The nature of the longest gamma-ray burstsAndrew LevanUniversity of Warwick

Burst durations

Swift 1644+57

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 distributionOthers are low luminosity GRBs (shock breakout?)Some initially classified as GRBs are clearly

different (e.g. GRB 110328A/Swift 1644+57 – 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. 20122; Woosley & Heger 2012)

Two more recent examples discussed here (101225A, 111209A)

GRB 101225A

Visibility from Bethlehem

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

Optical afterglow

Scenario I: Galactic

Tidal shredding of asteroid mass body around a NSCampana et al. 2011

Scenario II: Collapsar

Fryer et al. 1999; Thoene et al. 2011

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 111209A

10000s

OIII (4959) OIII (5007)

X-ray afterglow

X-ray afterglow

Supernova search

HST grism spectrumJ-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

SummaryVery 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

SGRBsLGRB

unknown