An extremely luminous X-ray outburst at the birth of a supernova

Abstract

Spectral analysis of Swift/XRT dataWe use the xspec v11.3.2 X-ray spectral fitting package to fit both a power law and a blackbody model to the XRT outburst data. In both models we allow for excess neutral hydrogen absorption (N H ) above the Galactic value along the line of sight to NGC 2770, N H,Gal = 1.7 × 10 20 cm −2 . The best-fit power law model (χ 2 = 7.5 for 17 degrees of freedom; probability, P = 0.98) has a photon index, Γ = 2.3 ± 0.3 (or, F ν ∝ ν −1.3±0.3 ) and N H = 6.9 +1.8 −1.5 × 10 21 cm −2 . The best-fit blackbody model is described by kT = 0.71 ± 0.08 keV and N H = 1.3 +1.0 −0.9 × 10 21 cm −2 . However, this model provides a much poorer fit to the data (χ 2 = 26.0 for 17 degrees of freedom; probability, P = 0.074). We therefore adopt the power law model as the best description of the data. The resulting count rate to flux conversion is 1 counts s −1 = 5 × 10 −11 erg cm −2 s −1 . The outburst undergoes a significant hard-to-soft spectral evolution as indicated by the ratio of counts in the 0.3 − 2 keV band and 2 − 10 keV band. The hardness ratio decreases from 1.35 ± 0.15 during the peak of the flare to 0.25 ± 0.10 about 400 s later. In the context of the power law model this spectral softening corresponds to a change from Γ = 1.70 ± 0.25 to 3.20 ± 0.35 during the same time interval. High resolution optical spectroscopyWe obtained the spectrum with the High Resolution Echelle Spectrometer (HIRES) mounted on the Keck I 10-m telescope beginning at Jan 17.46 UT. A total of four 1800-s exposures were obtained with a spectral resolution, R = 48, 000, and a slit width of 0.86 arcsec. The data reach a signal-to-noise ratio of 18 per pixel. We reduced the data with the MAKEE reduction package. We are interested in the Na I D and K I absorption features since they are sensitive to the gas column density, and hence extinction, along the line of the sight to the SN. Rejecting a Relativistic Origin for XRO 080109We investigate the possibility that XRO 080109 is the result of a relativistic outflow similar to that in GRBs. In this context the emission is non-thermal synchrotron radiation. The outburst flux density is 7.5 × 10 2 µJy at 0.3 keV. Simultaneously, we find 3σ limits on the flux density in the UBV bands (∼ 3 eV) of F ν < 9.0 × 10 2 µJy, indicating that the peak of the synchrotron spectrum must be located between the UV and X-ray bands. In the standard synchrotron model this requires the frequencies corresponding to electrons with the minimum and cooling Lorentz factors to obey ν m ≈ ν c ≈ 3 × 10 16 Hz, while the peak of the spectrum is F ν,p ≈ 3 mJy.The inferred values of ν m and ν c allow us to constrain 47 the outflow parameters and thus to check for consistency with the hypothesis of relativistic expansion. The relevant parameters are the bulk Lorentz factor (γ), the magnetic field (B), and the shock radius (R sh ). From the value of ν c we find γB 3 ≈ 8.3 × 10 3 , and since γ > 1 we conclude that B < 20 G. In addition, using ν m we find ǫ 2 e γ 3 B ≈ 3 × 10 4 ; here ǫ e is the fraction of posts...

Stellar Forensics with the Supernova‐GRB Connection

“…For the first time, the very earlytime peak (at ∼ 1 day, see Fig 5) could be observed for this kind of SN, from which one can deduce the progenitor radius, since that peak is due to black-body emission from the cooling and expanding stellar envelope. Using our reliable and early-time measurements of the bolometric output of this SN in conjunction with models by Waxman et al (2007) and Chevalier & Fransson (2008), as well as published values of kinetic energy and ejecta mass, we derived a progenitor radius of 1.2 ± 0.7 R (in agreement with Soderberg et al 2008) and 12 ± 7 R , respectively, the latter being more in line with typical WN stars. We furthermore showed that the observed X-ray emission by which it was discovered (Soderberg et al 2008) is different from those of X-ray flashes, the weaker cousins of GRBs, which demonstrates that even normal SN Ib, surprisingly, can give rise to highenergy phenomena (but see Mazzali et al 2008).…”

“…This happened for SN 2008D/XRT080109 (where XRT stands for X-ray Transient) which was discovered by Soderberg et al (2008). From the early light of the explosion, one can reconstruct a massive star's pre-explosion composition and radius, which provides a powerful tool to closely investigate a single star out to cosmological distances.…”

“…Besides its utility, the story of SN 2008D/XRT 080109 reminds us of the importance of serendipity in science. SN 2008D/XRT 080109 was discovered by Soderberg et al (2008) in X-rays via the Swif t satellite, because they were monitoring another SN, SN 2007uy, in the same galaxy, when suddenly XRT080109 erupted and lasted ∼ 600 sec in X-rays. Furthermore, our program was also monitoring SN 2007uy, and the rest of the host galaxy, in the optical and NIR from the ground, providing us with stringent limits on the optical emission just hours before the onset of X-ray transient.…”

“…In Modjaz et al (2009), we gathered extensive panchromatic observations (X-ray, UV, Optical, NIR) from 13 different telescopes to determine the nature of SN 2008D, its accompanying Swift X-ray Transient 080109, and its progenitor (see also Soderberg et al 2008). We first established that SN 2008D is a spectroscopically normal SN Ib (i.e., showing conspicuous He lines, see Fig 4), which implies the progenitor star had an intact He layer, but had not retained its outermost H envelope.…”

Extreme luminosities in ejecta produced by intermittent outflows around rotating black holes

“…Relativistic shocks at breakout have been associated with the smooth light curves of LLGRBs, e.g., GRB 980425/SN1998bw, GRB060218/SN1006aj (Campana et al 2006) and GRB080109/SN2008D (Soderberg et al 2008;Mazzali et al 2008), here attributed to the breakout of moderately relativistic magnetized ejecta from the inner accretion disk around a rotating black hole. Disk winds and their ejecta hereby provide an efficient aspherical explosion mechanism with and without association to GRBs (Maeda et al 2006;Taubenberger et al 2009), that may be especially efficient when baryon rich (van Putten et al 2011).…”