The nature of the longest gamma-ray bursts Andrew Levan University of Warwick

Burst durations Swift Very long bursts are often image triggers (1000+s)

 Correction from Swift observed rates to astrophysical rates (at Swift limits) could be a factor 10 Not as rare as you think……. For same integrated fluence, more difficult to detect long events Data courtesy J. Kennea

Should they be different?  Some very long bursts probably are the “tail” of the “normal” GRB distribution  Others are low luminosity GRBs (shock breakout?)  Some initially classified as GRBs are clearly different (e.g. GRB A/Swift – Burrows talk)  Jet breakout time – longer GRBs could mean bigger stars?  Suggestion of longer lived central engines, perhaps powered by outer layers of stars (e.g. Quataert et al ; Woosley & Heger 2012)  Two more recent examples discussed here (101225A, A)

GRB A Visibility from Bethlehem

X-ray spectrum = Powerlaw + blackbody (1 keV)

Optical afterglow

Scenario I: Galactic Tidal shredding of asteroid mass body around a NS Campana et al. 2011

Scenario II: Collapsar Fryer et al. 1999; Thoene et al Supernova? He-NS merger Bang

Thoene et al. 2011

HST and late time observations Note: No proper motion in 6 months (v < 250 km/s/kpc) If host the afterglow is “on” the nucleus HST ACS/F435W (Jan)Gemini GMOS (July) g-band Significant host contribution at ~1 month, but not resolved

Host constraints

GRB A 10000s

OIII (4959) OIII (5007)

X-ray afterglow

Supernova search HST grism spectrum J-band u-band VLT + Gemini Clear reddening, no obvious SN. Host? Chromatic afterglow?

HST observations WFC3/F336W WFC3/F110W Host galaxy still largely unresolved (<800pc), very compact if bright

Summary  Very long GRBs are more astrophysically common that we might expect. (but too long for Swift and too faint for BATSE, GBM etc).  The longest bursts appear to have distinct prompt, afterglow (and host?) properties from the majority of GRBs.  They are probably cosmological, but evidence for SN within them remains weak  Understanding the nature of the longest gamma- ray transients should remain an important task.

SGRs TDEs? Galactic sources (SGR, LMXB, HMXB, micro-quasar, gamma- ray pulsar) LLGRBs SGRBs LGRB unknown