Detection of the optical flash from the gamma-ray burst coming simultaneously with gamma-rays by small telescopes, Pi of the Sky, TORTORA and RAPTOR provides new clues for understanding the physics of the most powerful cosmic explosions and proves potential of continuous sky monitoring by detectors of a new type.
Every night several dozens of robotic telescopes monitor the night sky looking for interesting astrophysical phenomena. The day 2008.03.19 was unusual for some of them. At 2:19 a.m. EDT the NASA Swift satellite detected gamma rays from a powerful explosion in the constellation Bootes. This event, named GRB 080319B, probably originated from a violent death of a super-massive star in a so-called hypernova explosion ending with a black-hole creation. It happened 7.5 billion light years from the Earth, halfway across the visible Universe.
In a paper to appear in Thursday's September 11th issue of the journal Nature, a team of 92 authors, including 10 members of the "Pi of the Sky" team from Poland, report observations across the spectrum gathered by a simultaneous effort of many research groups. Swift, "Pi of the Sky", the Russian KONUS detector on NASA's Wind satellite and optical camera TORTORA, run by a Russian-Italian collaboration together with a handful of large telescopes conducted observations that began 30 minutes before the explosion and followed its afterglow for months. The equipment which was used to gather the precise data varied from very small to very large. Large telescopes can provide detailed information about the spectrum and measure the distance to burst. Small telescopes can continuously monitor the whole sky. They are the first to see unusual phenomena, which was famously demonstrated with GRB080319B.
Such explosions are observed about a hundred times per year and are known as gamma-ray burst, since they are observed mainly in gamma rays. GRB 080319B was unusually strong among them. It proved to be exceptional in one more aspect: the burst in gamma rays was accompanied by an optical flash, which - for a few seconds - could have been seen even by a naked human eye, if somebody had happened to look in its direction. For the first time the light was recorded by optical telescopes with fast cameras. The first to detect the optical light of GRB 080319B, simultaneously with the detection of gamma rays by Swift, was "Pi of the Sky".
The "Pi of the Sky" apparatus, located at the Las Campanas Observatory in Chile is operated by a consortium of Polish institutes. Its telescope has a diameter of 71 mm only. However, it was designed precisely for events like this one. The apparatus monitors continuously large fraction of the sky taking 10 s exposures and detects optical flashes independently, while the satellite information confirms the origin of the flash. This strategy made an early optical detection of GRB 080319B possible.
During the night of 2008.03.18/19 "Pi of the Sky" was routinely monitoring the sky looking in the same direction as the Swift satellite and taking 10 s long images. The image stated at 6:12:47 UT showed a new object. It was automatically detected by the burst recognition algorithm. At 6:12:49 UT Swift received the first gamma rays. This moment marked as T=0 stands for the beginning of the burst. Only two seconds later (T0+2s) another robotic telescope RAPTOR took an image of the object. At T0+16s the object became visible to TORTORA optical camera which made a wonderful movie of the peak of the burst with high temporal resolution. At T0+17s Swift sent out the alert about the burst and many telescopes on the ground turned towards the target. Swift's on-board telescope UVOT begin observation at T0+51s. One hour later the Very Large Telescope in Chile measured the distance to the burst, which was confirmed by the Hobby-Eberly Telescope in Texas.
Next day, it was already clear that a new important step towards understanding of physics of gamma-ray bursts has been made. It was a matter of a long lasting discussion whether the optical afterglow of a gamma-ray burst appears simultaneously with the gamma rays. In most of the presently accepted models gamma rays and optical light originate from a thin jet of matter escaping from an exploding star. However, it was not clear whether both types of radiation are produced by the same process at the same place. The simultaneous detection of both explosions - optical and gamma - has proven that indeed, they come from the same region. However, surprisingly enough, "Pi of the Sky" measured, that the intensity of the optical light is 10000 higher compared to gamma rays. This menas, that they cannot be produced in the same process.
These observations challenge many existing models and opens a new perspective for understanding the physics of the most powerful explosions in the Universe. In the Thursday's Nature Racusin and collaborators explain unexpected features of GRB 080319B, suggesting that its jet directed toward Earth contained an ultra-fast component just 0.4 degree across and this core resided within a slightly less energetic jet about 20 times wider.
Interestingly, the publication of the results of observations of GRB 080319B comes in time with the start-up of the Large Hadron Collider (LHC) - the world's largest particle accelerator at CERN, in which some members of the "Pi of the Sky" projects are also involved. Thus, a new area of investigations of the biggest and smallest explosions in the Universe is now being opened simultaneously underlying complementarity of space and laboratory observations.
So far "Pi of the Sky" apparatus consisted of two cameras installed in Las Campanas Observatory in Chile. They cover 20°x20° of the sky. To increase the chance of observing a GRB a new set of 32 cameras is under construction. They will cover 1/3 of the visible sky continuously. Original plan was to cover p steradians of the sky, justifying the name for the project. The name recalls also the title of John Barrow's book "Pi in the sky" arguing that the phenomena we observe are governed by physical laws expressed in mathematical language. "Pi of the Sky" is a member of a growing family of small, low-cost, robotic devices, like BOOTES, MASTER, RAPTOR, REM/TORTORA, ROTSE, SkyNet, TAROT and many others, which night by night patiently follow the sky searching for prey like the extraordinary bright gamma ray burst. Sophisticated technology allows them to operate almost without human intervention. Everything, starting from weather control, through target selection, till self diagnostics and repair, is performed automatically.
The idea of continuous monitoring of the whole sky was advanced by a late professor Bogdan Paczynski, a renown Polish astrophysicist working at the Princeton University. He also postulated to use wide field robotic telescopes for early optical observations of gamma ray bursts, this idea was finally confirmed by extraordinary observations of GRB 080319B. The number of robotic telescopes increases every year and their results are becoming more and more important. We can expect much more exciting discoveries from automatic observatories in next years.
The project is conducted by collaboration of Polish research institutes:* Soltan Institute for Nuclear Studies (Warsaw)
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