Review: Relativistic shock breakouts

The paper selected for the fourth Astronomy Twitter Journal Club meeting (8:10 p.m. UK time, 7:10 p.m. UT, Thursday 7th July 2011) was Relativistic shock breakouts – a variety of gamma-ray flares: from low luminosity gamma-ray bursts to type Ia supernovae by Nakar and Sari, June 13 2011 (arXiv:1106.2556v1). The paper was suggested by Derek Fox.

Please see the preview of the paper for more details. You can also see the unedited transcript of all Tweets from the meeting.

The authors of the paper state that relativistic shock breakout could explain a variety gamma-ray flares seen in observational data, and that this opens a new window for the study and detection of a variety of stellar explosions. They propose that the following stages occur:

  1. A gamma-ray flare is generated by the breakout shell when it becomes transparent.
  2. Due to light travel time effects, the breakout flare contains photons from faster and lighter shells, producing a high energy power-law spectrum. It also contains photons emitted by the breakout shell after it becomes transparent and cools adiabatically.
  3. The flare ends with a sharp decay at a time that coincides with the transition to the spherical phase.
  4. After the flare ends, spherical evolution dictates a steady decay of the luminosity.
  5. The post flare temperature decays at first at a steady rate, generated during the spherical phase. A sharp drop in the temperature is observed when shells which were not loaded by pairs during the shock crossing dominate the emission. The drop is typically from the X-ray range to the UV.
  6. The energy emitted before the steep temperature drop is often comparable to that emitted during the breakout flare. Thus, a signature of relativistic breakout in many scenarios is a bright and short gamma-ray flare and a delayed X-ray emission with comparable energy.
Mysteries solved?

The paper attempts to explain lots of unusual electromagnetic signatures that have been observed. The main discussion of the paper focused on previously unexplained mysteries that may now have been solved.

Great thing about this paper is that it explains multiple mysteries as aspects of a single phenomena – @partialobs

First, one of the great mysteries of GRBs, what explains the g-ray emission of SN 1998bw? – @partialobs

The g-ray luminosity of 1998bw was 1e-4 times that of the z~1 GRBs. So this paper explains as shock breakout – not a jet. – @partialobs

So, are you convinced that these relativistic shocks are a good explanation for all these mysterious observations?- @astronomyjc

In fact, it turns out that all “low-luminosity” GRBs from z<~0.3 exhibit shock-breakout according to model predictions – @partialobs

Derek (@partialobs) goes on to state that this paper has answered questions in his own research:

GRB 031203: z~0.1 discovered in gamma-rays, but with a peculiar signature of bright contemporaneous X-ray emission – @partialobs

I wrote a paper about the peculiar X-ray emission… couldn’t figure it out… now we know it’s shock breakout. – @partialobs

Apart from GRB 031203 and SN 1998bw, other observations that may now have been explained include: GRB 060218, GRB 100316D, SN 2008x.

Criticisms

The only real criticism of the paper was the lack of plots:

I’d love to see an actual gamma ray time plot against an equivalent optical light curve to actually get a feel for it. – @LeonBaloo

Lots of mention of light curves but non included that demonstrated their model vs. observations. – @kashfarooq

My chief complaint about paper would be its lack of figures. I think I could make a nice one showing the “fit” to data – @partialobs

Agreed. Figures would have made a fast reading of the paper much simpler!- @astronomyjc

Derek also thought that the theoretical discussion in the paper was is a bit sparing in credit to other recent advances.

Questions and Answers

Question:

Gamma rays, GRB’s: is there an exact definition of terms? I mean, a gammaray-source like pulsar PSR B1259-63 isn’t a classical GRB. – @AdrianusV

Answer:

Important Q, ‘gamma-ray burst’ is observationally defined – now we see it encompasses multiple phenomena  – @partialobs

Question:

What would cause phenomena that result in relativistic shock breakout to have *low* luminosity? I mean, why is it low? – @kashfarooq

Answer:

Cosmological (z>1) gamma-ray bursts with 1e54 erg isotropic-equiv’t luminosity are not shock breakouts – @partialobs

GRBs much too luminous for this model. Explained as highly-relativistic jets (Meszaros, Rees) – @partialobs

“low-luminosity” GRBs only qualify as “low-lum” by comparison. Seen from the low-z Universe, z<~0.5 – @partialobs

Question:

When they talk about ‘local’ gamma-ray bursts, how local do they mean? Within our galaxy?- @astronomyjc

Answer:

Re: “local”, this would be z<0.1 I think. Here they are interested in new phenomena, e.g. type Ia SNe shock breakout – @partialobs

They point out that a type Ia shock breakout would be lower-luminosity than anything we have seen. – @partialobs

Question:

What is the difference between Ia and .Ia SNe? – @kashfarooq

Answer:

The “type .Ia SN” (Bildsten term) is geomatric mean of classical nova & type Ia – big bomb on WD surface – @partialobs

Question:

Do neutrino’s play a role in these shock breakouts? – @AdrianusV

Answer:

these would not be associated with the shock itself (not dense enough) but maybe w/ the bomb – @partialobs

Question:

Paper mentions we may already have the data that matches their predictions. Has anyone started looking? – @kashfarooq

Can INTEGRAL have seen the data, perhaps? – @AdrianusV

Answer:

Just to be clear: Most events they are interpreting are Swift BAT detections, so these already in the bag – @partialobs

And we can expect more, for as long as Swift is operating. INTEGRAL also (at lower rate). – @partialobs

Conclusions

The paper discusses a very promising new area and what it is has achieved can be summed up in one Tweet:

My favorite part of the paper is burst-by-burst discussion, extraordinary to see them all brought under one umbrella – @partialobs

A full summary of the paper from Derek:

I would characterize it as an important synthesis of recent advances in understanding the shock breakout phenomenon that advances our theoretical understanding, explains multiple X- and g-ray transient phenomena, and predicts new populations of high-energy transient that have yet to be discovered. – @partialobs

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