We present the first physically motivated background model for the Gamma-ray Burst Monitor (GBM) on board 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, this model can also lead to detections of new transient events, since long and weak, or slowly rising, events 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 the following four applications for (1) a canonical GRB, (2) the ultra-long GRB 091024, (3) the V404 Cygni outburst in June 2015, and (4) the ultra-long GRB 130925A.

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Context. In the era of time-domain, multi-messenger astronomy, the detection of transient events on the high-energy electromagnetic sky has become more important than ever. Previous attempts to systematically search for onboard, untriggered events in the data of Fermi-GBM have been limited to short-duration signals with variability time scales smaller than ≈ 1 min. This is due to the dominance of background variations on longer timescales. Aims. In this study, we aim to achieve a detection of slowly rising or long-duration transient events with high sensitivity and a full coverage of the GBM spectrum. Methods. We made use of our earlier developed physical background model, which allows us to effectively decouple the signal from long-duration transient sources from the complex varying background seen with the Fermi-GBM instrument. We implemented a novel trigger algorithm to detect signals in the variations of the time series that is composed of simultaneous measures in the light curves of the different Fermi-GBM detectors in different energy bands. To allow for a continuous search in the data stream of the satellite, the new detection algorithm was embedded in a fully automatic data analysis pipeline. After the detection of a new transient source, we also performed a joint fit for spectrum and location using the BALROG algorithm.Results. The results from extensive simulations demonstrate that the developed trigger algorithm is sensitive down to sub-Crab intensities (depending on the search timescale) and has a near-optimal detection performance. During a two month test run on real Fermi-GBM data, the pipeline detected more than 300 untriggered transient signals. We verified, for one of these transient detections, that it originated from a known astrophysical source, namely, the Vela X-1 pulsar, showing pulsed emission for more than seven hours. More generally, this method enables a systematic search for weak or long-duration transients.Key words. surveys -methods: data analysis -techniques: miscellaneous -gamma-ray burst: general -pulsars: generalinstabilities 1 https://swift.gsfc.nasa.gov/results/transients/ Article number,

The spectrometer on the international gamma-ray astrophysics laboratory (INTEGRAL/SPI) is a coded mask instrument observing since 2002 in the keV to MeV energy range, which covers the peak of the νFν spectrum of most gamma-ray bursts (GRBs). Since its launch in 2008, the gamma-ray burst monitor (GBM) on board the Fermi satellite has been the primary instrument for analysing GRBs in the energy range between ≈10 keV and ≈10 MeV. Here, we show that the spectrometer on board INTEGRAL, named ‘SPI’, which covers 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 over GBM is the energy resolution of ≈0.2% at 1.3 MeV compared to ≈10% for GBM. Therefore, SPI is an ideal instrument for precisely measuring 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 new open-source analysis software, PySPI. We apply these new techniques to GRB 120711A in order to validate and showcase the capabilities of this software. We show that PySPI improves the analysis of SPI GRB data compared to the INTEGRAL off-line scientific analysis software (OSA). In addition, we demonstrate that the GBM and the SPI data for this particular GRB can be fitted well with a physical synchrotron model. This demonstrates that SPI can play an important role in GRB spectral model fitting.

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.