Fermi ‘sharpens’ the view
The NASA satellite software has been rewritten, created with important Italian content. The LAT is now even more powerful
It is called PASS 8 and it is the new software that reconstructs the interaction between gamma photons and Fermi’s detectors. Now, seven years after its launch, the NASA satellite dedicated to the study of high and very high energy gamma radiation - in which Italy collaborates with the Italian Space Agency (ASI), the National Institute of Astrophysics (INAF) and the National Institute of Nuclear Physics (INFN) - has been enhanced and is now in a position to provide unprecedented performances.
In every astronomical instrument, the hardware is as important as the software used to analyse the data collected. The groups that construct instrumentation for ground-based telescopes plan regular upgrades of their instruments right from the start. For example, sensors can be changed, using more recent models, like larger ones that can be read more rapidly, or alignment and cooling systems, etc., can be modified. In parallel, it will be necessary to adapt the software to the new sensors, to the requests of users and to the new generations of computers, without forgetting new astronomical problems that may have emerged in the meantime.
If the performance of instruments in orbit is to be improved, the software needs to be modified. Obviously, the potential interest in software upgrades increases according to the extent to which the results depend upon the software used. For example, for an instrument that collects images, new on-board software can speed up reading and transmission, but is unlikely to improve the intrinsic quality of the data. The situation is different for the Large Area Telescope (LAT) aboard the NASA mission. The data relative to each gamma ray must be rebuilt starting from the trace that each electron-positron couple (result of the interaction of the gamma ray with matter from the detector) has left in the tracker. Passing from the traces of particles to the direction of arrival and to the energy of the gamma photon is the task of complex reconstruction software, which is an integral part of the Fermi telescope. This software was conceived well before the launch and although it functions very well, over time it has started to show some defects. In fact, with the increase in statistics, it is clear that the reconstruction programme was “losing” events at the lowest and highest energies. To resolve this problem, patching up the old software was not enough, making it necessary to rethink everything from the beginning.
“So, a decision was made to totally rewrite the gamma event construction software” explains Elisabetta Cavazzuti, who is in charge of the Fermi collaboration for the Italian Space Agency. “This was a difficult task that required the efforts of numerous brains over a very intense four-year period. Pass 8 changed (for the better) the performance of the Fermi mission, the data of which was entirely reanalysed using the new software. Seven and a half years after the launch of the LAT, it is now a more powerful instrument.”
The most important improvements are seen for the lowest energies (where the angular resolution of the instrument is, however, rather inadequate) and also the highest ones (where the angular resolution is at its best). For this reason, the PASS 8 data was initially used to compile a catalogue of sources revealed by Fermi at energies between 50 GeV and 2 TeV, an energy range that had been limited to ground-based gamma telescopes until now. By analysing 61,000 gamma rays with energies of over 50 GeV, Fermi revealed 360 sources, three quarters of which can be traced back to active galaxies, while 11% are linked to particles accelerated by pulsars or by supernova remains. The remaining 14% of the sources is not linked to known objects at other wavelengths.
When comparing the new Fermi catalogue to the compilation of gamma sources seen from the ground, by using special telescopes that take advantage of the Cherenkov effect, it is immediately clear that Fermi sees more than double the number of sources revealed by the gamma telescopes on the ground.
“The difference between Fermi LAT and the Cherenkov telescopes - explains Patrizia Caraveo, person in charge at INAF for the Fermi mission and CTA (Cherenkov Telescope Array) project board member - is all in the area of sky being explored. Fermi has a broad field of vision (about two steradians) and, when operating in scanning mode, it covers the whole sky every 3 hours, while the Cherenkov telescopes on the ground have a much smaller field of vision (a few square degrees) and must be pointing in a specific direction, covering a much more limited area of sky. Only 90 of the 360 sources in the Fermi catalogue correspond to sources seen by the ground-based telescopes (we must not forget that many of the extra-galactic gamma sources are extremely variable), demonstrating that Fermi can act as a pioneer, showing the ground-based telescopes the most interesting regions to study. PASS 8 creates new synergy between space gamma astronomy and the ground-based type, which will be very beneficial to the mini Array of small Cherenkov telescopes that INAF will construct as a forerunner to the Cherenkov Telescope Array.”
“Our team’s commitment to Pass 8 has produced results in a great deal of research conducted by Fermi” says Luca Latronico, person in charge of the Fermi LAT programme for INFN, where all of the telescope’s silicon trackers were integrated and many of the algorithms of the new reconstruction software were conceived in their laboratories. “Increased sensitivity makes it possible to detect very weak sources, or to considerably lower the limits of the minimum detectable flow, like in the case of dwarf galaxies, where failure to observe gamma photons translates directly into a very serious limitation on the mass and the probability of annihilation of the particles of dark matter that dominates these systems.” “The increased energy extension of the observations - Latronico continues - now makes it possible on the one hand to record electrons of 2 TeV with sufficient efficiency for seeking local sources, and photons of a few dozen MeV on the other, in search of polarized gamma sources or gamma ray bursts coming from the edges of the Universe.”