* Astronomy

Title: GRB060218 and GRBs associated with Supernovae Ib/c
Authors: Maria Giovanna Dainotti, Maria Grazia Bernardini, Carlo Luciano Bianco, Letizia Caito, Roberto Guida, Remo Ruffini

We plan to fit the complete gamma- and X-ray light curves of the long duration GRB060218, including the prompt emission, in order to clarify the nature of the progenitors and the astrophysical scenario of the class of GRBs associated to SNe Ib/c. The initial total energy of the electron-positron plasma E_{e^ ±^{tot}=2.32 x 10^{50} erg has a particularly low value similarly to the other GRBs associated with SNe. For the first time we observe a baryon loading B=10^{-2} which coincides with the upper limit for the dynamical stability of the fireshell. The effective CircumBurst Medium (CBM) density shows a radial dependence n_{cbm} \propto r^{-\alpha} with 1.0<\alpha<1.7 and monotonically decreases from 1 to 10^{-6} particles/cm³. Such a behaviour is interpreted as due to a fragmentation in the fireshell. Analogies with the fragmented density and filling factor characterising Novae are outlined. The fit presented is particularly significant in view of the complete data set available for GRB060218 and of the fact that it fulfils the Amati relation. We fit GRB060218, usually considered as an X-Ray Flash (XRF), as a "canonical GRB" within our theoretical model. The smallest possible black hole, formed by the gravitational collapse of a neutron star in a binary system, is consistent with the especially low energetics of the class of GRBs associated with SNe Ib/c. We give the first evidence for a fragmentation in the fireshell. Such a fragmentation is crucial in explaining both the unusually large T_{90} and the consequently inferred abnormal low value of the CBM effective density.

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Title: Multicolour Infrared Observations of SN 2006aj, the Supernova Associated with XRF 060218 - Paper I
Authors: Daniel Kocevski, Maryam Modjaz, Joshua S. Bloom, Ryan Foley, Daniel Starr, Cullen H. Blake, Michael Wood-Vasey, Emilio E. Falco, Nathaniel R. Butler, Mike Skrutskie, Andrew Szentgyorgyi

We report simultaneous multicolour near-infrared (NIR) observations of the supernova associated with x-ray Flash 060218 during the first 16 days after the high energy event. We find that the light curve rises and peaks relatively fast compared to other SN Ic, with the characteristic broad NIR peak seen in all three bands. We find that the rise profile before the peak is largely independent of NIR wavelength, each band appearing to transition into a plateau phase around day 10--13. Since the light curve is in the plateau phase when our observations end at day 16, we can only place limits on the peak absolute magnitudes, but we estimate that SN 2006aj is one of the lowest NIR luminosity XRF/GRB associated SNe observed to date. The broad peaks observed in the JHK_s bands point to a large increase in the NIR contribution of the total flux output from days 10--16. This evolution can be seen in the broad colour and SED diagrams constructed using UBVRIJHK_s monochromatic flux measurements for the first 16 days of the event. Ultimately, a 10-day rise time would make SN 2006aj an extremely fast rise SN Ic event, faster than SN 1998bw and SN 2003dh, which combined with its underluminous nature, indicates a lower amount of ^{56}Ni ejected by the progenitor compared to other XRF/GRB-SNe. Furthermore, the lack of significant colour change during the rise portion of the burst points to little or no spectral evolution over the first 10 days of activity in the NIR.

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Title: No supernovae from two nearby long gamma ray bursts
Authors: Johan P. U. Fynbo, Darach Watson, Christina C. Thoene, Jesper Sollerman, Joshua S. Bloom, Tamara M. Davis, Jens Hjorth, Pall Jakobsson, Uffe G. Joergensen, John F. Graham, Andrew S. Fruchter, David Bersier, Lisa Kewley, Arnaud Cassan, José María Castro Cerón, Suzanne Foley, Javier Gorosabel, Tobias C. Hinse, Keith D. Horne, Brian L. Jensen, Sylvio Klose, Daniel Kocevski, Jean-Baptiste Marquette, Daniel Perley, Enrico Ramirez-Ruiz, Maximilian D. Stritzinger, Paul Vreeswijk, Ralph A. M. Wijers, Kristian G. Woller, Dong Xu, Marta Zub
(Version 2)

It is now accepted that long duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. The standard "collapsar" model predicts that a broad-lined and luminous Type Ic core-collapse supernova (SN) accompanies every long-duration GRB. This association has been confirmed in observations of several nearby GRBs. Here we present observations of two nearby long-duration GRBs that challenge this simple view. In the GRBs 060505 and 060614 we demonstrate that no SN emission accompanied these long-duration bursts, down to limits hundreds of times fainter than the archetypal SN 1998bw that accompanied GRB 980425, and fainter than any Type Ic SN ever observed. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration and show that the bursts originated in star-forming regions. The absence of a SN to such deep limits is qualitatively different from all previous nearby long GRBs and suggests a new phenomenological type of massive stellar death.

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Astronomers say they have witnessed a stellar explosion - or supernova - unfolding in real time.

Their observations, outlined in the journal Nature, offer the most detailed picture yet of these cosmic explosions.



An initial release of energy from the star was picked up by the US Swift satellite in February, allowing experts to train their telescopes on the event.
This short, sharp outburst, known as an X-ray flash, is effectively an early warning signal that the star was going to turn supernova.

Source

A link between long gamma-ray bursts (GRBs) and supernovae has been established, but whether there is a similar relationship between the weaker and softer X-ray flashes and supernovae is unclear. GRB/XRF 060218, spotted by the Swift satellite on 18 February this year, may supply that missing link. In the first of four papers on this novel burster, Campana et al. report the sighting of the X-ray signature of a shock break-out, possible evidence of a supernova in progress. Pian et al. report the optical discovery of a type Ic supernova 2006aj associated with GRB/XRF 060218. Soderberg et al. report radio and X-ray observations that show that XRF 060218 is 100 times less energetic than, but of a type that is ten times more common than cosmological GRBs. Mazzali et al. modelled the spectra and light curve of SN 2006aj to show that it had a much smaller explosion energy and ejected much less mass than other GRB-supernovae, suggesting that it was produced by a star with a mass was only about 20 times that of the Sun, leaving behind a neutron star, rather than a black hole.

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Title: Supernova 2006aj and the associated X-Ray Flash 060218
Authors: Sollerman, J.; Jaunsen, A. O.; Fynbo, J. P. U.; Hjorth, J.; Jakobsson, P.; Stritzinger, M.; Feron, C.; Laursen, P.; Ovaldsen, J. -E.; Selj, J.; Thöne, C. C.; Xu, D.; Davis, T.; Gorosabel, J.; Watson, D.; Duro, R.; Ilyin, I.; Jensen, B. L.; Lysfjord, N.; Marquart, T.; Nielsen, T. B.; Näränen, J.; Schwarz, H. E.; Walch, S.; Wold, M.; Östlin, G.

We have studied the afterglow of the gamma-ray burst (GRB) of February 18, 2006. This is a nearby long GRB, with a very low peak energy, and is therefore classified as an X-ray Flash (XRF). XRF 060218 is clearly associated with a supernova -- dubbed SN 2006aj. We present early spectra for SN 2006aj as well as optical lightcurves reaching out to 50 days past explosion. Our optical lightcurves define the rise times, the lightcurve shapes and the absolute magnitudes in the U, V and R bands, and we compare these data with data for other relevant supernovae. SN 2006aj evolved quite fast, somewhat similarly to SN 2002ap, but not as fast as SN 1994I. Our spectra show the evolution of the supernova over the peak, when the U-band portion of the spectrum rapidly fades due to extensive line blanketing. We compare to similar spectra of very energetic Type Ic supernovae. Our first spectra are earlier than spectra for any other GRB-SN. The spectrum taken 12 days after burst in the rest frame is similar to somewhat later spectra of both SN 1998bw and SN 2003dh, implying a rapid early evolution. This is consistent with the fast lightcurve. From the narrow emission lines from the host galaxy we derive a redshift of z=0.0331±0.0007. This makes XRF 060218 the second closest gamma-ray burst detected. The flux of these emission lines indicate a high-excitation state, and a modest metallicity and star formation rate of the host galaxy.

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