Extragalactic Astronomy in the News: GRB 130427A

Apr 29, 2013
updated Apr 29, 2013

On Apr 27, 2013, several satellites in orbit around the Earth noticed a burst of gamma rays. Not just any old burst, but one of the (apparently) brightest bursts ever detected. Let's look at what we currently know about this Gamma-Ray Burst (GRB), just two days after its discovery, and what we might learn from it in the future.

The position of the burst is (based on optical afterglow measurements, NOT on the initial Swift BAT report, which was affected by spacecraft motion)

  J2000.0   RA 11:32:32.84        Dec +27:41:56.2
               173.13683               27.69894 

If we look at this region of the sky in the POSS II F plate (thanks to Aladin tool), we see nothing.

But SOMETHING went ka-boom. The BAT instrument on the Swift satellite saw a brief, very intense burst, followed by a longer but less intense period of emission.

Aboard the Integral spacecraft, the SPI-ACS instrument also detected a huge number of gamma rays from this event; the graph below shows just the brief initial "spike." The large number of brief peaks and valleys is typical of a hard, high-luminosity event.

Continued observations show that the X-ray flux from this event decreased continuously over the next couple of days. Click on the graph to get a wealth of ASCII data.

The hardness ratio -- the relative amount of high-energy to low-energy X-rays -- decreased slightly at first, then rose a bit. If you look closely at the graph above, you'll see that the dip occurs from about 200 to 300 seconds after the trigger: at roughly that same time, the light curve undergoes a change in slope. I'll let a real GRB expert explain.

The dip in the hardness ratio is a common phenomenon. You'll see it pretty much corresponds to the steep part of the decay. That is so-called high-latitude emission, produced by a rather complicated relativistic process that occurs when the prompt emission "turns off". Such emission decays steeply while at the same time softening (reddening) rapidly. Then the actual X-ray afterglow takes over, and such emission is usually rather hard (blue). Everything after that steep-to-shallow break is afterglow, and it is expected to not spectrally evolve, so the hardness remains constant. Purely in terms of spectral and temporal evolution, this X-ray afterglow is actually very run-of-the-mill. It's just hugely bright.
Explanation thanks to GRB expert Alexander Kann.

So, lots and lots and LOTS of gamma-ray and X-ray photons, making this one of the (apparently) brightest GRBs recorded since we've been looking. The only other events which might beat it (depending on exactly what one measures) are

But is there any data at other wavelengths? Yes!

The image on the left below is taken from the POSS II digitized sky survey; the image on the right was taken several hours later by Swedish amateur astronomers with a 16-inch telescope.

If we zoom in on the Swedish picture (or click on the image below to see the full-size version), we see a bright blueish object at the position of the GRB.

These measurements indicate that the GRB was apparent magnitude 17 or so around 23 hours UT on April 26, roughly 16 hours after the burst. Other optical measurements from the RAPTOR quick-response telescope suggest that the GRB reached optical magnitude R = 7.4, which would make it the second-brightest GRB in recorded history (after GRB 080319B , which reached magnitude 5.3 or so).

Patrick Wiggins used a 15-inch telescope in Stansbury Park, Utah, to measure the optical afterglow for several hours after the initial burst.

Note that the light curve appears linear if one graphs magnitude against the logarithm of time; what does that mean about the change in brightness with time?

Although the host galaxy is not bright enough for its redshift to have been measured by earlier galaxy surveys, gas in the host did produce absorption lines in the optical spectrum of the GRB itself. As the circular below reports, the redshift measured from those absorption lines is z = 0.34. The picture below is a gri color composite closeup of the host galaxy from the SDSS.

NUMBER:  14455
SUBJECT: GRB 130427A: Gemini-North redshift
DATE:    13/04/27 11:43:31 GMT
FROM:    Andrew Levan at U.of Leicester  

A.J. Levan (U. Warwick), S.B. Cenko (U.C. Berkeley), D.A. Perley
(Caltech) and N.R. Tanvir (U. Leicester) report for a larger

We obtained spectroscopy of the afterglow of GRB 130427A (Maselli
et al. GCN 14448, Elenin et al. GCN 14450, Perley et al. GCN 14451)
with Gemini-North / GMOS, begininnig at 09:19 UT roughly 1.5 hours
after the burst. Two different central wavelengths were observed
giving a coverage from ~3100-6700 A. The resulting spectra are of
very high signal to noise given the brightness of the afterglow.
In these spectra we identify absorption lines due to Ca H and K,
Mg I as well as the Mg II doublet at a common redshift of z=0.34.
We suggest this to be the redshift of GRB 130427A.

We do not see evidence for emission lines from an underlying host,
although given the brightness of the afterglow this is not surprising.
The absolute magnitude of object in SDSS, if at z=0.34 is M_R ~
-19.7, relatively bright for a GRB host.

We thank the Gemini-staff for their help in rapidly obtaining these

Text of GCN Circular 14455

While that is a long, long way from Earth, it's relatively close for a GRB.

Data taken from Nysewander, Fruchter, and Pe'er ApJ 701, 824-836 (2009)

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