Modeling Supernova-like Explosions Associated with Gamma-ray

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Modeling Supernova-like Explosions Associated with Gamma-ray Bursts with Short Durations S.R.Kulkarni 1ABSTRACT There is now good evidence linking short-hard GRBs with both elliptical and spiral galaxies at relatively low redshifts,redshift of about 0.2.This contrasts with the average redshift of about 2of long-duration events,which also occur only in star-forming galaxies.The diversity of hosts is reminiscent of type Ia supernovae,which are widely (but not universally)believed to originate from the coalescence of white dwarfs.By analogy,it has been postulated that short-hard bursts originate from neutron star mergers.Mergers,as well as stellar core-collapse events (type II SNe and long-duration GRBs)are accompanied by long-lived sub-relativistic components powered by radioactive decay of unsta-ble elements produced in the explosion.It is therefore interesting to explore whether short duration events also have ejecta powered by radioactivity (i.e.that are supernova-like).Observations already inform us that any supernova like component in the ?rst few well studied short hard bursts must be fainter than those typical of type Ia or core-collapse supernovae.Rather than refer to weaker supernova-like component as “mini-super nova”,an etymologically in-defensible term,I use the term macronova .I investigate the observability of macronovae powered by neutron decay and by radioactive Nickel.Separately,I note that a macronova will reprocess energetic emission arising from a long lived central source.I ?nd that surprisingly interesting limits on the basic parameters

of macronovae can be obtained provided observations are obtained with current 10-m class telescopes over a range of one hour to one day following the burst.Detection or even strong constraints provided by macronova observations can be expected to yield the most direct clue (barring data from futuristic gravitational wave interferemeters)to the origin of short hard bursts.It is this prize that,over the course of coming few years,will undoubtedly motivate astronomers to mount ambitious macronova campaigns.

Subject headings:gamma rays:bursts —supernovae:general —stars:novae

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1.Background

There is currently a great deal of excitement and sense of great progress in our under-standing of the mysterious class of short duration,hard spectrum gamma-ray bursts.GRB 050509b was the?rst short hard burst to be localized to moderate precision(<10′′).This was made possible thanks to the prompt slewing of Swift and subsequent discovery of a rapidly fading X-ray source(Gehrels et al.2005).The X-ray afterglow was noted to coincide with a nearby cluster of galaxies(Gehrels et al.2005)and lay in the outskirts of a bright elliptical galaxy at z=0.22(Bloom et al.2005).Fortunately the next two short hard bursts were localized to arcsecond accuracy thanks to the discovery of their X-ray,optical or radio afterglows and,as a result,greatly advanced the?eld of short hard bursts.GRB050709was detected by the High Energy Transient Explorer(Villasenor et al.2005).The optical(Hjorth et al.2005)and X-ray(Fox et al.2005)afterglow places this burst on a star-forming galaxy at z=0.16(Fox et al.2005).GRB050724was detected by Swift(Covino et al.2005).The radio and optical afterglow localization places this event on an elliptical galaxy(Berger et al. 2005)at z=0.26(Prochaska et al.2005).

Collectively these three short bursts indicate both elliptical and spiral host galaxies. These associations lend credence to other less well localized associations e.g.Nakar et al. (2005).It is however a recent development that is of potentially greater interest to this paper,namely the claim by Tanvir et al.(2005)of a population of short bursts that are even closer.

It is intriguing that the hosts of short hard bursts are as diverse as those of Ia supernovae.

A model(popular in the past but not ruled out by observations)that is invoked for Ia explosions involves double degenerate binary systems which at some point coalesce and then explode.An early and enduring model for short hard bursts is the coalescence of two neutron stars(or a neutron star and a black hole).The coalescence is expected to produce a burst of neutrinos,of gamma-ray and also eject neutron-rich ejecta(Eichler et al.1989).

Several years ago Li&Paczy′n ski(1998)speculated the ejecta would result in a supernova-like explosion i.e.a sub-relativistic explosion with radioactivity.The purpose of this paper is to revisit this topic now that the rough distance scale has been determined.Furthermore, the possibility that some events may be even very close makes it doubly attractive to develop quantitative models of associated supernova-like explosions.

Coalescence models are not silent on the issue of associated non-relativistic out?ows(e.g. Janka&Ru?ert2002;Rosswog2005).Indeed,there appears to be a multiplicity of reasons for such out?ows:tidal tails(which appear inevitable in all numerical models investigated to date),a wind driven by neutrino emission of a central massive neutron star and explosion of

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a striped neutron star.To this list I add another possible mechanism:ejection of the outer regions of the accretion disk owing to conservation of angular momentum.

The composition of the ejecta is a matter of fundamental theoretical interest but also of great observational consequence.For the explosion to be observationally detected the ejecta must contain a long-lived source of power.Otherwise the ejecta cools rapidly and is not detectable with the sensitivity of current telescopes.Indeed,the same argument applies to ordinary SN.Unfortunately,theoretical mode …… 此处隐藏：30516字，全部文档内容请下载后查看。喜欢就下载吧 ……