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Post Info TOPIC: Strange explosion


L

Posts: 131433
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RE: Strange explosion
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Title: GRB060218 as a Tidal Disruption of a White Dwarf by an Intermediate Mass Black Hole
Authors: Roman V. Shcherbakov, Asaf Pe'er, Christopher S. Reynolds, Roland Haas, Tanja Bode, Pablo Laguna

A highly unusual pair of a gamma-ray burst (GRB) GRB060218 and an associated supernova SN2006aj has puzzled theorists for years. A supernova shock breakout and a jet from a newborn stellar mass compact object were put forward to explain its multiwavelength signature. We propose that the source is naturally explained by another channel, a tidal disruption of a white dwarf (WD) by an intermediate mass black hole (IMBH). The tidal disruption is accompanied by a tidal pinching, which leads to the ignition of a WD and a supernova. Some debris falls back onto the IMBH, forms a disk, which quickly amplifies the magnetic field, and launches a jet. We successfully fit soft X-ray spectrum with the Comptonised blackbody emission from a jet photosphere. The optical/UV emission is consistent with self-absorbed synchrotron from the expanding jet front. The accretion rate temporal dependence Mdot(t) in a tidal disruption provides a good fit to soft X-ray lightcurve. The IMBH mass is found to be about 10^4Msun in three independent estimates: (1) fitting tidal disruption Mdot(t) to soft X-ray lightcurve; (2) computing the jet base radius in a jet photospheric emission model; (3) inferring the central BH mass based on a host dwarf galaxy stellar mass. The supernova position is consistent with the center of the host galaxy, while low supernova ejecta mass is consistent with a WD mass. High expected rate of tidal disruptions in dwarf galaxies is consistent with one source observed by Swift satellite over several years at GRB060218 distance of 150Mpc. The encounters with the WDs provide a lot of fuel for IMBH growth.

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Posts: 131433
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GRB060218
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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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L

Posts: 131433
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RE: Strange explosion
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Two brilliant flashes of light from nearby galaxies are puzzling astronomers and could indicate that gamma-ray bursts, which signal the birth of a black hole, are more diverse than once thought.
The two new gamma-ray bursts are of the long variety but, surprisingly, did not show any evidence of supernova activity. This flies in the face of what was an emerging consensus about the origin of long bursts, according to University of California, Berkeley's Joshua Bloom, assistant professor of astronomy. To Bloom, this indicates that there are more than two ways to produce a gamma-ray flash and a black hole.

"Instead of simplicity and clarity, we're seeing a rich diversity emerge - there are more ways than we thought for producing flashes of gamma-rays" - Joshua Bloom.

Bloom and 30 colleagues from around the world report observations of two of these peculiar gamma-ray bursts, labelled GRB 060505 and GRB 060614, in the Dec. 21 issue of the British journal Nature. Three other papers in the same issue report details of GRB 060614. Both bursts were detected by NASA's Swift satellite - the first on May 5, 2006, the second on June 14, 2006.

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GRB 060505: Gamma ray burst without a supernova.

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L

Posts: 131433
Date:
Supernova 2006aj
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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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L

Posts: 131433
Date:
GRB060218
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The intense cosmic outbursts of high-energy radiation known as gamma-ray bursts (GRBs) - among the most energetic astrophysical processes known - are thought to arise from the collapse of massive stars in a supernova. But not all supernovae generate GRBs, and it remains unclear what causes some but not others to produce these bursts. Four international teams of astronomers, including several from the Carnegie Observatories, recently reported in Nature different views of a burst event that occurred in February 2006, which promises to yield new insights into the process.
This burst, GRB060218, which occurred 440 million light years away in the constellation of Aries, was a mild type of GRB known as an X-ray flash. These are not as bright as normal GRBs, and don't produce as many gamma rays. It was previously unclear whether X-ray flashes were generated by the same basic process of stellar collapse during a supernova. But the new results seem to make that link secure.
A team led by Elena Pian, and another headed by Sergio Campana, show that GRB060218 was associated with a supernova called SN2006aj. The researchers think that this supernova was less energetic than those that produce fully fledged GRBs, but brighter than supernovae that don't generate such bursts at all. Alicia Soderberg and her colleagues, including Carnegie's Edo Berger, Eric Persson, and Pat McCarthy, have used observations of GRB060218 at radio and X-ray wavelengths to deduce that it was around a hundred times less energetic than ordinary GRBs - but they think such events should be ten times more common. The burst was unusual in that it went on producing X-rays for several weeks, which the researchers say was the result of high-energy debris thrown out by the explosion.

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L

Posts: 131433
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Long gamma ray bursts
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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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RE: Strange explosion
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Title: An optical supernova associated with the X-ray flash XRF 060218
Authors: E. Pian, P.A. Mazzali, N. Masetti, P. Ferrero, S. Klose, E. Palazzi, E. Ramirez-Ruiz, S.E. Woosley, C. Kouveliotou, J. Deng, A.V. Filippenko, R. Foley, J. Fynbo, D.A. Kann, W. Li, J. Hjorth, K. Nomoto, F. Patat, D. Sauer, J. Sollerman, P.M. Vreeswijk, E.W. Guenther, A. Levan, P. O'Brien, N. Tanvir, R.A.M.J. Wijers, C. Dumas, O. Hainaut, D.S. Wong, D. Baade, L. Wang, L. Amati, E. Cappellaro, A.J. Castro-Tirado, S. Ellison, F. Frontera, A.S. Fruchter, J. Greiner, K. Kawabata, C. Ledoux, K. Maeda, P. Moller, L. Nicastro, E. Rol, R. Starling
Comments: Final published version

Long-duration gamma-ray bursts (GRBs) are associated with type Ic supernovae that are more luminous than average and that eject material at very high velocities. Less-luminous supernovae were not hitherto known to be associated with GRBs, and therefore GRB-supernovae were thought to be rare events. Whether X-ray flashes - analogues of GRBs, but with lower luminosities and fewer gamma-rays - can also be associated with supernovae, and whether they are intrinsically 'weak' events or typical GRBs viewed off the axis of the burst, is unclear. Here we report the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. Supernova 2006aj is intrinsically less luminous than the GRB-supernovae, but more luminous than many supernovae not accompanied by a GRB. The ejecta velocities derived from our spectra are intermediate between these two groups, which is consistent with the weakness of both the GRB output and the supernova radio flux. Our data, combined with radio and X-ray observations, suggest that XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB observed off-axis. This extends the GRB-supernova connection to X-ray flashes and fainter supernovae, implying a common origin. Events such as XRF 060218 are probably more numerous than GRB-supernovae.

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L

Posts: 131433
Date:
Supernova 2006aj
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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.

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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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RE: Strange explosion
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Expand (166kb, 560 x 461)
This half-degree-wide finder field in Aries shows the GRB's location near the 9.6 magnitude star at the top right.


Position(2000): RA = 03:21:39.71 Dec = +16:52:02.6
30 and 60 arc minutes field of view.

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Astronomers believe the bursts of radiation are emitted from a star about to implode; this may be a rare glimpse of a black-hole birth.

Using large telescopes tuned to different parts of the electromagnetic spectrum, scientists should be able to discover the chemistry of an exploding star for the first time, from within 100 seconds of the blast's beginning to its end.
So far, the blast seems to herald a "Type 1C" supernova. This means that the star has consumed all its nuclear fuel, leaving behind only an iron core, which then implodes.
The recorded Gamma rays smoothly increased in the 30-minute blast, a departure from the usual pattern of short, sharp peaks. That may indicate that the axis of the supernova, which typically produces jets of exploding material, was pointed away from Earth.

For a gamma ray burst, this is close. The only GRB with a closer confirmed distance was observed by the Italian-Dutch BeppoSAX satellite on April 25, 1998, at a distance of about 120 million light-years. That event was also associated with a supernova (SN 1998bw), and was one of the crucial events that bolstered the idea that long GRBs are linked to supernovae.

You'll need a least a 16” telescope to see it, though it may brighten considerably over the next week. You won't be able to go out in your backyard and see it with binoculars.

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