Long-lasting emission of short gamma-ray bursts (GRBs) is crucial to reveal the physical origin of the central engine as well as to detect electromagnetic (EM) counterparts to gravitational waves ...(GWs) from neutron star binary mergers. We investigate 65 X-ray light curves of short GRBs, which is six times more than previous studies, by combining both Swift/BAT and XRT data. The light curves are found to consist of two distinct components at >5 with bimodal distributions of luminosity and duration, i.e., extended (with a timescale of 103 s) and plateau emission (with a timescale of 103 s), which are likely the central engine activities, but not afterglows. The extended emission has an isotropic energy comparable to the prompt emission, while the plateau emission has ∼0.01-1 times this energy. Half (50%) of our sample has both components, while the other half is consistent with having both components. This leads us to conjecture that almost all short GRBs have both the extended and plateau emission. The long-lasting emission can be explained by the jets from black holes with fallback ejecta, and could power macronovae (or kilonovae) like GRB 130603B and GRB 160821B. Based on the observed properties, we quantify the detectability of EM counterparts to GWs, including the plateau emission scattered to the off-axis angle, with CALET/HXM, INTEGRAL/SPI-ACS, Fermi/GBM, MAXI/GSC, Swift/BAT, XRT, the future ISS-Lobster/WFI, Einstein Probe/WXT, and eROSITA.
ABSTRACT
The nature of the shallow decay phase in the X-ray afterglow of the gamma-ray burst (GRB) is not yet clarified. We analyse the data of early X-ray afterglows of 26 GRBs triggered by Burst ...Alert Telescope onboard Neil Gehrels Swift Observatory and subsequently detected by Fermi Large Area Telescope (LAT) and/or Imaging Atmospheric Cherenkov Telescopes. It is found that nine events (including two out of three very-high-energy gamma-ray events) have no shallow decay phase and that their X-ray afterglow light curves are well described by single power-law model except for the jet break at later epoch. The rest are fitted by double power-law model and have a break in the early epoch (around ks), however, eight events (including a very-high-energy gamma-ray event) have the pre-break decay index larger than 0.7. We also analyse the data of well-sampled X-ray afterglows of GRBs without LAT detection and compare their decay properties with those of high-energy and very-high-energy gamma-ray events. It is found that for the GeV/TeV bursts, the fraction of events whose X-ray afterglows are described by single power law is significantly larger than those for non-GeV/TeV GRBs. Even if the GeV/TeV GRBs have shallow decay phase, their decay slope tends to be steeper than non-GeV/TeV bursts, that is, they have less noticeable shallow decay phase in the early X-ray afterglow. A possible interpretation along with the energy injection model is briefly discussed.
ABSTRACT To date, the Burst Alert Telescope (BAT) onboard Swift has detected ∼1000 gamma-ray bursts (GRBs), of which ∼360 GRBs have redshift measurements, ranging from z = 0.03 to z = 9.38. We ...present the analyses of the BAT-detected GRBs for the past ∼11 years up through GRB 151027B. We report summaries of both the temporal and spectral analyses of the GRB characteristics using event data (i.e., data for each photon within approximately 250 s before and 950 s after the BAT trigger time), and discuss the instrumental sensitivity and selection effects of GRB detections. We also explore the GRB properties with redshift when possible. The result summaries and data products are available at http://swift.gsfc.nasa.gov/results/batgrbcat/index.html. In addition, we perform searches for GRB emissions before or after the event data using the BAT survey data. We estimate the false detection rate to be only one false detection in this sample. There are 15 ultra-long GRBs (∼2% of the BAT GRBs) in this search with confirmed emission beyond ∼1000 s of event data, and only two GRBs (GRB 100316D and GRB 101024A) with detections in the survey data prior to the starting of event data.
Abstract
The observed durations of prompt gamma-ray emission from gamma-ray bursts (GRBs) are often used to infer the progenitors and energetics of the sources. Inaccurate duration measurements will ...have a significant impact on constraining the processes powering the bursts. The “tip-of-the-iceberg” effect describes how the observed signal is lost into background noise; lower instrument sensitivity leads to higher measurement bias. In this study, we investigate how observing conditions, such as the number of enabled detectors, background level, and incident angle of the source relative to the detector plane, affect the measured duration of GRB prompt emission observed with the Burst Alert Telescope on board the Neil Gehrels Swift Observatory (Swift/BAT). We generate “simple-pulse” light curves from an analytical fast rise exponential decay function and from a sample of eight real GRB light curves. We fold these through the Swift/BAT instrument response function to simulate light curves Swift/BAT would have observed for specific observing conditions. We find duration measurements are highly sensitive to observing conditions and the incident angle of the source has the highest impact on measurement bias. In most cases duration measurements of synthetic light curves are significantly shorter than the true burst duration. For the majority of our sample, the percentage of duration measurements consistent with the true duration is as low as ∼25%–45%. In this article, we provide quantification of the tip-of-the-iceberg effect on GRB light curves due to Swift/BAT instrumental effects for several unique light curves.
This study is based on 5.5 years of continuous observation of sprites from Sagamihara, Japan. Up to February 2022, we detected 537 sprites and found that the most significant number of sprites were ...observed during the winter (303 sprites); on the other hand, there were only 46 sprites in summer. The hourly distribution of the number of observed sprites peaked at midnight JST (15:00 and 16:00 UTC). To understand the seasonal and the hourly distribution of sprites, we estimate the number of sprites considering the energy and the polarity of lightning, the temporal changes of surrounding environments of sprites, and the conditions for generating sprites. We found that the energy of lightning, the monthly ratio of a positive cloud-to-ground discharge, and the hourly change in the electron number density are essential factors to match the observed sprite distributions.
ABSTRACT We verified the off-axis jet model of X-ray flashes (XRFs) and examined a discovery of off-axis orphan gamma-ray burst (GRB) afterglows. The XRF sample was selected on the basis of the ...following three factors: (1) a constraint on the lower peak energy of the prompt spectrum , (2) redshift measurements, and (3) multicolor observations of an earlier (or brightening) phase. XRF 020903 was the only sample selected on the basis of these criteria. A complete optical multicolor afterglow light curve of XRF 020903 obtained from archived data and photometric results in the literature showed an achromatic brightening around 0.7 days. An off-axis jet model with a large observing angle (0.21 rad, which is twice the jet opening half-angle, ) can naturally describe the achromatic brightening and the prompt X-ray spectral properties. This result indicates the existence of off-axis orphan GRB afterglow light curves. Events with a larger viewing angle ( ) could be discovered using an 8 m class telescope with wide-field imagers such as the Subaru Hyper-Suprime-Cam and the Large Synoptic Survey Telescope.
We have performed a systematic study of gamma-ray bursts (GRBs), which have various values in the peak energy of the F spectrum of prompt emission, Epeak, observed by the Swift/Burst Alert Telescope ...(BAT) and Fermi/Gamma-ray Burst Monitor, investigating their prompt and X-ray afterglow emissions. We cataloged long-lasting GRBs observed by Swift between 2004 December and 2014 February in three categories according to the classification by Sakamoto et al.: X-ray flashes (XRFs), X-ray-rich GRBs (XRRs), and classical GRBs (C-GRBs). We then derived , as well as if viable, of the Swift spectra of their prompt emission. We also analyzed their X-ray afterglows and found that GRB events with a lower , i.e., softer GRBs, are fainter in 0.3-10 keV X-ray luminosity and decay more slowly than harder GRBs. The intrinsic event rates of the XRFs, XRRs, and C-GRBs were calculated using the Swift/BAT trigger algorithm. Those of the XRRs and XRFs are larger than that of the C-GRBs. If we assume that the observational diversity of Epeak is explained using the off-axis model, these results yield a jet half-opening angle of Δθ ∼ 0 3, and a variance of the observing angles θobs 0 6. This implies that this tiny variance would be responsible for the Epeak diversity observed by Swift/BAT, which is unrealistic. Therefore, we conclude that the Epeak diversity is not explained with the off-axis model, but is likely to originate from some intrinsic properties of the jets.
Traditionally, gamma-ray bursts (GRBs) are classified in the T
90–hardness ratio two-dimensional plane into long/soft and short/hard GRBs. In this paper, we suggest to add the ‘amplitude’ of GRB ...prompt emission as the third dimension as a complementary criterion to classify GRBs, especially those of short durations. We define three new parameters f, f
eff and f
eff, z
as ratios between the measured/simulated peak flux of a GRB/pseudo-GRB and the flux background, and discuss the applications of these parameters to GRB classification. We systematically derive these parameters to find that most short GRBs are likely not ‘tip-of-iceberg’ of long GRBs. However, one needs to be cautious if a short GRB has a relatively small f (e.g. f < 1.5), since the chance for an intrinsically long GRB to appear as a ‘disguised’ short GRB is higher. Based on available data, we quantify the probability of a disguised short GRB below a certain f value is as
$P (<f)\sim 0.78^{+0.71}_{-0.4} f^{-4.33\pm 1.84}$
. By progressively ‘moving’ a long GRB to higher redshifts through simulations, we also find that most long GRBs would show up as rest-frame short GRBs above a certain redshift.
Abstract
We report on the hard X-ray burst and the first ∼100 days of NICER monitoring of the soft X-ray temporal and spectral evolution of the newly discovered magnetar Swift J1818.0−1607. The burst ...properties are typical of magnetars with a duration of
T
90
= 10 ± 4 ms and a temperature of
kT
= 8.4 ± 0.7 keV. The 2–8 keV pulse shows a broad, single-peak profile with a pulse fraction increasing with time from 30% to 43%. The NICER observations reveal strong timing noise with
varying erratically by a factor of 10, with an average long-term spin-down rate of
s
−2
, implying an equatorial surface magnetic field of 2.5 × 10
14
G and a young characteristic age of ∼470 yr. We detect a large spin-up glitch at MJD 58928.56 followed by a candidate spin-down glitch at MJD 58934.81, with no accompanying flux enhancements. The persistent soft X-ray spectrum of Swift J1818.0−1607 can be modeled as an absorbed blackbody with a temperature of ∼1 keV. Its flux decayed by ∼60% while the modeled emitting area decreased by ∼30% over the NICER observing campaign. This decrease, coupled with the increase in the pulse fraction, points to a shrinking hot spot on the neutron star surface. Assuming a distance of 6.5 kpc, we measure a peak X-ray luminosity of 1.9 × 10
35
erg s
−1
, lower than its spin-down luminosity of 7.2 × 10
35
erg s
−1
. Its quiescent thermal luminosity is ≲1.7 × 10
34
erg s
−1
, lower than those of canonical young magnetars. We conclude that Swift J1818.0−1607 is an important link between regular magnetars and high-magnetic-field, rotation-powered pulsars.
The unprecedentedly bright optical afterglow of GRB 130606A located by Swift at a redshift close to the reionization era (z = 5.913) provides a new opportunity to probe the ionization status of the ...intergalactic medium (IGM). Here we present an analysis of the red Lyα damping wing of the afterglow spectrum taken by Subaru/FOCAS during 10.4–13.2 hr after the burst. We find that the minimal model including only the baseline power-law and H i absorption in the host galaxy does not give a good fit, leaving residuals showing concave curvature in 8400–8900 Å with an amplitude of about 0.6% of the flux. Such a curvature in the short wavelength range cannot be explained either by extinction at the host with standard extinction curves, intrinsic curvature of afterglow spectra, or by the known systematic uncertainties in the observed spectrum. The red damping wing by intervening H i gas outside the host can reduce the residual by about 3 σ statistical significance. We find that a damped Lyα system is not favored as the origin of this intervening H i absorption, from the observed Lyβ and metal absorption features. Therefore absorption by diffuse IGM remains as a plausible explanation. A fit by a simple uniform IGM model requires an H i neutral fraction of f
H i
∼ 0.1–0.5 depending on the distance to the GRB host, implying high f
H i
IGM associated with the observed dark Gunn–Peterson (GP) troughs. This gives new evidence that the reionization is not yet complete at z = 6.
PDF
