View Full Version : Yet Another Thrilling GRB! GRB 130702A and SN 2013dx.

Don Alexander
2013-Jul-25, 10:17 PM
This year is developing pretty spectacularly for Gamma-Ray Burst science! There was, of course, GRB 130427A, the most intense burst in three decades (http://cosmoquest.org/forum/showthread.php?143754-GRB-130427A-burst-of-the-(quarter)-century). Then, in early June, we had the first short GRB (130603B) ever for which a good absorption afterglow spectrum could be taken (additionally, it may feature a never-before-seen transient phenomenon called a "kilonova"). Three days later, a GRB (130606A) with a very bright afterglow at a redshift of 5.9 was discovered, allowing the best GRB spectra so far for an explosion at the end of the reionization era to be taken. Both GRBs have already produced papers.

But I want to talk about another event that occurred three weeks ago, GRB 130702A. I did not see the two recent papers featured in the "fun arXiv papers" thread, so I will present them here:

L. P. Singer et al.: Discovery and redshift of an optical afterglow in 71 square degrees: iPTF13bxl and GRB 130702A (http://arxiv.org/abs/1307.5851)

We report the discovery of the optical afterglow of the gamma-ray burst (GRB) 130702A, identified upon searching 71 square degrees surrounding the Fermi Gamma-ray Burst Monitor (GBM) localization. Discovered and characterized by the intermediate Palomar Transient Factory (iPTF), iPTF13bxl is the first afterglow discovered solely based on a GBM localization. Real-time image subtraction, machine learning, human vetting, and rapid response multi-wavelength follow-up enabled us to quickly narrow a list of 27,004 optical transient candidates to a single afterglow-like source. Detection of a new, fading X-ray source by Swift and a radio counterpart by CARMA and the VLA confirmed the association between iPTF13bxl and GRB 130702A. Spectroscopy with the Magellan and Palomar 200-inch telescopes showed the afterglow to be at a redshift of z=0.145, placing GRB 130702A among the lowest redshift GRBs detected to date. The prompt gamma-ray energy release and afterglow luminosity are intermediate between typical cosmological GRBs and nearby sub-luminous events such as GRB 980425 and GRB 060218. The bright afterglow and emerging supernova offer an opportunity for extensive panchromatic follow-up. Our discovery of iPTF13bxl demonstrates the first observational proof-of-principle for ~10 Fermi-iPTF localizations annually. Furthermore, it represents an important step towards overcoming the challenges inherent in uncovering faint optical counterparts to comparably localized gravitational wave events in the Advanced LIGO and Virgo era.

P. L. Kelly et al.: Evidence that Gamma-ray Burst 130702A Exploded in a Dwarf Satellite of a Massive Galaxy (http://arxiv.org/abs/1307.5103)

GRB 130702A is a nearby long-duration gamma-ray burst (LGRB) discovered by the Fermi satellite whose associated afterglow was detected by the Palomar Transient Factory. Subsequent photometric and spectroscopic monitoring has identified a coincident broad-lined Type Ic supernova (SN), and nebular emission detected near the explosion site is consistent with a redshift of z=0.145. The SN-GRB exploded at an offset of ~7.6" from the center of an inclined r=18.1 mag red disk-dominated galaxy, and ~0.6" from the center of a much fainter r=23 mag object. We obtained Keck-II DEIMOS spectra of the two objects and find a 2{\sigma} upper limit on their line-of-sight velocity offset of ~<60 km/s. If we project the SN-GRB coordinates onto the plane of the inclined massive disk galaxy, the explosion would have a ~61+-10 kpc offset, or ~6 times the galaxy's half-light radius. This large estimated nuclear offset suggests that the faint source is not a star-forming region of the massive red galaxy but is instead a dwarf galaxy. The star-formation rate of the dwarf galaxy is ~0.05 solar masses per year, and we place an upper limit on its oxygen abundance of 12 + log(O/H) < 8.16 dex. The identification of an LGRB in a dwarf satellite of a massive, metal-rich primary galaxy suggests that recent detections of LGRBs spatially coincident with metal-rich galaxies may be, in some cases, superpositions.

What Singer and colleagues at the intermediate Palomar Transient Factory have done is extremely impressive!:clap: After all, the GRB community has worked for literally decades toward rapidly available, small error circles, which has culminated in Swift giving us arcsecond error circles within usually just 120 seconds or so!! And now these people come along and discover an afterglow in a Fermi GBM error circle with 4 degrees radius... (I shall note that: a) really bright GRBs have error circles of one degree radius, roughly 16 times smaller. b) There is a systematic error component involved in GBM error circles which is usually at least several degrees, and the GBM position CAN quite often be TEN or more degrees wrong. This one was very precise, essentially the statistical error circle had no systematics.)

And not just did they find this moderately bright needle in an absolutely gigantic haystack... No, the GRB turns out to be anything but a run-of-the-mill event. It seems to belong to a slowly emerging class of "transition GRBs" which bridge the high luminsoity cosmological events (like GRB 130427A) with the class of local universe low-luminosity GRBs. GRB 130702A had a clear, albeit not overly luminous afterglow component while at the same time morphing into a bright, classical broad-lined Type Ic SN. Which, at the ridiculously low redshift of z=0.145, will yield much better spectroscopic results than SN 2013cq associated with GRB 130427A (also due to the extremely faint underlying host). And even said host system is pretty much unprecedented, an extremely underluminous dwarf galaxy in a large-scale association.

I'm really looking forward to the SN results on this one, they should turn out to be quite spectacular! :)

2013-Jul-27, 07:28 PM
Yes this is all good stuff Don (or David). It
must be 4 or 5 gamma bursts connected with
supernova now over the years. You guys at the
coal face so to speak will keep the data piling
up. My own little thoughts are about the origin
of the high energy photons, the supernova or
something associated. I wonder if neutrinos
passing through white dwarf stars in line
almost from the supernova form a tight beam
through relatavistic focussing, ie the neutrinos
just emerging that hit particles give the photons.
But beams that can cover intergalactic distances?
Maybe a big ask!

2013-Jul-29, 02:02 PM
Yes - kudos for David, and thanks for taking us by the hand and explaining how significant the research is. Now we have a great chance of observing the incipient stages of a nearby type Ia; assuming there is an associated gamma ray burst, very near to us. All we need is for it to happen....