arXiv subjects

A. M. Soderberg

Publications and source records attributed to A. M. Soderberg.

104 records · Page 6Linked to original sources

arXiv2004

The Radio and X-ray Luminous Type Ibc Supernova 2003L

We present extensive radio observations of SN 2003L, the most luminous and energetic Type Ibc radio supernova with the exception of SN 1998bw (associated with GRB 980425). Observations from the Very Large Array are well described by a fitting a synchrotron self-absorption model to the emission spectrum. This model implies a sub-relativistic ejecta velocity, v ~ 0.2c, and a size of r ~ 4.3 x 10^{15} cm at t ~ 10 days. The circumstellar density is suitably fit with a stellar wind profile, n_e ~ r^{-2} and a constant mass loss rate of M_dot ~ 7.5 x 10^{-6} M_{solar}/year. Moreover, the magnetic field follows B ~ r^{-1} and the kinetic energy of the radio bright ejecta is roughly E ~ 10^{48} erg assuming equipartition of energy between relativistic electrons and magnetic fields. Furthermore, we show that free-free absorption does not contribute significantly to the radio spectrum, since it implies ejecta velocities which are inconsistent with size constraints derived from Very Long Baseline Array observations. In conclusion, we find that although SN 2003L has a radio luminosity comparable to that seen in SN 1998bw, it shows no evidence for a significant amount of energy coupled to relativistic ejecta. Using SN 2003L as an example, we comment briefly on the coupling of ejecta velocity and energy in Type Ibc supernovae.

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arXiv2004

The Caltech Core-Collapse Project (CCCP)

The cosmological utility of type Ia Supernovae (SNe) prompted numerous studies of these events, and they are now well characterized observationally, both as individual objects and as a population. In contrast, all other types of SNe (i.e., core-collapse events) are not as well observationally characterized. While some individual events have been studied in great detail (e.g., SN 1987A or SN 1998bw), the global properties of the core-collapse SN population are little known. However, in recent years, major drivers for change have emerged, among them the verification of the connection between core-collapse SNe and long-duration Gamma-Ray Bursts (GRBs), the possible utility of some core-collapse SNe (type II-P) as independent cosmological probes, and studies of core-collapse SNe as high redshift targets for missions like SNAP and JWST. The Caltech Core-Collapse Project (CCCP) is a large observational program using the Hale 200'' and the robotic 60'' telescopes at Palomar observatory to obtain optical photometry, spectroscopy and IR photometry of ~50 nearby core-collapse SNe. The program is designed to provide a complete sample of core-collapse events, with well-defined selection criteria and uniform, high-quality optical/IR observations, as well as radio and X-ray light curves for some events. We will use this sample to characterize the little-studied properties of core-collapse SNe as a population. The sample will be used as a comparison set for studies of SNe associated with Gamma-Ray Bursts, to promote and calibrate the use of SNe II-P for cosmography, and to set the stage for investigations of SNe at high-z using coming space missions such as SNAP and JWST.

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arXiv2004

The sub-energetic GRB 031203 as a cosmic analogue to GRB 980425

Over the six years since the discovery of the gamma-ray burst GRB 980425, associated with the nearby (distance, ~40 Mpc) supernova 1998bw, astronomers have fiercely debated the nature of this event. Relative to bursts located at cosmological distances, (redshift, z~1), GRB 980425 was under-luminous in gamma-rays by three orders of magnitude. Radio calorimetry showed the explosion was sub-energetic by a factor of 10. Here, we report observations of the radio and X-ray afterglow of the recent z=0.105 GRB 031203 and demonstrate that it too is sub-energetic. Our result, when taken together with the low gamma-ray luminosity, suggest that GRB 031203 is the first cosmic analogue to GRB 980425. We find no evidence that this event was a highly collimated explosion viewed off-axis. Like GRB 980425, GRB 031203 appears to be an intrinsically sub-energetic gamma-ray burst. Such sub-energetic events have faint afterglows. Intensive follow-up of faint bursts with smooth gamma-ray light curves (common to both GRBs 031203 and 980425) may enable us to reveal their expected large population.

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arXiv2004

Accurate Calorimetry of GRB 030329

We report late-time observations of the radio afterglow of GRB 030329. The light curves show a clear achromatic flattening at 50 days after the explosion. We interpret this flattening as resulting from the blast wave becoming trans-relativistic. Modeling of this transition enables us to make estimates of the energy content of the burst, regardless of the initial jet structure or the distribution of initial Lorentz factors of the ejecta. We find, in accordance with other events, that GRB 030329 is well-described by an explosion with total energy of a few times 10^51 erg expanding into a circumburst medium with a density of order unity.

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arXiv2004

The J-band Light Curve of SN 2003lw, Associated with GRB 031203

At z=0.1055, the gamma-ray burst GRB 031203 is one of the two nearest GRBs known. Using observations from the Very Large Array (VLA) and Chandra X-ray Observatory, we derive sub-arcsecond localizations of the radio and X-ray afterglow of this GRB. We present near-infrared observations of the supernova SN 2003lw, which exploded in the host galaxy of the GRB 031203. Our deep, high resolution Magellan/PANIC data establish that this SN is spatially coincident with the radio and X-ray localizations of the afterglow of GRB 031203 to sub-arcsecond precision, and is thus firmly associated with the GRB. We use image differencing to subtract the bright emission from the host galaxy, and measure the time evolution of the SN between ~5 and ~50 days after the GRB. The resulting light curve has a shape which is quite different from that of the two SNe previously associated with GRBs, SN 1998bw and SN 2003dh. With SN 2003lw securely associated with this burst, we confirm that all three GRBs with redshifts z<0.3 were accompanied by SN explosions.

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arXiv2003

Flaring up: radio diagnostics of the kinematic, hydrodynamic and environmental properties of GRBs

The specific incidence of radio flares appears to be significantly larger than that of the prompt optical emission. This abundance, coupled with the reverse shock interpretation suggests that radio flares add a unique probe on the physics of GRB shocks. Motivated thus, we estimate the strength of the reverse shock expected for bursts in which multi-wavelength observations have allowed the physical parameters of the forward shock to be determined. We use all 6 bursts (980519, 990123, 990510, 991208, 991216, 000418) which are found to be adiabatic and thus predicted to have a strong reverse shock. We aim to constrain the hydrodynamic evolution of the reverse shock and the initial bulk Lorentz factor -- which we found to be between $10^{2}$ and $10^{3}$ and well above the lower limits derived from the requirement that gamma-ray bursts be optically thin to high-energy photons. In half of the cases we improve the description of the early afterglow lightcurves by adding a contribution from the reverse shock. Modelling of this early emission provides the opportunity to investigate the immediate surroundings of the burst. For 991216 and 991208, the expected $1/r^2$ density structure for a stellar wind is not compatible with the early afterglow lightcurves. Considering the radial range relevant to these GRBs, we discuss the conditions under which the inclusion of a wind termination shock may resolve the absence of a $1/r^2$ density profile.

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arXiv2003

The bright optical afterglow of the nearby gamma-ray burst of 29 March 2003

Many past studies of cosmological gamma-ray bursts (GRBs) have been limited because of the large distance to typical GRBs, resulting in faint afterglows. There has long been a recognition that a nearby GRB would shed light on the origin of these mysterious cosmic explosions, as well as the physics of their fireballs. However, GRBs nearer than z=0.2 are extremely rare, with an estimated rate of localisation of one every decade. Here, we report the discovery of bright optical afterglow emission from GRB 030329. Our prompt dissemination and the brilliance of the afterglow resulted in extensive followup (more than 65 telescopes) from radio through X-ray bands, as well as measurement of the redshift, z=0.169. The gamma-ray and afterglow properties of GRB 030329 are similar to those of cosmological GRBs (after accounting for the small distance), making this the nearest known cosmological GRB. Observations have already securely identified the progenitor as a massive star that exploded as a supernova, and we anticipate futher revelations of the GRB phenomenon from studies of this source.

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arXiv2003

A Common Origin for Cosmic Explosions Inferred from Fireball Calorimetry

Past studies suggest that long-duration gamma-ray bursts (GRBs) have a standard energy of E_gamma ~ 10^51 erg in ultra-relativistic ejecta when corrected for asymmetry ("jets"). However, recently a group of sub-energetic bursts, including the peculiar GRB 980425 associated with SN 1998bw (E_gamma ~ 10^48 erg), has been identified. Here we report radio observations of GRB 030329, the nearest burst to date, which allow us to undertake calorimetry of the explosion. Our observations require a two-component explosion: a narrow (5 degrees) ultra-relativistic component responsible for the gamma-rays and early afterglow, and a wide, mildly relativistic component responsible for the radio and optical afterglow beyond 1.5 days. While the gamma-rays are energetically minor, the total energy release, dominated by the wide component, is similar to that of other GRBs. Given the firm link of GRB 030329 with SN 2003dh our result suggests a common origin for cosmic explosions in which, for reasons not understood, the energy in the highest velocity ejecta is highly variable

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arXiv2003

A Radio Survey of Type Ib and Ic Supernovae: Searching for Engine Driven Supernovae

The association of gamma-ray bursts (GRBs) and core-collapse supernovae (SNe) of Type Ib and Ic was motivated by the detection of SN 1998bw in the error box of GRB 980425 and the now-secure identification of a SN 1998bw-like event in the cosmological GRB 030329. The bright radio emission from SN 1998bw indicated that it possessed some of the unique attributes expected of GRBs, namely a large reservoir of energy in (mildly) relativistic ejecta and variable energy input. The two popular scenarios for the origin of SN 1998bw are a typical cosmological burst observed off-axis or a member of a new distinct class of supernova explosions (gSNe). In the former, about 0.5% of local Type Ib/c SNe are expected to be similar to SN1998bw; for the latter no such constraint exists. Motivated thus, we began a systematic program of radio observations of most reported Type Ib/c SNe accessible to the Very Large Array. Of the 33 SNe observed from late 1999 to the end of 2002 at most one is as bright as SN 1998bw. From this we conclude that the incidence of such events is <3%. Furthermore, analysis of the radio emission indicates that none of the observed SNe exhibit clear engine signatures. Finally, a comparison of the SN radio emission to that of GRB afterglows indicates that none of the SNe could have resulted from a typical GRB, independent of the initial jet orientation. Thus, while the nature of SN 1998bw remains an open question, there appears to be a clear dichotomy between the majority of hydrodynamic and engine-driven explosions.

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arXiv2003

Cosmological Uses of Gamma-Ray Bursts

Studies of the cosmic gamma-ray bursts (GRBs) and their host galaxies are starting to provide interesting or even unique new insights in observational cosmology. GRBs represent a new way of identifying a population of star-forming galaxies at cosmological redshifts. GRB hosts are broadly similar to the normal field galaxy populations at comparable redshifts and magnitudes, and indicate at most a mild luminosity evolution out to z ~ 1.5 - 2. GRB optical afterglows seen in absorption provide a powerful new probe of the ISM in dense, central regions of their host galaxies, complementary to the traditional studies using QSO absorbers. Some GRB hosts are heavily obscured, and provide a new way to select a population of cosmological sub-mm sources, and a novel constraint on the total obscured fraction of star formation over the history of the universe. Finally, detection of GRB afterglows at z > 6 may provide a unique way to probe the primordial star formation, massive IMF, early IGM, and chemical enrichment at the end of the cosmic reionization era.

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arXiv2003

A Radio Flare from GRB 020405: Evidence for a Uniform Medium Around a Massive Stellar Progenitor

We present radio observations of GRB 020405 starting 1.2 days after the burst, which reveal a rapidly-fading ``radio flare''. Based on its temporal and spectral properties, we interpret the radio flare as emission from the reverse shock. This scenario rules out a circumburst medium with a radial density profile ρ~ r^{-2} expected around a mass-losing massive star, since in that case the reverse shock emission decays on the timescale of the burst duration t~100 s. Using published optical and X-ray data, along with the radio data presented here, we further show that a self-consistent model requires collimated ejecta with an opening angle of 6 degrees (t_j~0.95 days). As a consequence of the early jet break, the late-time (t>10 days) emission measured with the Hubble Space Telescope significantly deviates from an extrapolation of the early, ground-based data. This, along with an unusually red spectrum, F_ν\~ ν^{-3.9}, strengthens the case for a supernova that exploded at about the same time as GRB 020405, thus pointing to a massive stellar progenitor for this burst. This is the first clear association of a massive progenitor with a uniform medium, indicating that a ρ~ r^{-2} profile is not a required signature, and in fact may not be present on the lengthscales probed by the afterglow in the majority of bursts.

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arXiv2003

Discovery of Early Optical Emission from GRB 021211

We report our discovery and early time optical, near-infrared, and radio wavelength follow-up observations of the afterglow of the gamma-ray burst GRB 021211. Our optical observations, beginning 21 min after the burst trigger, demonstrate that the early afterglow of this burst is roughly three magnitudes fainter than the afterglow of GRB 990123 at similar epochs, and fainter than almost all known afterglows at an epoch of 1d after the GRB. Our near-infrared and optical observations indicate that this is not due to extinction. Combining our observations with data reported by other groups, we identify the signature of a reverse shock. This reverse shock is not detected to a 3-sigma limit of 110 uJy in an 8.46-GHz VLA observation at t=0.10d, implying either that the Lorentz factor of the burst gamma <~ 200, or that synchrotron self-absorption effects dominate the radio emission at this time. Our early optical observations, near the peak of the optical afterglow (forward shock), allow us to characterize the afterglow in detail. Comparing our model to flux upper limits from the VLA at later times, t >~ 1 week, we find that the late-time radio flux is suppressed by a factor of two relative to the >~ 80 uJy peak flux at optical wavelengths. This suppression is not likely to be due to synchrotron self-absorption or an early jet break, and we suggest instead that the burst may have suffered substantial radiative corrections.

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arXiv2003

The Cosmic Gamma-Ray Bursts and Their Host Galaxies in a Cosmological Context

Studies of the cosmic gamma-ray bursts (GRBs) and their host galaxies are now starting to provide interesting or even unique new insights in observational cosmology. Observed GRB host galaxies have a median magnitude R ~ 25 mag, and show a range of luminosities, morphologies, and star formation rates, with a median redshift z ~ 1. They represent a new way of identifying a population of star-forming galaxies at cosmological redshifts, which is mostly independent of the traditional selection methods. They seem to be broadly similar to the normal field galaxy populations at comparable redshifts and magnitudes, and indicate at most a mild luminosity evolution over the redshift range they probe. Studies of GRB optical afterglows seen in absorption provide a powerful new probe of the ISM in dense, central regions of their host galaxies, which is complementary to the traditional studies using QSO absorption line systems. Some GRB hosts are heavily obscured, and provide a new way to select a population of cosmological sub-mm sources. A census of detected optical tranistents may provide an important new way to constrain the total obscured fraction of star formation over the history of the universe. Finally, detection of GRB afterglows at high redshifts (z > 6) may provide a unique way to probe the primordial star formation, massive IMF, early IGM, and chemical enrichment at the end of the cosmic reionization era.

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arXiv2002

Optical Spectropolarimetry of the GRB 020813 Afterglow

The optical afterglow of gamma-ray burst 020813 was observed for 3 hours with the LRIS spectropolarimeter at the Keck-I telescope, beginning 4.7 hours after the burst was detected by HETE-2. The spectrum reveals numerous metal absorption lines that we identify with two systems at z=1.223 and z=1.255. We also detect an O II 3727 emission line at z=1.255 and we identify this galaxy as the likely host of the GRB. After a correction for Galactic interstellar polarization, the optical afterglow has a linear polarization of 1.8-2.4% during 4.7-7.9 hours after the burst. A measurement of p = 0.80% +/- 0.16% on the following night by Covino et al. demonstrates significant polarization variability over the next 14 hours. The lack of strong variability in the position angle of linear polarization indicates that the magnetic field in the jet is likely to be globally ordered rather than composed of a number of randomly oriented cells. Within the framework of afterglow models with collimated flows, the relatively low observed polarization suggests that the magnetic field components perpendicular and parallel to the shock front are only different by about 20%.

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