Hello, everyone, this fresh off the press, you read it here first™!!
This morning, a GRB occurred, fittingly labeled GRB 130427A, which is turning out to be very, very extraordinary.
It triggered the Swift satellite, which slewed to the GRB, finding an extremely luminous X-ray afterglow, at > 6,000 counts/s (uncorrected for pile-up, I expect the final value to go to about 10,000). This is among the highest ever measured, but still beneath some like 100621A which peaked at > 100,000 ct/s.
The Swift BAT light curve over a broader time range (http://gcn.gsfc.nasa.gov/notices_s/s...20000msbx.jpeg) reveals a huge peak about 50 seconds before Swift actually triggered. Turns out Swift was slewing to a pre-planned target when the main activity occurred (triggering is turned off during slews), then triggered on the tail of the main peak, before another smaller peak at roughly 100 secs occurred, which is the one seen in X-rays.
The main Swift peak, even in this raw view, exceeds 100,000 counts per second in BAT, which is higher than any GRB detected except for some SGR 1900+14 flares ("Storm episode" on 060329).
The extreme fluence of this GRB was revealed in full by the SPI-ACS detector onboard INTEGRAL: http://www.isdc.unige.ch/integral/ib...-05802-39824-0
The last time a GRB reached about 90k counts per 50 msec in SPI-ACS, it was GRB 110918A, which to this day holds the record as the highest peak luminosity ever measured for a GRB.
And by now, the Swift BAT automatic refined analysis has appeared. Hidden in all these numbers and figures are a few astonishing facts.
- The 15-350 keV fluence is measured to be 6.5E-4 erg/cm^2. To put this into perspective, the famous "naked-eye burst" 080319B, one of the most intense GRBs ever detected, had a fluence in the same window of 1.6E-4, so a factor 4 lower. The "broadband" fluence of 080319B (measured over a much broader spectral window up to many MeV) is about identical to this one's fluence in the narrow Swift energy window.
- The peak count rate is close to 50 photons/s/detector. The current record holder for Swift, GRB 090424, reached about 11, which equated to ~ 77 photons/cm^2. So this one will likely have ~ 400 ph/s/det.
- The spectrum in the Swift energy window is an absolutely straight power law with a slope of 1.2. This indicates that a) the peak energy of the spectrum lies above the Swift window and b) it lies significantly above the Swift window. So Sift is just seeing a relatively small fraction of the entire emission.
All this leads me to predict a total fluence in a broadband window of > 1x10^-3 erg/cm^2 (remember, the Naked Eye-Burst had ~ 6 x 10^4, and even GRB 110918A had "only" ~ 8 x 10^-4), with something in the range of 2 - 3 x 10^-3 possible. The last time a GRB with such a fluence was detected was 25 years ago, GRB 881024. Only two other events (830801B and 840384) have fluences in the range of a few millierg/cm^2.
Therefore GRB 130427A is likely the most energetic (observed) GRB of the last 25 years, and maybe among the top 3 GRBs EVER (well, ever since the late 1960s, of course).
Of course, the location of this GRB was rapidly observed with ground-based telescopes, Several robots were on target within minutes, detecting a very bright afterglow at about 11th magnitude. This is actually "moderately unextraordinary", considering GRB 080319B reached mag 5.3. Spectroscopy with Gemini-North by Levan et al. reveals a redshift of 0.34, making this one of the closer GRBs known and explaining its extraordinary brightness. Continuing follow-up shows an afterglow which in the first hours is brighter than that of any Swift GRB, but still significantly fainter than that of the famous GRB 030329 which occurred at redshift 0.16.
Some very "back of the envelope" calculations I have done show that the GRB and its afterglow are, intrinsically and compared to the whole sample we know, actually rather run-of-the-mill. If this GRB hat happened at a redshift of 2, it would have been good but nothing spectacular - only the low distance makes this such an incredible event.
It lies at (J2000) RA = 11:32:32.84, Dec. = +27:41:56.2, for anyone with a good (> 30 cm) amateur telescope and a CCD. It is not, and never was, a naked-eye object, and was visible in binoculars only for some minutes.
EDIT: Oh, two more things:
- Sadly, this burst is not going to have very good Swift data. The satellite relies on field stars to lock its star tracker (similar to guiding for ground-based telescopes) and the field is very empty. Initial star tracker loss-of-lock caused an error in the aspect, and the reported X-ray position was 50 arcseconds (!) off the correct one.
- In much better news, unlike GRB 110918A, this one was detected by the GBM instrument on Fermi (with the mindbogglingly ludicrous detection significance of 5014 sigma... :| ), and it seems to be in the field-of-view of the LAT instrument, which hopefully was also slewed to the GRB position. Therefore, this GRB may yield the best high-energy data set ever taken.