Fermi GBM Research Articles

In-flight Energy Calibration of the GECAM Gamma-ray Detectors

The Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission is designed to monitor the Gamma-Ray Bursts (GRBs) associated with gravitational waves and other high-energy transient sources. The mission consists of two microsatellites which are planned to operate at the opposite sides of the Earth. Each GECAM satellite could detect and localize GRBs in about 8 keV–5 MeV with its 25 Gamma-Ray Detectors (GRDs). In this work, we report the in-flight energy calibration of GRDs using the characteristic gamma-ray lines in the background spectra, and show their performance evolution during the commissioning phase. Besides, a preliminary cross-calibration of energy response with Fermi GBM data is also presented, validating the energy response of GRDs.

Fermi GBM Observations of the Galactic Magnetar SGR 1935+2154 during Its 2022 January Activity

The X-ray and radio burst-emitting soft gamma-ray repeater SGR 1935+2154 has been the most active magnetar during the past decade, with at least one active episode each year since 2019. We report on the 2022 January activity as observed by the Fermi Gamma-Ray Burst Monitor and present the temporal and time-integrated spectral properties of the observed 112 X-ray bursts. No radio bursts were reported during the activity. The mean burst duration is comparable to most of the previous activities but is shorter than that of 2020, during which the magnetar emitted radio bursts. The mean burst spectrum is softer than the earlier activities and a long-term softening trend since 2016 is evident and is accompanied by a rise in the mean burst fluence and blackbody emission radii. The power-law indices of the RBB2−kTBB correlation also show an evolution during 2019–2022, where the soft component started becoming steeper, whereas the hard component shows a less steep trend. The burst properties show no significant evolution within the activity, which lasted for ∼ 45 days, and none of the observed bursts showed properties reminiscent of those of the hard, long X-ray counterpart to the 2020 fast radio burst FRB 200428 from the source. We discuss the results comparatively with the 8 yr history of the source since its discovery in 2014.

Some statistical remarks on GRBs jointly detected by Fermi and Swift satellites

ABSTRACT We made statistical analysis of the Fermi GBM and Swift BAT observational material, accumulated over 15 yr. We studied how GRB parameters (T90 duration, fluence, and peak flux) that were observed by only one satellite differ from those observed by both. In the latter case, it was possible to directly compare the values of the parameters that both satellites measured. The GRBs measured by both satellites were identified using the k-nearest neighbours algorithm in Euclidean distance. In the parameter space we determined the direction in which the jointly detected GRBs differ most from those detected by only one of the instruments using linear discriminant analyses. To get the strength of the relationship between the parameters obtained from the GBM and BAT, a canonical correlation was performed. The GBM and BAT T90 distributions were fitted with a linear combination of lognormal functions. The optimal number of such functions required for fit is two for GBM and three for BAT. Contrary to the widely accepted view, we found that the number of lognormal functions required for fitting the observed distribution of GRB durations does not allow us to deduce the number of central engine types responsible for GRBs.

The spectral analysis and study of GRB 120709A, a burst with three distinct emission episodes

The reason for quiescent intervals and isolated emission episodes in gamma-ray burst (GRB) light curves is still not well understood. Probing these questions further requires the study of the spectral characteristics and properties of the outflow of such GRBs. GRB 120709A is a burst with three distinct emission episodes, separated by quiescent periods lasting several seconds. The episodes are comparable with each other in terms of peak flux and duration. The GRB was observed by the Fermi LAT and GBM instruments, covering over six orders of magnitude of energy. In this work, we carry out the time-integrated and time-resolved spectral analysis of GRB 120709A. We use these results to study the temporal variation of its parameters. In particular, we find that the peak energy of the spectrum of each episode traces the flux of those episodes. We also study and parameterise the correlation between the isotropic luminosity and intrinsic peak energy. Finally, we determine the parameters of the outflow based on gamma-gamma opacity calculations. We find that if we require the MeV emission to occur above the photosphere, then the ratio of the GeV and MeV emission radii can not be greater than ∼1.2. This implies an internal origin of the GeV emission. This also allows us to estimate the average value of the Lorentz factor to be in the ∼100–200 range. We also evaluate the photospheric and MeV emission radii and find them to be in the (4.5–5.3) ×1012 cm and (2.1–5.2) ×1014 cm ranges, respectively. We note the limitations of this exercise due to the relative faintness of the GRB which prevents us from doing a very fine time-resolved analysis or fitting the spectra with multi-component models. We also discuss GRB 120709A in the context of models for distinct emission episodes and quiescent intervals.

A Search for Millilensing Gamma-Ray Bursts in the Observations of Fermi GBM

Millilensing of gamma-ray bursts (GRBs) is expected to manifest as multiple emission episodes in a single triggered GRB with similar light-curve patterns and similar spectrum properties. Identifying such lensed GRBs could help improve constraints on the abundance of compact dark matter. Here we present a systemic search for millilensing among 3000 GRBs observed by the Fermi GBM up to 2021 April. Eventually we find four interesting candidates by performing an autocorrelation test, hardness test, and time-integrated/resolved spectrum test. GRB 081126A and GRB 090717A are ranked as the first-class candidates based on their excellent performance in both temporal and spectrum analysis. GRB 081122A and GRB 110517B are ranked as the second-class candidates (suspected candidates), mainly because their two emission episodes show clear deviations in part of the time-resolved spectrum or in the time-integrated spectrum. Considering a point-mass model for the gravitational lens, our results suggest that the density parameter of lens objects with mass M L ∼ 106 M ⊙ is larger than 1.5 × 10−3.

A connection between spectral sharpness and energies as well as flux in fermi gamma-ray bursts

ABSTRACT We revisit the sharpness angle (θ) under the peak or break of gamma-ray burst (GRB) spectra with the best peak flux P and time-integrated F spectral data provided by the Fermi GBM Burst Catalogue. We compute the sharpness angles of best-fit model spectra and check some interesting relations between θ and physics quantities. It is found that (i) a positive correlations between θ and the observed fluence as well as the isotropic radiated energy holds among GRBs, especially for the F spectra; (ii) when checking the correlation between θ and energy flux a weaker anti-correlation holds among GRBs and a tighter anti-correlation holds within GRBs, especially in single pulses. Our results further show that the spectral shape is related to the energy and flux by cross-checking other measures of spectral curvature. The correlated relationship between spectral sharpness and energy flux can be well explained as a thermal origin for GRB prompt emission: A large entropy around the peak of the light curve makes the photosphere approach the saturation radius, resulting in an intense emission with a narrow spectrum; as the entropy decreases, the photosphere deviates from the saturation radius, resulting in weaker emission with a broader spectrum.

Differential Source Count for Gamma-Ray Bursts

Abstract Different forms of long gamma-ray burst (GRB) luminosity functions are considered on the basis of an explicit physical model. The inferred flux distributions are compared with the observed ones from two samples of GRBs, Swift and Fermi GBM. The best-fit parameters of the luminosity functions are found, and the physical interpretations are discussed. The results are consistent with the observation of a comparable number of flat-phase afterglows and monotonically decreasing ones.

Prospects of Observing Gamma-Ray Bursts with Orbital Detectors of Ultra-High-Energy Cosmic Rays

TUS (Tracking Ultraviolet Set-up), the first orbital telescope of ultra-high energy cosmic rays (UHECRs), has demonstrated that instruments of this kind have much broader capabilities and can also detect meteors, transient luminous events, anthropogenic glow and other processes taking place in Earth atmosphere in the UV frequency range. In this short letter, we address the question if an orbital UHECR detector can also register gamma-ray bursts (GRBs) via the fluorescent glow of irradiated nocturnal atmosphere. An analysis of the Fermi GBM catalog of GRBs and properties of several active and perspective instruments reveals that a detector with parameters of the optical system similar to those of the KLYPVE-EUSO (K-EUSO) or POEMMA telescopes and an appropriate \slow-mode" trigger will be able to observe only two GRBs in average every year of operation. POEMMA will register up to 3-4 GRBs in a year of operation in the stereo mode.

Electromagnetic counterpart to gravitational waves from coalescence of binary black hole with magnetic monopole charge

A Satellite Fermi GBM detected recent putative short Gamma Ray Bursts (GRBs) in coincident with the gravitational wave signal GW 150914 produced by the merger of binary black hole (BH). If at least one BH possess magnetic monopole charge in the binary BH system then the short-duration GRBs may produce during the final phase of a binary BH merger. The detection of gravitational waves GW 150914, GW 151226 and LVT 151012 by LIGO gave the evidence that merging of the compact object like binary BH often happens in our universe. In this paper, we report the qualitative model to discuss the generation of electromagnetic radiation from the merging of two BHs with equal masses and at least one BH carrying the magnetic monopole charge in the binary system. In this model, BH possess a magnetic monopole charge that may not be neutralized before the coalescence. During the inspiralling process, the magnetic monopole charge on the BH would produced the electric dipole moment. Short duration GRB would produce by the rapidly evolution of the electric dipole moment which may detectable on Earth. We predict that this model would be beneficial in the future to explain the generation of gravitational wave (GW) plus a electromagnetic signal of multi-wavelength from mergers of magnetically charged BHs.

Subsecond Spikes in Fermi GBM X-Ray Flux as a Probe for Solar Flare Particle Acceleration.

Solar flares are known to release a large amount of energy into accelerating electrons. Studying small timescale (⩽2s) fluctuations in nonthermal X-ray flux offers the opportunity to probe the nature of those acceleration mechanisms. By comparing the durations, differences in timing between energy bands, and the periodicity of these spikes against the relevant timescales called for by various acceleration mechanisms, a test for each mechanism's validity can be made. This work details the analysis of fast fluctuations in Fermi Gamma-ray Burst Monitor (Fermi GBM) data from two M9.3 class solar flares that occurred on SOL2011-07-30 and SOL2011-08-04. This study shows the usefulness of Fermi GBM data as a means of examining these small timescale spikes and presents a rigorous method of identifying, counting, and measuring the temporal properties of these subsecond X-ray spikes. In the two flares examined we found spikes to primarily occur in spans of 60-100 s in the impulsive phase. The relative spike intensity averaged between 6% and 28% when compared to the slowly varying component of the X-ray flux. The average spike durations were 0.49 and 0.38 s for the 2 flares. The spike duration distribution for the SOL2011-08-04 flare was found to follow a power law with a -1.2 ± 0.3 index. Of the three spiking intervals identified, only one was found to have a periodicity, showing significant power at the 1.7 ± 0.1 Hz frequency.

A new fitting function for GRB MeV spectra based on the internal shock synchrotron model

Aims. The physical origin of the gamma-ray burst (GRB) prompt emission is still a subject of debate. Internal shock models have been widely explored, owing to their ability to explain most of the high-energy properties of this emission phase. While the Band function or other phenomenological functions are commonly used to fit GRB prompt emission spectra, we propose a new parametric function that is inspired by an internal shock physical model. We use this function as a proxy of the model to compare it easily to GRB observations. Methods. We built a parametric function that represents the spectral form of the synthetic bursts provided by our internal shock synchrotron model (ISSM). We simulated the response of the Fermi instruments to the synthetic bursts and fit the obtained count spectra to validate the ISSM function. Then, we applied this function to a sample of 74 bright GRBs detected by the Fermi GBM, and we computed the width of their spectral energy distributions around their peak energy. For comparison, we also fit the phenomenological functions that are commonly used in the literature. Finally, we performed a time-resolved analysis of the broadband spectrum of GRB 090926A, which was jointly detected by the Fermi GBM and LAT. This spectrum has a complex shape and exhibits a power-law component with an exponential cutoff at high energy, which is compatible with inverse Compton emission attenuated by gamma-ray internal absorption. Results. This work proposes a new parametric function for spectral fitting that is based on a physical model. The ISSM function reproduces 81% of the spectra in the GBM bright GRB sample, versus 59% for the Band function, for the same number of parameters. It gives also relatively good fits to the GRB 090926A spectra. The width of the MeV spectral component that is obtained from the fits of the ISSM function is slightly larger than the width from the Band fits, but it is smaller when observed over a wider energy range. Moreover, all of the 74 analyzed spectra are found to be significantly wider than the synthetic synchrotron spectra. We discuss possible solutions to reconcile the observations with the internal shock synchrotron model, such as an improved modeling of the shock microphysics or more accurate spectral measurements at MeV energies.

A Search for Lensed Gamma-Ray Bursts in 11 yr of Observations by Fermi GBM

Abstract Macrolensing of gamma-ray bursts (GRBs) is expected to manifest as a GRB recurring with the same light curve and spectrum as a previous one, but with a different flux and a slightly offset position. Identifying such lensed GRBs may give important information about the lenses, the cosmology, and the GRBs themselves. Here we present a search for lensed GRBs among ∼2700 GRBs observed by the Fermi Gamma-ray Burst Monitor during 11 yr of operations. To identify lensed GRBs, we perform initial cuts on position, time-averaged spectral properties, and relative duration. We then use the cross-correlation function to assess the similarity of light curves, and finally we analyze the time-resolved spectra of the most promising candidates. We find no convincing lens candidates. The most similar pairs are single-pulsed GRBs with relatively few time bins for the spectral analysis. This is best explained by similarities within the GRB population rather than lensing. However, the null result does not rule out the presence of macrolensed GRBs in the sample. In particular, we find that observational uncertainties and Poisson fluctuations can lead to significant differences within a pair of lensed GRBs.

X-Ray Emission Evolution of the Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124 during the 2017–2018 Outburst Observed by the MAXI GSC

Abstract This paper reports on the X-ray emission evolution of the ultraluminous Galactic X-ray pulsar Swift J0243.6+6124 during the giant outburst from 2017 October to 2018 January as observed by the MAXI GSC all-sky survey. The 2–30 keV light curve and the energy spectra confirm the source luminosity L X assuming an isotropic emission reached 2.5 × 1039 erg s−1, 10 times higher than the Eddington limit for a 1.4 M ⊙ neutron star. When the source was luminous with L X ≳ 0. 9 × 1038 erg s−1, it generally exhibited a negative correlation on a hardness-intensity diagram. However, two hardness ratios, a soft color (=4–10 keV/2–4 keV) and a hard color (=10–20 keV/4–10 keV), showed somewhat different behavior across a characteristic luminosity of L c ≃ 5 × 1038 erg s−1. The soft color changed more than the hard color when L X < L c, whereas the opposite was observed above L c. The spectral change above L c was represented by a broad enhanced feature at ∼6 keV on top of the canonical cutoff power-law continuum. The pulse profiles, derived daily, made the transition from a single-peak to a double-peak as the source brightened across L c. These spectral and pulse-shape properties can be interpreted by a scenario in which the accretion columns on the neutron-star surface, producing the Comptonized X-ray emission, gradually became taller as L X increases. The broad 6 keV enhancement could be a result of cyclotron-resonance absorption at ∼10 keV, corresponding to a surface magnetic field B s ≃ 1.1 × 1012 G. The spin-frequency derivatives calculated with the Fermi GBM data showed a smooth positive correlation with L X up to the outburst peak, and its linear coefficient is comparable to those of typical Be binary pulsars whose B s are (1–8) × 1012 G. These results suggest that the B s of Swift J0243.6+6124 is a few times 1012 G.

Observations of a GX 301–2 Apastron Flare with the X-Calibur Hard X-Ray Polarimeter Supported by NICER, the Swift XRT and BAT, and Fermi GBM

Abstract The accretion-powered X-ray pulsar GX 301−2 was observed with the balloon-borne X-Calibur hard X-ray polarimeter during late 2018 December, with contiguous observations by the Neutron star Interior Composition Explorer Mission (NICER) X-ray telescope, the Swift X-ray Telescope and Burst Alert Telescope, and the Fermi Gamma-ray Burst Monitor spanning several months. The observations detected the pulsar in a rare apastron flaring state coinciding with a significant spin up of the pulsar discovered with the Fermi Gamma-ray Burst Monitor. The X-Calibur, NICER, and Swift observations reveal a pulse profile strongly dominated by one main peak, and the NICER and Swift data show strong variation of the profile from pulse to pulse. The X-Calibur observations constrain for the first time the linear polarization of the 15–35 keV emission from a highly magnetized accreting neutron star, indicating a polarization degree of % (90% confidence limit) averaged over all pulse phases. We discuss the spin up and the X-ray spectral and polarimetric results in the context of theoretical predictions. We conclude with a discussion of the scientific potential of future observations of highly magnetized neutron stars with the more sensitive follow-up mission XL-Calibur.

A Connection between Spectral Width and Energetics As Well As Peak Luminosity in Fermi Gamma-Ray Bursts

We have revisited the spectral width in the EF E spectrum of gamma-ray bursts with the BEST peak flux P and time-integrated F spectral data provided by the Fermi GBM Burst Catalog. We first compute the BEST spectral widths to compare with some typical physics models. Our analysis results consist with the previous results: blackbody emission alone cannot explain the observed spectrum and most of the observed spectra cannot be interpreted by the synchrotron radiation. We then check the correlations between the spectral width and the observable model-independent burst properties of duration, fluence, and peak flux and find that positive correlations exist between them for both the P and F spectra. Moreover, the short burst appears to extend the correlation found for the long burst. We further demonstrate that these positive correlations also exist in the cosmological rest frame; that is, the spectral width correlates with the isotropic-equivalent energy E iso as well as the isotropic-equivalent peak luminosity L iso for different energy bands and timescales. Our results show that the wider bursts have larger energy and luminosity. Moreover, short bursts would appear to extend this trend qualitatively. Taking the Amati relation into account, we tend to believe that the spectral shape is related to energy and luminosity.

Multidimensional analysis of Fermi GBM gamma-ray bursts

The Fermi GBM catalog provides a large database with many measured variables that can be used to explore and verify gamma-ray burst classification results. We have used Principal Component Analysis and statistical clustering techniques to look for clustering in a sample of 801 gamma-ray bursts described by sixteen classification variables. The analysis recovers what appears to be the Short class and two long-duration classes that differ from one another via peak flux, with negligible variations in fluence, duration and spectral hardness. Neither class has properties entirely consistent with the Intermediate GRB class. Spectral hardness has been a critical Intermediate class property. Rather than providing spectral hardness, Fermi GBM provides a range of fitting variables for four different spectral models; it is not intuitive how these variables can be used to support or disprove previous GRB classification results.

Limits on Electromagnetic Counterparts of Gravitational-wave-detected Binary Black Hole Mergers

Abstract Unlike mergers of two compact objects containing a neutron star (NS), binary black hole (BBH) mergers are not accompanied by the production of tidally disrupted material and hence lack the most direct source of accretion to power a jet and generate electromagnetic (EM) radiation. However, following a tentative detection by the Fermi GBM of a γ-ray counterpart to GW150914, several ideas were proposed for driving a jet and producing EM radiation. If such jets were in fact produced, they would, however, lack the cocoon emission that makes jets from binary NSs also bright at large viewing angles. Here, via Monte Carlo simulations of a population of BBH mergers with properties consistent with those inferred from the existing LIGO/Virgo observations and the angular emission characteristic of jets propagating into the interstellar medium, we derive limits on the allowed energetics and Lorentz factors of such jets from EM follow-ups to GW-detected BBH merger events to date, and we make predictions that will help tighten these limits with broadband EM follow-ups to events in future LIGO/Virgo runs. The condition that ≲1 event out of 10 GW-detected BBH mergers be above the Fermi/GBM threshold imposes that any currently allowed emission model has to satisfy the condition (E iso/1049 erg)(θ jet/20°) ≲ 1.

Fermi GBM Observations of GRB 150101B: A Second Nearby Event with a Short Hard Spike and a Soft Tail

Abstract In light of the joint multimessenger detection of a binary neutron star merger as the gamma-ray burst GRB 170817A and in gravitational waves as GW170817, we reanalyze the Fermi Gamma-ray Burst Monitor data of one of the closest short gamma-ray bursts (SGRBs): GRB 150101B. We find that this burst is composed of a short hard spike followed by a comparatively long soft tail. This apparent two-component nature is phenomenologically similar to that of GRB 170817A. While GRB 170817A was distinct from the previously known population of SGRBs in terms of its prompt intrinsic energetics, GRB 150101B is not. Despite these differences, GRB 150101B can be modeled as a more on-axis version of GRB 170817A. Identifying a similar signature in two of the closest SGRBs suggests that the soft tail is common, but generally undetectable in more distant events. If so, it will be possible to identify nearby SGRBs from the prompt gamma-ray emission alone, aiding the search for kilonovae.

Possible GeV counterpart at the ground level associated with Fermi LAT gamma-ray bursts

From June 2014 to February 2017, the Fermi LAT detected 46 gamma-ray bursts (GRBs) with photon energies above 20 MeV, and the trigger coordinates of seven of them were within the FoV of New-Tupi detector located in the central region of the South Atlantic Anomaly (SAA).We show in this paper that two of these seven GRBs have a probable GeV counterpart observed at ground level by New-Tupi detector. The first is GRB 160609A, a short duration GRB with a bright emission of photons over a broad energy range extending up to GeV energies. The second is GRB 160625B, a very long duration GRB, for which the Fermi LAT detected more than 300 photons with energies above 100 MeV in the ∼1 ks interval after the GBM trigger. In the first case, the signal at New-Tupi has a nominal significance of 3.5σ in the counting rate time profiles, within the T90(=5.6 s) duration on Fermi GBM. However, the effective significance is only 3.0σ. In the second case, New-Tupi detector registered at least two excess (peaks) with a nominal statistical significance of 4.8σ and 5.0σ at 438 s and 558 s after the trigger. The first is within the T90(=460 s) on Fermi GBM. Even so, the effective significance is only ∼2.0σ. In addition, from a Monte Carlo analysis, we show that the expected signal-to-noise ratio is compatible with the observation of GRB 160709A, only if the differential index of the GRB energy spectrum be equal or higher than −2.2 (a non-steep spectrum).

Fermi GBM GRBs' multivariate statistics

Studying their gamma‐ray spectra may reveal some physical information on γ‐ray bursts (GRBs). The Fermi satellite has observed more than 2,000 GRBs. The FERMIGBRST catalog contains the GRB parameters (peak energy, spectral indices, intensity) estimated for both the total emission (fluence) and the emission during the interval of the peak flux. We found a relationship with linear discriminant analysis between the spectral categories and the model‐independent physical data. We compared the Swift and Fermi spectral types, and found a connection between the Fermi fluence spectra and the Swift spectra, but the result of the peak flux spectra can be disputable. We found that those GRBs that were observed by both Swift and Fermi can similarly discriminate as the complete Fermi sample. We conclude that the common observation probably did not find any trace of selection effects in the spectral behavior of GRBs.