What are Gamma Ray Bursts?
Everyday, a part of the sky suddenly outshines the rest, to eyes sensitive to gamma light. These gamma-ray bursts (GRB) are flashes of the Universe's most extreme energy, which last seconds to a few minutes. Luckily for us, everything seemed to suggest that they belonged to the cosmological Universe, in other words several billion light years away in distance and thus in time.
Two neutron stars, finally meeting after courting each other in a frenetic dance, or the last scream of matter stretched and heated to millions of degrees before being engulfed by a black hole, are possible scenarios for what causes such bursts. Yet what triggers them remains a mystery and observing them from the ground remains impossible (the atmosphere acts as a shield).
How does INTEGRAL detect GRBs?
The detection of GRB was among the secondary objectives of Integral's mission: thanks to the SPI Anti-Coincidence System (ACS) which provides an omnidirectional field of detection, the European satellitehas spotted more than 400 GRBs to date.
When they come into its fields of view, at a rate of approximately one per month, the Integral Burst Alert System (IBAS) immediately alerts via Internet the world's scientific community and within a few tens of seconds other observatories can point to the given coordinates. This method ensures follow-up observations of GRBs at different wavelengths (mainly X-ray and optical) and observation of the afterglow. Such a spectacular coordination has been demonstrated more than once, as ESA's in-orbit XMM-Newton X-ray Observatory is pointed shortly after the burst started.
In most case, only the ACS detects GRBs at a rate of one in two days, and plots its light curve (intensity of the source as it evolves in time). The exact location of the burst is obtained by the triangulation method as part of the 3rd interplanetary Network (IPN) of gamma-ray burst detectors which includes NASA's Swift and Italy's BeppoSAX.
What have Integral's results brought to the scientific community?
On 3 December 2003, Integral detected the specific activity of a gamma ray burst named GRB031203. Less than 20 seconds later, IBIS communicated its approximate coordinates, and a few hours later XMM-Newton determined a more precise position. This turned out to be not only the closest GRB ever seen, but also the faintest. This invalidates the use of GRBs as "standard candles", objects of known set brightness used to determine astronomical distances derived from the apparent brightness. It also means they do not belong exclusively to the domain of cosmological universe. Until now, scientists thought GRBs were remnants of the past, even though GRB980425 was an odd point on the graph. But now, GRB980425 is joined by GRB031203, both defying the rule by being so close and sub-energetic. We can ignore a single outlier, but two calls for a revision of our theories.
Another key event in Integral's life was the launch in 2005 of the US Swift satellite dedicated to detecting and observing GRBs. The mission has been a success and already gathered new data, despite a temporary failure. However, it does not completely replace Integral which spotted GRB070707 when SWIFT did not.
1/ and 2/ Observation of GRB 030227 to the right of the Crab pulsar (Credits: CEA).
3/ Gamma ray burst as seen by Integral/ISGRI (Credits ESA).
4/ and 5/ Everyday, a part of the sky suddenly outshines the rest... (Credits: ESA)
Integral detects variable polarization from gamma-ray burst GRB041219A
Although Integral was not optimized for the detection of GRBs, many GRBs have gone off serendipitously inside the field of view of its instruments. When this happens, Integral's high sensitivity and unprecedented spatial resolution at these energies make the data it collects very valuable: the closer the GRB is to the centre of the field of view, the richer the temporal and spectral information Integral can reveal. On 19 December 2004, a GRB was detected very close (~3deg) to the centre of the field of view of the IBIS instrument, well within the region of its maximum sensitivity. Although IBIS is an imaging instrument, the two detectors, PICsIT (composed of scintillator material) and ISGRI (made of semiconductor crystals), can also be used as a Compton polarimeter if the brightness of the source being observed is high enough.
GRB041219A, as the event was named, turns out to be the brightest GRB detected by Integral to date. Because of this, the authors have been able to perform a time-resolved analysis of the polarization of the signal. This is an important feature of this analysis, because on most occasions the fluxes are not strong enough and the signal must be integrated over the entire duration of the GRB, and if the polarization of the signal changes with time, the modulation of the signal is smeared out, and eventually it falls below the threshold of detectability.
- Diego Götz, Philippe Laurent, and François Lebrun, Frédéric Daigne, and Željka Bošnjak "Variable polarization measured in the prompt emission of GRB 041219A using IBIS on board Integral", 2009, Astrophysical Journal Letters 695 (2009) L208-L212.
- Sazonov, S. Yu., A. A. Lutovinov, and R. A. Sunyaev. "An apparently normal gamma-ray burst with an unusually low luminosity." Nature 430 (2004): 646-648
- Soderberg, A. M. et al "The sub-energetic gamma-ray burst GRB 031203 as a cosmic analogue to the nearby GRB 980425." Nature 430 (2004): 648-650
- ISDC's GCN circular, "GRB 070707: A long GRB detected by Integral.", number 6605 to 6615