* Astronomy

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XRT 080109

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RE: Supernova 2008D

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Title: From Shock Breakout to Peak and Beyond: Extensive Panchromatic Observations of the Type Ib Supernova 2008D associated with Swift X-ray Transient 080109
Authors: Maryam Modjaz, W. Li, N. Butler, R. Chornock, D. Perley, S. Blondin, J. S. Bloom, A. V. Filippenko, R. P. Kirshner, D. Kocevski, D. Poznanski, M. Hicken, R. J. Foley, G. S. Stringfellow, P. Berlind, D. Barrado y Navascues, C. H. Blake, H. Bouy, W. R. Brown, P. Challis, H. Chen, W. H. de Vries, P. Dufour, E. Falco, A. Friedman, M. Ganeshalingam, P. Garnavich, B. Holden, G. Illingworth, J. Liebert, G. H. Marion, N. Lee, S. S. Olivier, J. X. Prochaska, J. M. Silverman, N. Smith, D. Starr, T. N. Steele, A. Stockton, G. G. Williams, W. M. Wood-Vasey
(Version v3)

We present extensive early photometric (ultraviolet through near-infrared) and spectroscopic (optical and near-infrared) data on supernova (SN) 2008D as well as X-ray data analysis on the associated Swift/X-ray transient (XRT) 080109. Our data span a time range of 5 hours before the detection of the X-ray transient to 150 days after its detection, and detailed analysis allowed us to derive constraints on the nature of the SN and its progenitor; throughout we draw comparisons with results presented in the literature and find several key aspects that differ. We show that the X-ray spectrum of XRT 080109 can be fit equally well by an absorbed power law or a superposition of about equal parts of both power law and blackbody. Our data first established that SN 2008D is a spectroscopically normal SN Ib (i.e., showing conspicuous He lines), and show that SN 2008D had a relatively long rise time of 18 days and a modest optical peak luminosity. The early-time light curves of the SN are dominated by a cooling stellar envelope (for \Delta t~0.1- 4 day, most pronounced in the blue bands) followed by 56^Ni decay. We construct a reliable measurement of the bolometric output for this stripped-envelope SN, and, combined with estimates of E_K and M_ej from the literature, estimate the stellar radius R_star of its probable Wolf-Rayet progenitor. According to the model of Waxman et al. and of Chevalier & Fransson, we derive R_star^{W07}= 1.2±0.7 R_sun and R_star^{CF08}= 12±7 R_sun, respectively; the latter being more in line with typical WN stars. Spectra obtained at 3 and 4 months after maximum light show double-peaked oxygen lines that we associate with departures from spherical symmetry, as has been suggested for the inner ejecta of a number of SN Ib cores.

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Title: The Early Asymmetries of Supernova 2008D / XRF 080109
Authors: J.R. Maund, J.C. Wheeler, D.Baade, F. Patat, P.A. Hoflich, L. Wang, A. Clocchiatti

Spectropolarimetry of the Type Ib SN 2008D, associated with the XRF 080109, at two separate epochs, are presented. The epochs of these observations correspond to V-band light curve maximum and 15 days after light curve maximum (or 21 and 36 days after the XRF). We find SN 2008D to be significantly polarised, although the largest contribution is due to the interstellar polarisation component of Q_ISP=0±0.1% and U_ISP=-1.2±0.1%. At the two epochs, the spectropolarimetry of SN 2008D is classified as being D1+L(HeI)+L(Ca II). The intrinsic polarization of continuum wavelength regions is <0.4%, at both epochs, implying an asymmetry of the photosphere of <10%. Similar to other Type Ibc SNe, such as 2005bf, 2006aj and 2007gr, we observed significant polarisation corresponding to the spectral features of Ca II, He I, Mg I, Fe II and, possibly, O I 7774, about a close-to-spherically-symmetric photosphere. We introduce a new plot showing the chemically distinct line forming regions in the ejecta and comment on the apparent ubiquity of highly polarised high-velocity Ca II features in Type Ibc SNe. The polarisation angle of Ca II IR triplet was significantly different, at both epochs, to those of the other species, suggesting high-velocity Ca II forms in a separate part of the ejecta. The apparent structure in the outer layers of SN 2008D has implications for the interpretation of the early-time X-ray emission associated with shock break-out.

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Type Ib Supernova 2008D

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RE: Supernova 2008D

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Title: Type Ib Supernova 2008D associated with the Luminous X-ray Transient 080109: An Energetic Explosion of a Massive Helium Star
Authors: Masaomi Tanaka, Nozomu Tominaga, Ken'ichi Nomoto, S. Valenti, D.K. Sahu, T. Minezaki, Y. Yoshii, M. Yoshida, G.C. Anupama, S. Benetti, G. Chincarini, M. Della Valle, P. A. Mazzali, E. Pian
(Version v3)

We present a theoretical model for supernova (SN) 2008D associated with the luminous X-ray transient 080109. The bolometric light curve and optical spectra of the SN are modelled based on the progenitor models and the explosion models obtained from hydrodynamic/nucleosynthetic calculations. We find that SN 2008D is a more energetic explosion than normal core-collapse supernovae, with an ejecta mass of Mej = 5.3 ± 1.0 Msun and a kinetic energy of E = 6.0 ± 2.5 x 10^{51} erg. The progenitor star of the SN has a 6-8 Msun He core with essentially no H envelope (< 5 x 10^{-4} Msun) prior to the explosion. The main-sequence mass of the progenitor is estimated to be Mms =20-25 Msun, with additional systematic uncertainties due to convection, mass loss, rotation, and binary effects. These properties are intermediate between those of normal SNe and hypernovae associated with gamma-ray bursts. The mass of the central remnant is estimated as 1.6 - 1.8 Msun, which is near the boundary between neutron star and black hole formation.

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Title: VLBI Observations of SN 2008D
Authors: M. F. Bietenholz, A. M. Soderberg, N. Bartel
(Version v2)

We report on two epochs of very-long-baseline interferometry (VLBI) observations of the Type Ib/c supernova SN 2008D, which was associated with the X-ray outburst XRF 080109. At our first epoch, at t = 30 days after the explosion, we observed at 22 and 8.4 GHz, and at our second, at t = 133 days, at 8.4 and 5.0 GHz. The VLBI observations allow us to accurately measure the source's size and position at each epoch, and thus constrain its expansion velocity and proper motion. We find the source at best marginally resolved at both epochs, allowing us to place a 3sigma upper limit of ~0.75c on the expansion velocity of a circular source. For an elongated source, our measurements are compatible with mildly relativistic expansion. However, our 3sigma upper limit on the proper motion is 4 micro-arcsec/day, corresponding to an apparent velocity of <0.6c, and is consistent with a stationary flux centroid. This limit rules out a relativistic jet such as an gamma-ray burst jet away from the line of sight, which would be expected to show apparent proper motion of >c. Taken together, our measurements argue against the presence of any long-lived relativistic outflow in SN 2008D. On the other hand, our measurements are consistent with the nonrelativistic expansion velocities of <30,000 km/s and small proper motions (<500 km/s) seen in typical supernovae.

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Title: Supernova 2008D: A Repetition of Supernova 1987A In a Binary
Authors: Gerald E. Brown, Chang-Hwan Lee

The Supernova 2008D is similar to that of SN 1987A without the H-envelope. Soderberg et al.(2008) reported the serendipitous discovery of the SN2008D at the time of the explosion, accompanied by an X-ray outburst XRF080109. The central remnant, which we believe to be the black-hole (BH) central engine in the Blandford-Znajek mechanism, is estimated, on the basis of the 7% 56Ni production, to have a mass of 1.6-1.8 M_sun. This is not much larger than the upper limit of 1.56 m_sun for the mass of the compact object in SN1987A found by Bethe & Brown (1995); also, on the basis of the 7.5% 56Ni production, they interpreted it as a low-mass BH. Redoing the light curve so as to take into account the absence of convective carbon burning from zero age main sequence 18-24 M_sun and replacing it by carbon shell burning, we see that the remnant in SN2008D must be less massive than in SN 1987A; there of 18 M_sun. Thus, the maximum neutron star mass is 1.5 M_sun. Note that the metallicity of the host galaxy of SN2008D is similar to that of our Galaxy.

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A European-led team of international astronomers have discovered clues that suggest that a star that recently went supernova may have exploded more violently than previously thought and collapsed into a black hole. This brutal explosion produced a series of gamma-ray bursts that were only just able to be registered by the astronomers. This discovery could mark an important milestone for astronomers everywhere, as it is providing them with more information of one of the galaxy's most violent events.

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