arXiv subjects

R. A. Chevalier

Publications and source records attributed to R. A. Chevalier.

At least 37 records · Page 2Linked to original sources

arXiv2007

GRB 060505: A possible short-duration gamma-ray burst in a star forming region at redshift of 0.09

On 2006 May 5, a four second duration, low energy, ~10^49 erg, Gamma-Ray Burst (GRB) was observed, spatially associated with a z=0.0894 galaxy. Here, we report the discovery of the GRB optical afterglow and observations of its environment using Gemini-south, Hubble Space Telescope (HST), Chandra, Swift and the Very Large Array. The optical afterglow of this GRB is spatially associated with a prominent star forming region in the Sc-type galaxy 2dFGRS S173Z112. Its proximity to a star forming region suggests that the progenitor delay time, from birth to explosion, is smaller than about 10 Myr. Our HST deep imaging rules out the presence of a supernova brighter than an absolute magnitude of about -11 (or -12.6 in case of ``maximal'' extinction) at about two weeks after the burst, and limits the ejected mass of radioactive Nickel 56 to be less than about 2x10^-4 solar mass (assuming no extinction). Although it was suggested that GRB 060505 may belong to a new class of long-duration GRBs with no supernova, we argue that the simplest interpretation is that the physical mechanism responsible for this burst is the same as for short-duration GRBs.

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arXiv2006

Circumstellar Interaction Around Type Ib/c Supernovae and the GRB Connection

Radio observations of Type Ib/c supernovae suggest that circumstellar interaction takes place with a wide range of wind densities, comparable to that seen in Galactic Wolf-Rayet stars. Efficient production of magnetic field in the shocked region is needed. The X-ray emission observed from some Type Ib/c supernovae is higher than would be expected by the thermal or inverse Compton mechanisms; a synchrotron interpretation requires a flattening of the electron energy spectrum at high energies, as might occur in a cosmic ray dominated shock wave. The wind density variations that are indicated in two supernovae may be due to a binary companion, although variable mass loss from a single star remains a possibility. Other than the optical supernova radiation, the emission from the nearby SN 2006aj/GRB 060218 appears to be powered by a central engine, while that from SN 1998bw/GRB 980425 could be powered by either a central engine or the outer supernova ejecta.

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arXiv2006

Relativistic ejecta from XRF 060218 and the rate of cosmic explosions

Over the last decade, long-duration gamma-ray bursts (GRBs) including the subclass of X-ray flashes (XRFs) have been revealed to be a rare variety of Type Ibc supernova (SN). While all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary Type Ibc SNe by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just an SN, remains the crucial open question. Here we present radio and X-ray observations of XRF 060218 (associated with SN 2006aj), the second nearest GRB identified to-date, which allow us to measure its total energy and place it in the larger context of cosmic explosions. We show that this event is 100 times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary Type Ibc SNe by the presence of 10^48 erg coupled to mildly-relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) which produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs/XRFs and ordinary SNe, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.

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arXiv2006

Multi-Wavelength Observations of GRB 050820A: An Exceptionally Energetic Event Followed from Start to Finish

We present observations of the unusually bright and long gamma-ray burst GRB 050820A, one of the best-sampled broadband data sets in the Swift era. The gamma-ray light curve is marked by a soft precursor pulse some 200 s before the main event; the lack of any intervening emission suggests that it is due to a physical mechanism distinct from the GRB itself. The large time lag between the precursor and the main emission enabled simultaneous observations in the gamma-ray, X-ray, and optical band-passes, something only achieved for a handful of events to date. While the contemporaneous X-rays are the low-energy tail of the prompt emission, the optical does not directly track the gamma-ray flux. Instead, the early-time optical data appear mostly consistent with the forward shock synchrotron peak passing through the optical, and are therefore likely the beginning of the afterglow. On hour time scales after the burst, the X-ray and optical light curves are inconsistent with an adiabatic expansion of the shock into the surrounding region, but rather indicate that there is a period of energy injection. Observations at late times allow us to constrain the collimation angle of the relativistic outflow to theta = 6.8 - 9.3 degrees. Our estimates of both the kinetic energy of the afterglow and the prompt gamma-ray energy release make GRB 050820A one of the most energetic events for which such values could be determined.

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arXiv2006

X-Ray Observations of Type Ia Supernovae with Swift: Evidence for Circumstellar Interaction for SN 2005ke

We present a study of the early (days to weeks) X-ray and UV properties of eight Type Ia supernovae (SNe Ia) which have been extensively observed with the X-Ray Telescope (XRT) and UV/Optical Telescope (UVOT) onboard Swift, ranging from 5-132 days after the outburst. SN 2005ke is tentatively detected (at a 3-3.6 sigma level of significance) in X-rays based on deep monitoring with the XRT ranging from 8 to 120 days after the outburst. The inferred X-ray luminosity [(2+/-1) x 10^{38} ergs/s; 0.3-2 keV band] is likely caused by interaction of the SN shock with circumstellar material (CSM), deposited by a stellar wind from the progenitor's companion star with a mass-loss rate of ~ 3 x 10^{-6} M_sun/yr (v_w/10 km/s). Evidence of CSM interaction in X-rays is independently confirmed by an excess of UV emission as observed with the UVOT onboard Swift, starting around 35 days after the explosion. The non-detection of SN 2005ke with Chandra 105 days after the outburst implies a rate of decline steeper than L_x \propto t^{-0.75}, consistent with the decline expected from the interaction of the SN shock with a spherically symmetric CSM (t^{-1}). None of the other seven SNe Ia is detected in X-rays or shows a UV excess, which allows us to put tight constraints on the mass-loss rates of the progenitor systems.

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arXiv2006

The Radio and X-ray Luminous SN 2003bg and the Circumstellar Density Variations Around Radio Supernovae

We report extensive radio and X-ray observations of SN 2003bg whose spectroscopic evolution shows a transition from a broad-lined Type Ic to a hydrogen-rich Type II and later to a typical hydrogen-poor Type Ibc. We show that the extraordinarily luminous radio emission is well described by a self-absorption dominated synchrotron spectrum while the observed X-ray emission at t~30 days is adequately fit by Inverse Compton scattering of the optical photons off of the synchrotron emitting electrons. Our radio model implies a sub-relativistic ejecta velocity, v ~ 0.24c, at t_0 ~ 10 days after the explosion which emphasizes that broad optical absorption lines do not imply relativistic ejecta. We find that the total energy of the radio emitting region evolves as E ~ 7.3 x 10^{48} (t/t_0)^{0.4} erg assuming equipartition of energy between relativistic electrons and magnetic fields (e_e=e_B=0.1). The circumstellar density is well described by a stellar wind profile with modest (factor of ~2) episodic density enhancements which produce abrupt achromatic flux variations. We estimate an average progenitor mass loss rate of M_dot ~ 3 x 10^{-4} solar masses per year (assuming a wind velocity of 1000 km/s), consistent with the observed values for Galactic Wolf-Rayet stars. Comparison with other events reveals that ~50% of radio supernovae show similar short timescale flux variations attributable to circumstellar density irregularities. Specifically, the radio light-curves of SN 2003bg are strikingly similar to those of the Type IIb SN 2001ig, suggestive of a common progenitor evolution. We conclude that their progenitors experienced quasi-periodic mass loss episodes just prior to the SN explosion. [ABRIDGED]

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arXiv2006

The Expansion Asymmetry and Age of the Cassiopeia A Supernova Remnant

HST ACS images of the young SN remnant Cas A are used to explore the expansion and spatial distribution of its highest velocity debris. Proper motions of over 1800 outlying ejecta knots are reported. The distribution of transverse expansion velocities for these knots shows a striking bipolar asymmetry with the highest velocity knots confined to nearly opposing northeast and southwest `jets'. The jets appear kinematically and chemically distinct with respect to the remnant's highest velocity debris seen in other directions. Significant gaps in the spatial distribution of outlying ejecta lie in directions which are approximately perpendicular to the jets. Extrapolations of 9 month proper motions for all outer ejecta knots and a subsample of 72 bright and compact knots suggest explosion dates (assuming no knot deceleration) of 1662 +/- 27 and 1672 +/- 18, respectively. We find some evidence for non-uniform deceleration in different directions with knots located along the northwestern limb among the least decelerated ejecta suggesting a convergence date of 1681 +/-19. The remnant's central X-ray point source lies some $7''$ to the southeast of the estimated expansion center (PA = 169 deg) indicating a projected motion of ~350 km/s toward the middle of the broad southern outer ejecta knot gap.

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arXiv2005

An HST Study of the Supernovae Accompanying GRB 040924 and GRB 041006

We present the results from a {\it Hubble Space Telescope/ACS} study of the supernovae associated with gamma-ray bursts 040924 ($z=0.86$) and 041006 ($z=0.71$). We find evidence that both GRBs were associated with a SN 1998bw-like supernova dimmed by $\sim 1.5$ and $\sim 0.3$ magnitudes, respectively, making GRB 040924 the faintest GRB-associated SN ever detected. We study the luminosity dispersion in GRB/XRF-associated SNe and compare to local Type Ibc supernovae from the literature. We find significant overlap between the two samples, suggesting that GRB/XRF-associated SNe are not necessarily more luminous nor produce more $^{56}$Ni than local SNe. Based on the current (limited) datasets, we find that the two samples may share a similar $^{56}$Ni production mechanism.

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arXiv2005

The afterglow of GRB050709 and the nature of the short-hard gamma-ray bursts

The final chapter in the long-standing mystery of the gamma-ray bursts (GRBs) centres on the origin of the short-hard class, suspected on theoretical grounds to result from the coalescence of neutron star or black hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical, and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst whose accurate position allows us to unambiguously associate it with a star-forming galaxy at redshift z=0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.

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arXiv2005

Discovery of Outlying, High-Velocity Oxygen-Rich Ejecta in Cassiopeia A

Hubble Space Telescope images of the young Galactic supernova remnant Cassiopeia A reveal a far larger population of outlying, high-velocity knots of ejecta with a broader range of chemical properties than previously suspected. We identify three main classes of outer ejecta: 1) Knots dominated by [N II] 6548,6583 emission; 2) Knots dominated by oxygen emission lines especially [O II] 7319,7330; and 3) Knots with emission line strengths similar to the [S II] strong FMK ejecta commonly seen in the main emission shell. The discovery of a significant population of O-rich ejecta situated in between the suspected N-rich outer photospheric layer and S-rich FMK-like ejecta suggests that the Cas A progenitor's chemical layers were not completely disrupted by the supernova explosion outside of the remnant's NE and SW high velocity `jet' regions. In addition, we find the majority of O-rich outer ejecta at projected locations out beyond (v = 6500 - 9000 km/s) the remnant's fastest moving Fe-rich X-ray emission material (6000 km/s) seen in Chandra and XMM data along the eastern limb. This suggests that penetration of Fe-rich material up through the S and Si-rich mantle did not extend past the progenitor's N or O-rich outer layers for at least this section of the remnant.

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arXiv2005

Limits from the Hubble Space Telescope on a Point Source in SN 1987A

We observed supernova 1987A (SN 1987A) with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) in 1999 September, and again with the Advanced Camera for Surveys (ACS) on the HST in 2003 November. No point source is observed in the remnant. We obtain a limiting flux of F_opt < 1.6 x 10^{-14} ergs/s/cm^2 in the wavelength range 2900-9650 Angstroms for any continuum emitter at the center of the supernova remnant (SNR). It is likely that the SNR contains opaque dust that absorbs UV and optical emission, resulting in an attenuation of ~35% due to dust absorption in the SNR. Taking into account dust absorption in the remnant, we find a limit of L_opt < 8 x 10^{33} ergs/s. We compare this upper bound with empirical evidence from point sources in other supernova remnants, and with theoretical models for possible compact sources. Bright young pulsars such as Kes 75 or the Crab pulsar are excluded by optical and X-ray limits on SN 1987A. Of the young pulsars known to be associated with SNRs, those with ages < 5000 years are all too bright in X-rays to be compatible with the limits on SN 1987A. Examining theoretical models for accretion onto a compact object, we find that spherical accretion onto a neutron star is firmly ruled out, and that spherical accretion onto a black hole is possible only if there is a larger amount of dust absorption in the remnant than predicted. In the case of thin-disk accretion, our flux limit requires a small disk, no larger than 10^{10} cm, with an accretion rate no more than 0.3 times the Eddington accretion rate. Possible ways to hide a surviving compact object include the removal of all surrounding material at early times by a photon-driven wind, a small accretion disk, or very high levels of dust absorption in the remnant.

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arXiv2005

An HST Search for Supernovae Accompanying X-ray Flashes

We present the results from an Hubble Space Telescope/ACS search for supernovae associated with X-ray flashes 020903, 040701, 040812 and 040916. We find strong evidence that XRF 020903 (z=0.25) was associated with a SN 1998bw-like supernova and confirm this using optical spectroscopy at t ~ 25 days. We find no evidence, however, for SN 1998bw-like supernovae associated with the other three events. In the case of XRF 040701 (z=0.21), we rule out even a faint supernova similar to SN 2002ap, using template light-curves for several local Type Ic supernovae. For the two cases in which the redshift is not known, XRFs 040812 and 040916, we derive robust redshift limits assuming they were accompanied by supernovae similar to SN 1998bw and compare these limits with photometric redshift constraints provided by their host galaxies. We supplement this analysis with results for three additional events (XRFs 011030, 020427 and 030723) and discuss the observed diversity of supernovae associated with X-ray flashes and gamma-ray bursts. We conclude that XRF-SNe exist, but can be significantly fainter than SN 1998bw, possibly consistent with the observed spread in local Type Ibc supernovae.

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arXiv2004

Hard X-ray Emission Clumps in the gamma-Cygni Supernova Remnant: an INTEGRAL-ISGRI View

Spatially resolved images of the galactic supernova remnant G78.2+2.1 (gamma-Cygni) in hard X-ray energy bands from 25 keV to 120 keV are obtained with the IBIS-ISGRI imager aboard the International Gamma-Ray Astrophysics Laboratory INTEGRAL. The images are dominated by localized clumps of about ten arcmin in size. The flux of the most prominent North-Western (NW) clump is (1.7 +/- 0.4) 10^{-11} erg/cm^2/s in the 25-40 keV band. The observed X-ray fluxes are in agreement with extrapolations of soft X-ray imaging observations of gamma-Cygni by ASCA GIS and spatially unresolved RXTE PCA data. The positions of the hard X-ray clumps correlate with bright patches of optical line emission, possibly indicating the presence of radiative shock waves in a shocked cloud. The observed spatial structure and spectra are consistent with model predictions of hard X-ray emission from nonthermal electrons accelerated by a radiative shock in a supernova interacting with an interstellar cloud, but the powerful stellar wind of the O9V star HD 193322 is a plausible candidate for the NW source as well.

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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

Circumstellar interaction of the type Ia supernova 2002ic

We propose a model to account for the bolometric light curve, quasi-continuum and the Ca II emission features of the peculiar type Ia supernova (SN) 2002ic, which exploded in a dense circumstellar envelope. The model suggests that the SN Ia had the maximum possible kinetic energy and that the ejecta expand in an approximately spherically symmetric (possibly clumpy) circumstellar environment. The Ca II and quasi-continuum are emitted by shocked SN ejecta that underwent deformation and fragmentation in the intershock layer. Modeling of the Ca II triplet implies that the contribution of the O I 8446 A line is about 25% of the 8500 A feature on day 234, which permits us to recover the flux in the Ca II 8579 A triplet from the flux of 8500 A blend reported by Deng et al. (2004). We use the Ca II doublet and triplet fluxes on day 234 to derive the electron temperature (~4400 K) in the Ca II line-emitting zone and the ratio of the total area of dense fragments to the area of the shell, S/S_0 ~ 100. We argue that Ca II bands and quasi-continuum originate from different zones of the shocked ejecta that reflect the abundance stratification of the supernova.

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arXiv2003

Discovery of GRB 020405 and its Late Red Bump

We present the discovery of GRB 020405 made with the Inter-Planetary Network (IPN). With a duration of 60 s, the burst appears to be a typical long duration event. We observed the 75-square acrminute IPN error region with the Mount Stromlo Observatory's 50-inch robotic telescope and discovered a transient source which subsequently decayed and was also associated with a variable radio source. We identify this source as the afterglow of GRB 020405. Subsequent observations by other groups found varying polarized flux and established a redshift of 0.690 to the host galaxy. Motivated by the low redshift we triggered observations with WFPC2 on-board the Hubble Space Telescope (HST). Modeling the early ground-based data with a jet model, we find a clear red excess over the decaying optical lightcurves that is present between day 10 and day 141 (the last HST epoch). This `bump' has the spectral and temporal features expected of an underlying supernova (SN). In particular, the red color of the putative SN is similar to that of the SN associated with GRB 011121, at late time. Restricting the sample of GRBs to those with z<0.7, a total of five bursts, red bumps at late times are found in GRB 970228, GRB 011121, and GRB 020405. It is possible that the simplest idea, namely that all long duration GRBs have underlying SNe with a modest dispersion in their properties (especially peak luminosity), is sufficient to explain the non detections.

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arXiv2002

The Faint Optical Afterglow and Host Galaxy of GRB 020124: Implications for the Nature of Dark Gamma-Ray Bursts

We present ground-based optical observations of GRB 020124 starting 1.6 hours after the burst, as well as subsequent Very Large Array (VLA) and Hubble Space Telescope (HST) observations. The optical afterglow of GRB 020124 is one of the faintest afterglows detected to date, and it exhibits a relatively rapid decay, $F_ν\propto t^{-1.60\pm 0.04}$, followed by further steepening. In addition, a weak radio source was found coincident with the optical afterglow. The HST observations reveal that a positionally coincident host galaxy must be the faintest host to date, R>29.5 mag. The afterglow observations can be explained by several models requiring little or no extinction within the host galaxy, A_V~0-0.9 mag. These observations have significant implications for the statistics of the so-called dark bursts (bursts for which no optical afterglow is detected), which are usually attributed to dust extinction within the host galaxy. The faintness and relatively rapid decay of the afterglow of GRB 020124, combined with the low inferred extinction indicate that some dark bursts are intrinsically dim and not dust obscured. Thus, the diversity in the underlying properties of optical afterglows must be observationally determined before substantive inferences can be drawn from the statistics of dark bursts.

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arXiv2002

GRB 010921: Strong Limits on an Underlying Supernova from HST

GRB 010921 was the first HETE-2 GRB to be localized via its afterglow emission. The low-redshift of the host galaxy, z=0.451, prompted us to undertake intensive multi-color observations with the Hubble Space Telescope with the goal of searching for an underlying supernova component. We do not detect any coincident supernova to a limit 1.34 mag fainter than SN 1998bw at 99.7% confidence, making this one of the most sensitive searches for an underlying SN. Analysis of the afterglow data allow us to infer that the GRB was situated behind a net extinction (Milky Way and the host galaxy) of A_V ~ 1.8 mag in the observer frame. Thus, had it not been for such heavy extinction our data would have allowed us to probe for an underlying SN with brightness approaching those of more typical Type Ib/c supernovae.

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