A Physical Background Model for the Fermi Gamma-ray Burst Monitor

We present the first physically motivated background model for the Gamma-Ray Burst Monitor (GBM) onboard the Fermi satellite. Such a physically motivated background model has the potential to significantly improve the scientific output of Fermi/GBM, as it can be used to improve the background estimate for spectral analysis and localization of Gamma-Ray Bursts (GRBs) and other sources. Additionally, it can also lead to detections of new transient events, since long/weak or slowly rising ones do not activate one of the existing trigger algorithms. In this paper we show the derivation of such a physically motivated background model, which includes the modeling of the different background sources and the correct handling of the response of GBM. While the goal of the paper is to introduce the model rather than developing a transient search algorithm, we demonstrate the ability of the model to fit the background seen by GBM by showing four applications, namely (1) for a canonical GRB, (2) for the ultra-long GRB 091024, (3) for the V404 Cygni outburst in June 2015, and (4) the ultra-long GRB 130925A.Comment: Accepted for publication in A&A, 15 pages, 22 figure

Time-resolved spectral catalogue of INTEGRAL/SPI gamma-ray bursts

Since its launch in 2002, the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite has detected many gamma-ray bursts (GRBs), which are summarised in the INTEGRAL Burst Alert System (IBAS) catalogue. This catalogue combines triggers from the data of the Imager on Board the INTEGRAL (IBIS) and of the anti-coincident shield (ACS) of the SPectrometer on INTEGRAL (SPI). Since the Germanium detectors of SPI also serve as a valuable GRB detector on their own, we present an up-to-date time-resolved catalogue covering all GRBs detected by SPI through the end of 2021 in this work. Thanks to SPI’s high energy coverage (20 keV−8 MeV) and excellent energy resolution, it can improve the modelling of the curvature of the spectrum around the peak and, consequently, it could provide clues on the still unknown emission mechanism of GRBs. We split the SPI light curves of the individual GRBs in time bins of approximately constant signals to determine the temporal evolution of spectral parameters. We tested both the empirical spectral models as well as a physical synchrotron spectral model against the data. For most GRBs, the SPI data cannot constrain the high-energy power law shape above the peak energy, but the parameter distributions for the cut-off power law fits are similar to those of the time-resolved catalogue of gamma-ray burst monitor (GBM) GRBs. We find that a physical synchrotron model can fit the SPI data of GRBs well. While checking against detections of other GRB instruments, we identified one new SPI GRB in the SPI field of view that had not been reported before

Improving INTEGRAL/SPI data analysis of GRBs

INTEGRAL/SPI is a coded mask instrument observing since 2002 in the keV to MeV energy range, which covers the peak of the $\nu F\nu$ spectrum of most Gamma-Ray Bursts (GRBs). Since its launch in 2008, Fermi/GBM has been the primary instrument for analyzing GRBs in the energy range between $\approx$ 10 keV to $\approx$ 10 MeV. Herein, we show that SPI, covering a similar energy range, can give equivalently constraining results for some parameters if we use an advanced analysis method. Also, combining the data of both instruments reduces the allowed parameter space in spectral fits. The main advantage of SPI as compared to GBM is the energy resolution of $\approx$ 0.2\% at 1.3 MeV compared to $\approx$ 10\% for GBM. Therefore, SPI is an ideal instrument to precisely measure the curvature of the spectrum. This is important, as it has been shown in recent years that physical models rather than heuristic functions should be fit to GRB data to obtain better insights into their still unknown emission mechanism, and the curvature of the peak is unique to the different physical models. To fit physical models to SPI GRB data and get the maximal amount of information from the data, we developed a new open source analysis software {\tt PySPI}. We apply these new techniques to GRB 120711A in order to validate and showcase {\tt PySPI}'s capabilities. We show that {\tt PySPI} improves the analysis of SPI GRB data compared to the {\tt OSA} analysis. In addition, we demonstrate that the GBM and the SPI data of this GRB can be fitted well with a physical synchrotron model. This evinces that SPI can play an important role in GRB spectral model fitting.Comment: 11 pages, 13 figures, Accepted by A&