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Register a new userProbing the Bright Radio Flare and Afterglow of GRB 130427A with the Arcminute Microkelvin Imager
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G. E. Anderson
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We present one of the best sampled early time light curves of a gamma-ray burst (GRB) at radio wavelengths. Using the Arcminute Mircrokelvin Imager (AMI) we observed GRB 130427A at the central frequency of 15.7 GHz between 0.36 and 59.32 days post-burst. These results yield one of the earliest radio detections of a GRB and demonstrate a clear rise in flux less than one day after the gamma-ray trigger followed by a rapid decline. This early time radio emission probably originates in the GRB reverse shock so our AMI light curve reveals the first ever confirmed detection of a reverse shock peak in the radio domain. At later times (about 3.2 days post-burst) the rate of decline decreases, indicating that the forward shock component has begun to dominate the light-curve. Comparisons of the AMI light curve with modelling conducted by Perley et al. show that the most likely explanation of the early time 15.7 GHz peak is caused by the self-absorption turn-over frequency, rather than the peak frequency, of the reverse shock moving through the observing bands.
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The Arcminute Microkelvin Imager is a pair of interferometer arrays operating with six frequency channels spanning 13.9-18.2 GHz, with very high sensitivity to angular scales 30-10. The telescope is aimed principally at Sunyaev-Zeldovich imaging of clusters of galaxies. We discuss the design of the telescope and describe and explain its electronic and mechanical systems.
The optical light that is generated simultaneously with the x-rays and gamma-rays during a gamma-ray burst (GRB) provides clues about the nature of the explosions that occur as massive stars collapse to form black holes. We report on the bright optical flash and fading afterglow from the powerful burst GRB 130427A and present a comparison with the properties of the gamma-ray emission that show correlation of the optical and >100 MeV photon flux light curves during the first 7,000 seconds. We attribute this correlation to co-generation in an external shock. The simultaneous, multi-color, optical observations are best explained at early times by reverse shock emission generated in the relativistic burst ejecta as it collides with surrounding material and at late times by a forward shock traversing the circumburst environment. The link between optical afterglow and >100 MeV emission suggests that nearby early peaked afterglows will be the best candidates for studying GRB emission at GeV/TeV energies.
The Arcminute Microkelvin Imager (AMI) telescopes located at the Mullard Radio Astronomy Observatory near Cambridge have been significantly enhanced by the implementation of a new digital correlator with 1.2 MHz spectral resolution. This system has replaced a 750-MHz resolution analogue lag-based correlator, and was designed to mitigate the effects of radio frequency interference, particularly from geostationary satellites that contaminate observations at low declinations. The upgraded instrument consists of 18 ROACH2 Field Programmable Gate Array platforms used to implement a pair of real-time FX correlators -- one for each of AMIs two arrays. The new system separates the down-converted RF baseband signal from each AMI receiver into two 2.3 GHz-wide sub-bands which are each digitized at 5-Gsps with 8 bits of precision. These digital data streams are filtered into 2048 frequency channels and cross-correlated using FPGA hardware, with a commercial 10 Gb Ethernet switch providing high-speed data interconnect. Images formed using data from the new digital correlator show over an order of magnitude improvement in dynamic range over the previous system. The ability to observe at low declinations has also been significantly improved.
We present the Arcminute Microkelvin Imager (AMI) Large Array catalogue of 139 gamma-ray bursts (GRBs). AMI observes at a central frequency of 15.7 GHz and is equipped with a fully automated rapid-response mode, which enables the telescope to respond to high-energy transients detected by Swift. On receiving a transient alert, AMI can be on-target within two minutes, scheduling later start times if the source is below the horizon. Further AMI observations are manually scheduled for several days following the trigger. The AMI GRB programme probes the early-time (< 1 day) radio properties of GRBs, and has obtained some of the earliest radio detections (GRB 130427A at 0.36 and GRB 130907A at 0.51 days post-burst). As all Swift GRBs visible to AMI are observed, this catalogue provides the first representative sample of GRB radio properties, unbiased by multi-wavelength selection criteria. We report the detection of six GRB radio afterglows that were not previously detected by other radio telescopes, increasing the rate of radio detections by 50% over an 18-month period. The AMI catalogue implies a Swift GRB radio detection rate of >15%, down to ~0.2 mJy/beam. However, scaling this by the fraction of GRBs AMI would have detected in the Chandra & Frail sample (all radio-observed GRBs between 1997 - 2011), it is possible ~44 - 56% of Swift GRBs are radio-bright, down to ~0.1 - 0.15 mJy/beam. This increase from the Chandra & Frail rate (~30%) is likely due to the AMI rapid-response mode, which allows observations to begin while the reverse-shock is contributing to the radio afterglow.
The complex multiwavelength emission of GRB afterglow 130427A (monitored in the radio up to 10 days, in the optical and X-ray until 50 days, and at GeV energies until 1 day) can be accounted for by a hybrid reverse-forward shock synchrotron model, with inverse-Compton emerging only above a few GeV. The high ratio of the early optical to late radio flux requires that the ambient medium is a wind and that the forward-shock synchrotron spectrum peaks in the optical at about 10 ks. The latter has two consequences: the wind must be very tenuous and the optical emission before 10 ks must arise from the reverse-shock, as suggested also by the bright optical flash that Raptor has monitored during the prompt emission phase (<100 s). The VLA radio emission is from the reverse-shock, the Swift X-ray emission is mostly from the forward-shock, but the both shocks give comparable contributions to the Fermi GeV emission. The weak wind implies a large blast-wave radius (8 t_{day}^{1/2} pc), which requires a very tenuous circumstellar medium, suggesting that the massive stellar progenitor of GRB 130427A resided in a super-bubble.
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