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Register a new userX-ray upper limits of GW151226 with MAXI
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The error region of the the gravitational-wave (GW) event GW151226 was observed with Monitor of All-sky X-ray Image (MAXI). MAXI was operated at the time of GW151226, and continuously observed to 4 minutes after the event. MAXI covered about 84% of the 90 percent error region of the GW event during the first 92 minutes orbit after the event. No significant X-ray transient was detected in the GW error region. A typical 3-$sigma$ GSC upper limit for a scan is 1.2 $times 10^{-9}$ ergs cm$^{-2}$ s$^{-1}$ in the 2-20 keV. The auto-detection (MAXI nova-search) systems detected a short excess event with a low significance (2.85$sigma$) from 5257 s to 5260 s after the GW trigger. Finally, we discuss the sensitivity of MAXI to long X-ray emissions of short gamma-ray bursts, which are expected to accompany GW events.
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We searched for X-ray candidates of the gravitational wave (GW) event GW150914 with Monitor of All-sky X-ray Image (MAXI). MAXI observed the error region of the GW event GW150914 from 4 minutes after the event and covered about 90% of the error region in 25 minutes. No significant time variations on timescales of 1 s to 4 days were found in the GW error region. The $3sigma$ upper limits for the X-ray emission associated with the GW event in 2--20 keV were 9.5 $times 10^{-10}$, 2.3 $times 10^{-10}$, and 0.8 $times 10^{-10}$ ergs cm$^{-2}$ s$^{-1}$ for the time scale of $sim$ 1000 s, 1 day, and 10 days, respectively. If GW events are associated with short GRBs like GRB 050709, MAXI will be able to detect X-ray emissions from the source.
We report the MAXI observation of the gravitational-wave (GW) event GW170817 and the electromagnetic counterpart of GW170817. GW170817 is a binary neutron star coalescence candidate detected by the Advanced LIGO and Advanced Virgo detectors, and it is the first event for which the optical counterpart has been discovered. In the MAXI observation, the Gas Slit Camera (GSC) covered approximately 62% of the sky region of the GW event within 90% probability during the first 92 min of orbit after the trigger. No significant X-ray transient was detected in the error region, and the upper limit of the average flux with a significance of 3 $sigma$ in the 2--10 keV band was 53/26 mCrab (one-orbit observation/one-day observation). In the optical counterpart of GW170817, the observational window of GSC at the position started at 20 s after the GW trigger, but the high voltage of GSC was unfortunately off at the time because the ISS was entering a high-particle-background region. The first observation of the position by GSC was eventually performed at 16797 sec (4.6 hours) since the GW trigger, yielding the 3 $sigma$ upper limit of 8.60$times$10$^{-9}$ erg cm$^{-2}$ s$^{-1}$ in the 2--10 keV band, though it was the earliest X-ray observation of the counterpart.
Fast Radio Bursts (FRBs) are short lived ($sim$ msec), energetic transients (having a peak flux density of $sim$ Jy) with no known prompt emission in other energy bands. We present results of a search for prompt X-ray emissions from 41 FRBs using the Cadmium Zinc Telluride Imager (CZTI) on AstroSat which continuously monitors $sim70%$ of the sky. Our searches on various timescales in the 20-200 keV range, did not yield any counterparts in this hard X-ray band. We calculate upper limits on hard X-ray flux, in the same energy range and convert them to upper bounds for $eta$: the ratio X-ray to radio fluence of FRBs. We find $eta leq 10^{8-10}$ for hard X-ray emission. Our results will help constrain the theoretical models of FRBs as the models become more quantitative and nearer, brighter FRBs are discovered.
We present upper limits in the hard X-ray and gamma-ray bands at the time of the LIGO gravitational-wave event GW 151226 derived from the CALorimetric Electron Telescope (CALET) observation. The main instrument of CALET, CALorimeter (CAL), observes gamma-rays from ~1 GeV up to 10 TeV with a field of view of ~2 sr. The CALET gamma-ray burst monitor (CGBM) views ~3 sr and ~2pi sr of the sky in the 7 keV - 1 MeV and the 40 keV - 20 MeV bands, respectively, by using two different scintillator-based instruments. The CGBM covered 32.5% and 49.1% of the GW 151226 sky localization probability in the 7 keV - 1 MeV and 40 keV - 20 MeV bands respectively. We place a 90% upper limit of 2 x 10^{-7} erg cm-2 s-1 in the 1 - 100 GeV band where CAL reaches 15% of the integrated LIGO probability (~1.1 sr). The CGBM 7 sigma upper limits are 1.0 x 10^{-6} erg cm-2 s-1 (7-500 keV) and 1.8 x 10^{-6} erg cm-2 s-1 (50-1000 keV) for one second exposure. Those upper limits correspond to the luminosity of 3-5 x 10^{49} erg s-1 which is significantly lower than typical short GRBs.
We report nine long X-ray bursts from neutron stars, detected with Monitor of All-sky X-ray Image (MAXI). Some of these bursts lasted for hours, and hence are qualified as superbursts, which are prolonged thermonuclear flashes on neutron stars and are relatively rare events. MAXI observes roughly 85% of the whole sky every 92 minutes in the 2-20 keV energy band, and has detected nine bursts with a long e-folding decay time, ranging from 0.27 to 5.2 hours, since its launch in 2009 August until 2015 August. The majority of the nine events were found to originate from transient X-ray sources. The persistent luminosities of the sources, when these prolonged bursts were observed, were lower than 1% of the Eddington luminosity for five of them and lower than 20% for the rest. This trend is contrastive to the 18 superbursts observed before MAXI, all but two of which originated from bright persistent sources. The distribution of the total emitted energy, i.e., the product of e-folding time and luminosity, of these bursts clusters around $10^{41}$-$10^{42}$ erg, whereas either of the e-folding time and luminosity ranges for an order of magnitude. Among the nine events, two were from 4U 1850-086 during the phases of relatively low persistent-flux, whereas it usually exhibits standard short X-ray bursts during outbursts.
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