MAXI observations of GRBs

TL;DR

MAXI detects a higher rate of soft-spectrum gamma-ray bursts, including X-ray flashes, due to its sensitivity in the 2-30 keV range, revealing their likely closer distances and potential confusion with galactic transients.

Contribution

This study presents the first systematic analysis of MAXI's GRB detections, highlighting its unique sensitivity to soft-spectrum bursts and their implications for GRB population studies.

Findings

MAXI detected 35 GRBs from 2009 to 2013.

One third of MAXI GRBs are also observed by other satellites.

MAXI's GRB rate is about three times higher than expected from BATSE data.

Abstract

Monitor of all-sky image (MAXI) Gas Slit Camera (GSC) detects gamma-ray bursts (GRBs) including the bursts with soft spectra, such as X-ray flashes (XRFs). MAXI/GSC is sensitive to the energy range from 2 to 30 keV. This energy range is lower than other currently operating instruments which is capable of detecting GRBs. Since the beginning of the MAXI operation on August 15, 2009, GSC observed 35 GRBs up to the middle of 2013. One third of them are also observed by other satellites. The rest of them show a trend to have soft spectra and low fluxes. Because of the contribution of those XRFs, the MAXI GRB rate is about three times higher than those expected from the BATSE log N - log P distribution. When we compare it to the observational results of the Wide-field X-ray Monitor on the High Energy Transient Explorer 2, which covers the the same energy range to that of MAXI/GSC, we find a possibility that many of MAXI bursts are XRFs with Epeak lower than 20 keV. We discuss the source of soft GRBs observed only by MAXI. The MAXI log N - log S distribution suggests that the MAXI XRFs distribute in closer distance than hard GRBs. Since the distributions of the hardness of galactic stellar flares and X-ray bursts overlap with those of MAXI GRBs, we discuss a possibility of a confusion of those galactic transients with the MAXI GRB samples.