gravitational lensing via the nearby galaxy.
We extend the standard fireball model, widely used to interpret gamma-ray burst (GRB) afterglow l... more We extend the standard fireball model, widely used to interpret gamma-ray burst (GRB) afterglow light curves, to include energy injections, and apply the model to the afterglow light curves of GRB 990510, GRB 000301C and GRB 010222. We show that discrete energy injections can cause temporal variations in the optical light curves and present fits to the light curves of GRB 000301C as an example. A continuous injection may be required to interpret other bursts such as GRB 010222. The extended model accounts reasonably well for the observations in all bands ranging from X-rays to radio wavelengths. In some cases, the radio light curves indicate that additional model ingredients may be needed.
Monthly Notices of The Royal Astronomical Society, 2011
We present high time resolution (1.09 s) photometry of GRB 080210 obtained with ULTRASPEC mounted... more We present high time resolution (1.09 s) photometry of GRB 080210 obtained with ULTRASPEC mounted on the ESO/3.6-m telescope, starting 68.22 min after the burst and lasting for 26.45 min. The light curve is smooth on both short (down to 2.18 s) and long time scales, confirmed by a featureless power spectrum. On top of the fireball power-law decay, bumps and wiggles at different time scales can, in principle, be produced by density fluctuations in the circumburst medium, substructures in the jet or by refreshed shocks. Comparing our constraints with variability limits derived from kinematic arguments, we exclude under-density fluctuations producing flux dips larger than 1 per cent with time scales \Deltat > 9.2 min (2 per cent on \Deltat > 2.3 min for many fluctuating regions). In addition, we study the afterglow VLT/FORS2 spectrum, the optical-to-X-ray spectral energy distribution (SED) and the time decay. The SED is best fit with a broken power law with slopes {\beta}opt = 0.71 \pm 0.01 and {\beta}X = 1.59 \pm 0.07, in disagreement with the fireball model, suggesting a non-standard afterglow for GRB 080210. We find AV = 0.18 \pm 0.03 mag optical extinction due to SMC-like dust and an excess X-ray absorption of log (NH/cm-2) = 21.58 +0.18 -0.26 assuming Solar abundances. The spectral analysis reveals a damped Ly{\alpha} absorber (log (NH I /cm-2) = 21.90 \pm 0.10) with a low metallicity ([X/H] = -1.21 \pm 0.16), likely associated with the interstellar medium of the GRB host galaxy (z = 2.641).
Monthly Notices of The Royal Astronomical Society, 1995
We investigate how a closed Friedmann-Robertson-Walker universe would appear to astronomers livin... more We investigate how a closed Friedmann-Robertson-Walker universe would appear to astronomers living at different cosmic epochs. For this purpose we apply the three observational tests of classical cosmology to closed universe models without a cosmological constant, both for the expanding and for the contracting phases of their evolution. In particular, we investigate how the Hubble diagram and other plots of observational quantities, such as angular size and number density of galaxies, change with cosmic time. Once recollapse has started, these diagrams become multivalued for certain ranges of the observational variables. Light from nearby galaxies is blueshifted whereas galaxies further away have redshifted spectra, these two regions of the observable universe being separated by a `surface of zero redshift'. In a contracting universe two different images of the same galaxy may appear in opposite directions of the sky since observers can, in principle, see beyond their own antipoles. Because of this and the fact that the cosmological diagrams are multivalued, interpretation of cosmological data could be considerably more difficult in the contracting phase than during the expanding phase of the evolution.
Monthly Notices of The Royal Astronomical Society, 2005
We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts ... more We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts (GRBs) 030226 (z= 1.986), 021004 (z= 2.335) and 020124 (z= 3.198). We find that the host galaxy of GRB 021004 is an extended (around 8 kpc) strong Lyα emitter with a rest-frame equivalent width (EW) of 68+12−11Å, and a star formation rate of 10.6 ± 2.0 M⊙ yr−1. We do not detect the hosts of GRB 030226 and GRB 020124, but the upper limits on their Lyα fluxes do not rule out large rest-frame EWs. In the fields of GRB 021004 and GRB 030226 we find seven and five other galaxies, respectively, with excess emission in the narrow-band filter. These galaxies are candidate Lyα-emitting galaxies in the environment of the host galaxies. We have also compiled a list of all z≳ 2 GRB hosts, and demonstrate that a scenario where they trace star formation in an unbiased way is compatible with current observational constraints. Fitting the z= 3 luminosity function (LF) under this assumption results in a characteristic luminosity of R*= 24.6 and a faint-end slope of α=−1.55, consistent with the LF measured for Lyman-break galaxies.
In this project we addresses the nature of host galaxies of gammaray bursts (GRBs) through numeri... more In this project we addresses the nature of host galaxies of gammaray bursts (GRBs) through numerical simulations of galaxy formation. GRBs are the most energetic events in the universe and those longer than about 2 seconds are thought to result from the core collapse and explosion of massive stars [Pir05]. Because of the cosmologically short lifetime of the massive progenitors, GRBs are generally considered to be powerful tracers of the massive star formation history of the universe. They can therefore also be expected to provide useful insights into the understanding of the galaxy formation process. The current interest in GRB research started in 1997 with the discovery of their optical afterglow emission. To date over 40 optical detections have been made, but the number of GRB host galaxy observations is still rather limited with only about 20 positive detections.
Several GRB afterglow light curves deviate strongly from the power law decay observed in most bur... more Several GRB afterglow light curves deviate strongly from the power law decay observed in most bursts. We show that these variations can be accounted for by including refreshed shocks in the standard fireball model previously used to interpret the overall afterglow behavior. As an example we consider GRB 021004 that exhibited strong light curve variations and has a reasonably well time-resolved polarimetry. We show that the light curves in the R-band, X-rays and in the radio can be accounted for by four energy injection episodes in addition to the initial event. The polarization variations are shown to be a consequence of the injections.
We have obtained U and R band observations of the depletion of background galaxies due to the gra... more 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.
GRB 021004 is one of the best sampled gamma-ray bursts (GRB) to date, however the nature of its l... more GRB 021004 is one of the best sampled gamma-ray bursts (GRB) to date, however the nature of its light curve is still being debated. A compilation of multiwavelength (from radio to X-rays) observations, including unpublished optical/near-infrared and millimetre observations, is used to fit a model based on 7 refreshed shocks that took place during the evolution of the afterglow. They imply a total energy release of ˜ 8×1051 erg. Analysis of the late photometry reveals that the GRB 021004 host is a low extinction ( A V ˜ 0.1) starburst galaxy with M B ≃ - 22.0.
When presenting numerical results we use the FRW world model with $\Omega_{m0} = 0.30$ and $\Omeg... more When presenting numerical results we use the FRW world model with $\Omega_{m0} = 0.30$ and $\Omega_{\Lambda0} = 0.70$ as our standard cosmological model. In this model the $\Lambda$-sphere is at a redshift of 0.67, and within a few Hubble times the event horizon will be stationary at a fixed proper distance of 5.1 Gpc (assuming $h_0 = 0.7$). All cosmological sources with present redshift larger than 1.7 have by now crossed the event horizon and are therefore completely out of causal contact.
Monthly Notices of The Royal Astronomical Society, 2005
We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts ... more We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts (GRBs) 030226 (z= 1.986), 021004 (z= 2.335) and 020124 (z= 3.198). We find that the host galaxy of GRB 021004 is an extended (around 8 kpc) strong Lyα emitter with a rest-frame equivalent width (EW) of 68+12-11Å, and a star formation rate of 10.6 +/- 2.0 Msolar yr-1. We do not detect the hosts of GRB 030226 and GRB 020124, but the upper limits on their Lyα fluxes do not rule out large rest-frame EWs. In the fields of GRB 021004 and GRB 030226 we find seven and five other galaxies, respectively, with excess emission in the narrow-band filter. These galaxies are candidate Lyα-emitting galaxies in the environment of the host galaxies. We have also compiled a list of all z>~ 2 GRB hosts, and demonstrate that a scenario where they trace star formation in an unbiased way is compatible with current observational constraints. Fitting the z= 3 luminosity function (LF) under this assumption results in a characteristic luminosity of R*= 24.6 and a faint-end slope of α=-1.55, consistent with the LF measured for Lyman-break galaxies.
ABSTRACT Using the standard fireball model, we create virtual populations of gamma-ray burst afte... more ABSTRACT Using the standard fireball model, we create virtual populations of gamma-ray burst afterglows and study their luminosity distributions. We find that the luminosity distributions can be described by a function similar to a log normal distribution with an exponential cutoff. The function parameters are frequency dependent but the choice of the model parameter distributions used to create the virtual populations has small effects on the results. Our results are compared to observations and indicate that a number of afterglows with low optical luminosities go unobserved.
Monthly Notices of The Royal Astronomical Society, 2007
We explore galaxy properties in general and properties of host galaxies of gamma-ray bursts (GRBs... more We explore galaxy properties in general and properties of host galaxies of gamma-ray bursts (GRBs) in particular, using N-body/Eulerian hydrodynamic simulations and the stellar population synthesis model, Starburst99, to infer observable properties. We identify simulated galaxies that have optical star formation rate (SFR) and SFR-to-luminosity ratio similar to those observed in a well-defined sample of ten host galaxies. Each of the numerical counterparts are found in catalogs at the same redshifts as the observed hosts. The counterparts are found to be low-mass galaxies, with low mass-to-light ratio, recent epoch of formation, and high ratio between the SFR and the average of the SFR. When compared to the overall galaxy population, they have colors much bluer than the high-mass star-forming galaxy population. Although their SFRs span a range of values, the specific rates of the numerical counterparts are equal to or higher than the median values estimated at the different redshifts. We also emphasize the strong relationships between the specific star formation rate (SFR) and quantities known to reflect the star formation history of galaxies, i.e. color and mass-to-light ratio: At intermediate redshift, the faintest and bluest galaxies are also the objects with the highest specific rates. These results suggest that GRB host galaxies are likely to be drawn from the high specific SFR sub-population of galaxies, rather than the high SFR galaxy population. Finally, as indicated by our catalogs, in an extended sample, the majority of GRB host galaxies is expected to have specific SFRs higher than found in the magnitude-limited sample studied here.
Results: We show that the luminosity functions can be described by a function similar to a log no... more Results: We show that the luminosity functions can be described by a function similar to a log normal distribution with an exponential cutoff. The function parameters are frequency dependent but not very dependent on the model parameter distributions used to create the virtual populations. Comparison with observations shows that while there is good general agreement with the data, it is difficult to explain simultaneously the X-ray and optical data. Possible reasons for this are discussed and the most likely one is that the standard fireball model is incomplete and that decoupling of the X-ray and optical emission mechanism may be needed.
We extend the standard fireball model, widely used to interpret gamma-ray burst (GRB) afterglow l... more We extend the standard fireball model, widely used to interpret gamma-ray burst (GRB) afterglow light curves, to include energy injections and apply the model to the afterglow light curves of GRB 990510, GRB 000301C, and GRB 010222. We show that discrete energy injections can cause temporal variations in the optical light curves, and we present fits to the light curves of GRB 000301C as an example. A continuous injection may be required to interpret other bursts, such as GRB 010222. The extended model accounts reasonably well for the observations in all bands ranging from X-rays to radio wavelengths. In some cases the radio light curves indicate that additional model ingredients may be needed.
Monthly Notices of The Royal Astronomical Society, 2004
We use numerical simulations of large scale structure formation to explore the cosmological prope... more We use numerical simulations of large scale structure formation to explore the cosmological properties of Gamma-Ray Burst (GRB) host galaxies. Among the different sub-populations found in the simulations, we identify the host galaxies as the most efficient star-forming objects, i.e. galaxies with high specific star formation rates. We find that the host candidates are low-mass, young galaxies with low to moderate star formation rate. These properties are consistent with those observed in GRB hosts, most of which are sub-luminous, blue galaxies. Assuming that host candidates are galaxies with high star formation rates would have given conclusions inconsistent with the observations. The specific star formation rate, given a galaxy mass, is shown to increase as the redshift increases. The low mass of the putative hosts makes them difficult to detect with present day telescopes and the probability density function of the specific star formation rate is predicted to change depending on whether or not these galaxies are observed.
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Papers by Einar Gudmundsson
The bulk of the paper discusses the cosmological ideas appearing in writings of learned Icelanders of the 17 th and the 18 th century. The most important sources for this story are disputations in Latin, written during the student years of the authors at the University of Copenhagen and printed there, albeit in small numbers. The subject has not been systematically studied until now, mainly due to the scarcity of combined knowledge of Latin and astronomical cosmology.
The bulk of the paper discusses the cosmological ideas appearing in writings of learned Icelanders of the 17 th and the 18 th century. The most important sources for this story are disputations in Latin, written during the student years of the authors at the University of Copenhagen and printed there, albeit in small numbers. The subject has not been systematically studied until now, mainly due to the scarcity of combined knowledge of Latin and astronomical cosmology.