arXiv subjects

J. C. Wheeler

Publications and source records attributed to J. C. Wheeler.

At least 127 records · Page 7Linked to original sources

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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arXiv2003

GRB 021004: a Massive Progenitor Star Surrounded by Shells

We present spectra of the optical transient of GRB021004 obtained with the Hobby-Eberly telescope starting 15.48, 20.31 hours, and 4.84 days after the burst and a spectrum obtained with the H. J. Smith 2.7 m Telescope starting 14.31 hours after the burst. GRB021004 is the first afterglow whose spectrum is dominated by absorption lines from high ionization species with multiple velocity components separated by up to 3000 km/s. We argue that these lines are likely to come from shells around a massive progenitor star. The high velocities and high ionizations arise from a combination of acceleration and flash-ionization by the burst photons and the wind velocity and steady ionization by the progenitor. We also analyze the broad-band spectrum and the light curve. We distinguish six components along the line of sight: (1) The z~2.293 absorption lines arise from the wind of a massive star. For a mass loss rate of ~6 x 10^{-5} solar masses per year, this component also provides the external medium to create the afterglow light. (2) A second shell produces absorption lines with a relative velocity of 560 km/s, and this is associated with the shell created by the fast massive star wind blowing a bubble in the preceding slow wind at a radial distance of order 10 pc. (3) More distant clouds within the host galaxy lie between 30-2500 pc, where they have been ionized by the burst. (4-6) The massive star wind has clumps with radii and over-densities of 0.022, 0.063, and 0.12 parsecs and 50%, 10%, and 10% respectively. The immediate progenitor of the burster could either be a WC-type Wolf-Rayet star or a highly evolved star whose original mass was just too small for it to become a WN-type Wolf-Rayet star.

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arXiv2003

Spectropolarimetry of GRB 021004 - Evidence for High Velocity Lyman-alpha Absorptions

We present spectropolarimetry observations of GRB 021004 obtained at the ESO VLT. We detect a remarkable increase of the degree of polarization blueward of the host galaxy rest frame Ly-alpha to about 5-10% that corresponds to a decrease in the continuum flux below the extrapolation of a power law. An increase of the degree of polarization is perhaps also recorded in some narrow lines, but of lower statistical significance. The broad polarization feature blueward of the Ly-alpha absorption line at 4040 Åis at least partially produced by hydrogen-rich material with velocities approaching 40,000 \kms\ located beyond the front of the afterglow shock, rather than due entirely to intergalactic Ly-alpha absorption systems on the line of sight to the GRB. The presence of the broad Ly-alpha absorption provides evidence that the GRB occurred inside the ejecta from a hydrogen-rich supernova that exploded before the GRB.

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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

Aspherical Supernovae Explosions

Core collapse supernovae(SN) are the final stages of evolution in massive stars during which the central region collapses. Recent explosion scenarios assumed that the ejection is due to energy deposition by neutrinos into the envelope but detailed models do not produce powerful explosions. There is new and mounting evidence for an asphericity and, in particular, for axial symmetry in several SNe which may be hard to reconcile within the spherical picture. This evidence includes the observed high polarization and its variation with time, pulsar kicks, high velocity iron-group and intermediate-mass elements material observed in remnants, direct observations of the debris of SN1987A etc. Some of the new evidence is discussed in more detail. To be in agreement with the observations, any successful mechanism must invoke some sort of axial symmetry for the explosion. We consider jet-induced/dominated explosions of core collapse SNe. Our study is based on detailed 3-D hydrodynamical and radiation transport models We find that the observations can be explained by low velocity, massive jets which stall well within the SN envelope. Such outflows may be produced by MHD- mechanisms, convective dominated accretion disks on the central object or asymmetric neutrino emissions. Asymmetric density/chemical distributions and, for SN2002ap, off-center energy depositions have been identified as crucial for the interpretation of the polarization.

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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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arXiv2002

Detection of a supernova signature associated with GRB 011121

Using observations from an extensive monitoring campaign with the Hubble Space Telescope we present the detection of an intermediate-time flux excess which is redder in color relative to the afterglow of GRB 011121, currently distinguished as the gamma-ray burst with the lowest known redshift. The red ``bump,'' which exhibits a spectral roll-over at ~7200 Angstrom, is well described by a redshifted Type Ic supernova that occurred approximately at the same time as the gamma-ray burst event. The inferred luminosity is about half that of the bright supernova 1998bw. These results serve as compelling evidence for a massive star origin of long-duration gamma-ray bursts. Models that posit a supernova explosion weeks to months preceding the gamma-ray burst event are excluded by these observations. Finally, we discuss the relationship between spherical core-collapse supernovae and gamma-ray bursts.

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arXiv2002

The Axially Symmetric Ejecta of Supernova 1987A

Extensive early observations proved that the ejecta of supernova 1987A (SN 1987A) are aspherical. Fifteen years after the supernova explosion, the Hubble Space Telescope has resolved the rapidly expanding ejecta. The late-time images and spectroscopy provide a geometrical picture that is consistent with early observations and suggests a highly structured, axially symmetric geometry. We present here a new synthesis of the old and new data. We show that the Bochum event, presumably a clump of $^{56}$Ni, and the late-time image, the locus of excitation by $^{44}$Ti, are most naturally accounted for by sharing a common position angle of about 14\degree, the same as the mystery spot and early speckle data on the ejecta, and that they are both oriented along the axis of the inner circumstellar ring at 45\degree to the plane of the sky. We also demonstrate that the polarization represents a prolate geometry with the same position angle and axis as the early speckle data and the late-time image and hence that the geometry has been fixed in time and throughout the ejecta. The Bochum event and the Doppler kinematics of the [Ca II]/[O II] emission in spatially resolved HST spectra of the ejecta can be consistently integrated into this geometry. The radioactive clump is deduced to fall approximately along the axis of the inner circumstellar ring and therefore to be redshifted in the North whereas the [Ca II]/[O II] 7300 Åemission is redshifted in the South. We present a jet-induced model for the explosion and argue that such a model can account for many of the observed asymmetries. In the jet models, the oxygen and calcium are not expected to be distributed along the jet, but primarily in an expanding torus that shares the plane and northern blue shift of the inner circumstellar ring.

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arXiv2002

Supernova 1998bw - The final phases

The probable association with GRB 980425 immediately put SN 1998bw at the forefront of supernova research. Here, we present revised late-time BVRI light curves of the supernova, based on template images taken at the VLT. To follow the supernova to the very last observable phases we have used HST/STIS. Deep images taken in June and November 2000 are compared to images taken in August 2001. The identification of the supernova is firmly established. This allows us to measure the light curve to about 1000 days past explosion. The main features are a rapid decline up to more than 500 days after explosion, with no sign of complete positron trapping from the Cobolt-56 decay. Thereafter, the light curve flattens out significantly. One possible explanation is powering by more long lived radioactive isotopes, if they are abundantly formed in this energetic supernova.

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arXiv2002

Extraordinary Late-Time Infrared Emission of Type IIn Supernovae

Near-Infrared (NIR) observations are presented for five Type IIn supernovae (SN 1995N, SN 1997ab, SN 1998S, SN 1999Z, and SN 1999el) that exhibit strong infrared excesses at late times (t >= 100 d). H- and K-band emission from these objects is dominated by a continuum that rises toward longer wavelengths. The data are interpreted as thermal emission from dust, probably situated in a pre-existing circumstellar nebula. The IR luminosities implied by single temperature blackbody fits are quite large,> 10^(41 - 42) erg s^-1, and the emission evolves slowly, lasting for years after maximum light. For SN 1995N, the integrated energy release via IR dust emission was 0.5 -- 1 * 10^50 erg. A number of dust heating scenarios are considered, the most likely being an infrared echo poweredby X-ray and UV emissions from the shock interaction with a dense circumstellar medium.

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arXiv2000

Preliminary Spectral Analysis of the Type II Supernova 1999em

We have calculated fast direct spectral model fits to two early-time spectra of the Type-II plateau SN 1999em, using the SYNOW synthetic spectrum code. The first is an extremely early blue optical spectrum and the second a combined HST and optical spectrum obtained one week later. Spectroscopically this supernova appears to be a normal Type II and these fits are in excellent agreement with the observed spectra. Our direct analysis suggests the presence of enhanced nitrogen. We have further studied these spectra with the full NLTE general model atmosphere code PHOENIX. While we do not find confirmation for enhanced nitrogen (nor do we rule it out), we do require enhanced helium. An even more intriguing possible line identification is complicated Balmer and He I lines, which we show falls naturally out of the detailed calculations with a shallow density gradient. We also show that very early spectra such as those presented here combined with sophisticated spectral modeling allows an independent estimate of the total reddening to the supernova, since when the spectrum is very blue, dereddening leads to changes in the blue flux that cannot be reproduced by altering the ``temperature'' of the emitted radiation. These results are extremely encouraging since they imply that detailed modeling of early spectra can shed light on both the abundances and total extinction of SNe II, the latter improving their utility and reliability as distance indicators.

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arXiv2000

Analysis of the Type IIn Supernova 1998S: Effects of Circumstellar Interaction on Observed Spectra

We present spectral analysis of early observations of the Type IIn supernova 1998S using the general non-local thermodynamic equilibrium atmosphere code \tt PHOENIX}. We model both the underlying supernova spectrum and the overlying circumstellar interaction region and produce spectra in good agreement with observations. The early spectra are well fit by lines produced primarily in the circumstellar region itself, and later spectra are due primarily to the supernova ejecta. Intermediate spectra are affected by both regions. A mass-loss rate of order $\dot M \sim 0.0001-0.001$\msol yr$^{-1}$ is inferred for a wind speed of 100-1000 \kmps. We discuss how future self-consistent models will better clarify the underlying progenitor structure.

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arXiv2000

Constraints on Type Ia Supernovae from Near Infrared Spectra

We describe a research program to improve the understanding of Type Ia Supernovae (SNe Ia) by modeling and observing near infrared (NIR) spectra of these events. The NIR between 0.9 microns and 2.5 microns is optimal for examining certain products of the SNe Ia explosion that may be blended or obscured in other spectral regions. NIR analysis will enable us to place important constraints on the physical properties of SNe Ia progenitors and their explosion dynamics. These are critical steps toward understanding the physics of Type Ia Supernovae. We have identified features in NIR spectra of SNe Ia that discriminate between Population I and Population II progenitors. These features can significantly restrict the evolutionary history of SNe Ia. We also examine certain products of the nuclear burning that enable us to place constraints on the propagation of nuclear burning during the explosion, and on the behavior of the burning front during the event. We will be able to differentiate between the several explosion models for SNe Ia.

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arXiv1999

Influence of the Stellar Population on Type Ia Supernovae: Consequences for the Determination of Omega

The influence of the metallicity at the main sequence on the chemical structure of the exploding white dwarf, the nucleosynthesis during the explosion and the light curves of an individual Type Ia supernovae have been studied. Detailed calculations of the stellar evolution, the explosion, and light curves of delayed detonation models are presented. Detailed stellar evolution calculations with a main sequence mass of 7 M_o have been performed to test the influence of the metallicity Z on the structure of the progenitor. A change of Z influences the central helium burning and, consequently, the size of the C/O core which becomes a C/O white dwarf and its C/O ratio. Subsequently, the white dwarf may grow to the Chandrasekhar mass and explode as a Type Ia supernovae. Consequently, the C/O structure of the exploding white dwarf depends on Z. Since C and O are the fuel for the thermonuclear explosion, Z indirectly changes the energetics of the explosion. In our example, changing Z from Pop I to Pop II causes a change in the zero point of the maximum brightness/decline relation by about $0.1^m$ and a change in the rise time by about 1 day. Combined with previous studies, We provide a relation between a change of the rise time in SNeIa and the off-set in the brightness/decline relation. We find that dM = dt where dM is the offset in the relation and dt is the change of the rise time in days. This relation provides a way to detect and test for evolutionary effects, and to estimate the possible consequences for the determination of the cosmological constant. However, due to uncertainties in the time evolution of the properties of the progenitors, we cannot predict whether evolution becomes important at z=0.5, 1 or 2. This question has to be answered by observations.

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arXiv1999

The Role of Kinetic Energy Flux in the Convective Urca Process

The previous analysis of the convective Urca neutrino loss process in degenerate, convective, quasi-static, carbon-burning cores by Barkat and Wheeler omitted specific consideration of the role of the kinetic energy flux. The arguments of Barkat and Wheeler that steady-state composition gradients exist are correct, but chemical equilibrium does not result in net cooling. Barkat and Wheeler included a "work" term that effectively removed energy from the total energy budget that could only have come from the kinetic energy, which must remain positive. Consideration of the kinetic energy in the thermodynamics of the convective Urca process shows that the convective Urca neutrinos reduce the rate of increase of entropy that would otherwise be associated with the input of nuclear energy and slow down the convective current, but, unlike the "thermal" Urca process do not reduce the entropy or temperature.

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arXiv1999

Jet-Induced Explosions of Core Collapse Supernovae

We numerically studied the explosion of a supernova caused by supersonic jets present in its center. The jets are assumed to be generated by a magneto-rotational mechanism when a stellar core collapses into a neutron star. We simulated the process of the jet propagation through the star, jet breakthrough, and the ejection of the supernova envelope by the lateral shocks generated during jet propagation. The end result of the interaction is a highly nonspherical supernova explosion with two high-velocity jets of material moving in polar directions, and a slower moving, oblate, highly distorted ejecta containing most of the supernova material. The jet-induced explosion is entirely due to the action of the jets on the surrounding star and does not depend on neutrino transport or re-acceleration of a stalled shock. The jet mechanism can explain the observed high polarization of Type Ib,c and Type II supernovae, pulsar kicks, very high velocity material observed in supernova remnants, indications that radioactive material was carried to the hydrogen-rich layers in SN1987A, and some others observations that are very difficult or impossible to explain by the neutrino energy deposition mechanism. The breakout of the jet from a compact, hydrogen- deficient core may account for the gamma-ray bursts and radio outburst associated with SN1998bw/GRB980425.

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arXiv1998

Aspherical Explosion Models for SN 1998bw/GRB 980425

The recent discovery of the unusual supernova SN1998bw and its apparent correlation with the gamma-ray burst GRB 980425 has raised new issues concerning both the GRB and supernovae. Although the spectra resemble those of TypeIc supernovae, there are distinct differences at early times and SN1998bw appeared to be unusually bright and red at maximum light. The apparent expansion velocities inferred by the Doppler shift of (unidentified) absorption features appeared to be high, making SN1998bw a possible candidate for a "hypernova" with explosion energies between 20 and 50E51 erg and ejecta masses in excess of 6 - 15 M_o. Based on light curve calculations for aspherical explosions and guided by the polarization observations of "normal" SNIc and related events, we present an alternative picture that allows SN1998bw to have an explosion energy and ejecta mass consistent with core collapse supernovae (although at the 'bright' end). We show that the LC of SN1998bw can be understood as result of an aspherical explosion along the rotational axis of a basically spherical, non-degenerate C/O core of massive star with an explosion energy of 2foe and a total ejecta mass of 2 M_o if it is seen from high inclinations with respect to the plane of symmetry. In this model, the high expansion velocities are a direct consequence of an aspherical explosion which, in turn, produces oblate iso-density contours. It suggests that the fundamental core-collapse explosion process itself is strongly asymmetric.

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arXiv1998

SN 1998bw/GRB 980425: Hypernova or Aspherical Explosion?

The recent discovery of the unusual supernova SN1998bw and its apparent correlation with the gamma-ray burst GRB 980425 has raised new issues concerning both the GRB and SNe. SN1998bw was unusually bright at maximum light and expansion velocities were large making SN1998bw a possible candidate for a "hypernova" with explosion energies exceeding 10^{52} erg. We show that the light curve of SN1998bw can be understood as the result of an aspherical explosion along the rotational axis of a basically spherical, non-degenerate C/O core of a massive star with an explosion energy of 2E51 erg, a total ejecta mass of 2 M_o, and a 56Ni mass of 0.2M_o if it is seen from high inclinations with respect to the plane of symmetry.In this model, the high expansion velocities are a direct consequence of the aspherical explosion which, in turn, produces oblate iso-density contours. This suggests that the fundamental core-collapse explosion process itself is strongly asymmetric.

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