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During a search for X-ray transients in the XMM–Newton archive within the EXTraS project, we discovered a new X-ray source that is detected only during an ∼5 min interval of an ∼21 hr-long observation performed on 2011 June 21 (EXMM 023135.0–603743, probability of a random Poissonian fluctuation: ∼1.4 × 10−27). With dedicated follow-up observations, we found that its position is consistent with a star-forming galaxy (SFR = 1–2 M⊙ yr−1) at redshift z = 0.092 ± 0.003 (d = 435 ± 15 Mpc). At this redshift, the energy released during the transient event was 2.8 × 1046 erg in the 0.3–10 keV energy band (in the source rest frame). The luminosity of the transient, together with its spectral and timing properties, make EXMM 023135.0–603743 a gripping analog to the X-ray transient associated to SN 2008D, which was discovered during a Swift/XRT observation of the nearby (d = 27 Mpc) supernova-rich galaxy NGC 2770. We interpret the XMM–Newton event as a supernova shock break-out or an early cocoon, and show that our serendipitous discovery is broadly compatible with the rate of core-collapse supernovae derived from optical observations and much higher than that of tidal disruption events. [ABSTRACT FROM AUTHOR]

We discuss the results of the analysis of multi-wavelength data for the afterglows of GRB 081007 and GRB 090424, two bursts detected by Swift. One of them, GRB 081007, also shows a spectroscopically confirmed supernova, SN 2008hw, which resembles SN 1998bw in its absorption features, while the maximum magnitude may be fainter, up to 0.7 mag, than observed in SN 1998bw. Bright optical flashes have been detected in both events, which allows us to derive solid constraints on the circumburst-matter density profile. This is particularly interesting in the case of GRB 081007, whose afterglow is found to be propagating into a constant-density medium, yielding yet another example of a gamma-ray burst (GRB) clearly associated with a massive-star progenitor which did not sculpt the surroundings with its stellar wind. There is no supernova component detected in the afterglow of GRB 090424, likely due to the brightness of the host galaxy, comparable to the Milky Way. We show that the afterglow data are consistent with the presence of both forward- and reverse-shock emission powered by relativistic outflows expanding into the interstellar medium. The absence of optical peaks due to the forward shock strongly suggests that the reverse-shock regions should be mildly magnetized. The initial Lorentz factor of outflow of GRB 081007 is estimated to be Γ ~ 200, while for GRB 090424 a lower limit of Γ > 170 is derived.We also discuss the prompt emission of GRB 081007, which consists of just a single pulse. We argue that neither the external forward-shock model nor the shock-breakout model can account for the prompt emission data and suggest that the single-pulse-like prompt emission may be due to magnetic energy dissipation of a Poynting-flux-dominated outflow or to a dissipative photosphere. [ABSTRACT FROM AUTHOR]

We present X-ray, UV/optical, and radio observations of the stripped-envelope, core-collapse supernova (SN) 2011ei, one of the least luminous SNe IIb or Ib observed to date. Our observations begin with a discovery within ~1 day of explosion and span several months afterward. Early optical spectra exhibit broad, Type II-like hydrogen Balmer profiles that subside rapidly and are replaced by Type Ib-like He-rich features on a timescale of one week. High-cadence monitoring of this transition suggests absorption attributable to a high-velocity (≳12,000 km s-1) H-rich shell, which is likely present in many Type Ib events. Radio observations imply a shock velocity of v ≈ 0.13 c and a progenitor star average mass-loss rate of M ≈ 1.4 × 10-5 M☉ yr-1 (assuming wind velocity vw = 103 km s-1). This is consistent with independent constraints from deep X-ray observations with Swift-XRT and Chandra. Overall, the multi-wavelength properties of SN 2011ei are consistent with the explosion of a lowermass (3-4 M☉), compact (R" ≲ 1 × 1011 cm), He-core star. The star retained a thin hydrogen envelope at the time of explosion, and was embedded in an inhomogeneous circumstellar wind suggestive of modest episodic mass loss. We conclude that SN 2011ei's rapid spectral metamorphosis is indicative of time-dependent classifications that bias estimates of the relative explosion rates for Type IIb and Ib objects, and that important information about a progenitor star's evolutionary state and mass loss immediately prior to SN explosion can be inferred from timely multi-wavelength observations. [ABSTRACT FROM AUTHOR]

We study the multi-dimensional geometry of supernova (SN) explosions by means of spectropolarimetric observations of stripped-envelope SNe, i.e., SNe without a hydrogen-rich layer. We perform spectropolarimetric observations of two stripped-envelope SNe, Type Ib SN 2009jf and Type Ic SN 2009mi. Both objects show non-zero polarization at the wavelength of the strong lines. They also show a loop in the Stokes Q - U diagram, which indicates a non-axisymmetric, three-dimensional ion distribution in the ejecta. We show that five out of six stripped-envelope SNe, which have been observed spectropolarimetrically so far, show such a loop. This implies that a three-dimensional geometry is common in stripped-envelope SNe. We find that stronger lines tend to show higher polarization. This effect is not related to the geometry, and must be corrected for to compare the polarization of different lines or different objects. Even after the correction, however, there remains a dispersion of polarization degree among different objects. Such a dispersion might be caused by three-dimensional clumpy ion distributions viewed from different directions. [ABSTRACT FROM AUTHOR]

We present the spectroscopic and photometric evolution of the nearby (z = 0.059) spectroscopically confirmed Type Ic supernova, SN 2010bh, associated with the soft, long-duration gamma-ray burst (X-ray flash) GRB 100316D. Intensive follow-up observations of SN 2010bh were performed at the ESO Very Large Telescope (VLT) using the X-shooter and FORS2 instruments. Thanks to the detailed temporal coverage and the extended wavelength range (3000-24800 Å), we obtained an unprecedentedly rich spectral sequence among the hypernovae, making SN 2010bh one of the best studied representatives of this SN class. We find that SN 2010bh has a more rapid rise to maximum brightness (8.0 ± 1.0 rest-frame days) and a fainter absolute peak luminosity (Lbol ≈ 3 × 1042 erg s–1) than previously observed SN events associated with GRBs. Our estimate of the ejected 56Ni mass is 0.12 ± 0.02 M☼. From the broad spectral features, we measure expansion velocities up to 47,000 km s–1, higher than those of SNe 1998bw (GRB 980425) and 2006aj (GRB 060218). Helium absorption lines He I λ5876 and He I 1.083 μm, blueshifted by ∼20,000-30,000 km s–1 and ∼28,000-38,000 km s–1, respectively, may be present in the optical spectra. However, the lack of coverage of the He I 2.058 μm line prevents us from confirming such identifications. The nebular spectrum, taken at ∼186 days after the explosion, shows a broad but faint [O I] emission at 6340 Å. The light curve shape and photospheric expansion velocities of SN 2010bh suggest that we witnessed a highly energetic explosion with a small ejected mass (Ek ≈ 1052 erg and Mej ≈ 3 M☼). The observed properties of SN 2010bh further extend the heterogeneity of the class of GRB SNe. [ABSTRACT FROM AUTHOR]