Search

The kilometer square array (KM2A) of the large high altitude air shower observatory (LHAASO) aims at surveying the northern γ-ray sky at energies above 10 TeV with unprecedented sensitivity. γ-ray observations have long been one of the most powerful tools for dark matter searches, as, e.g., high-energy γ rays could be produced by the decays of heavy dark matter particles. In this Letter, we present the first dark matter analysis with LHAASO-KM2A, using the first 340 days of data from 1/2-KM2A and 230 days of data from 3/4-KM2A. Several regions of interest are used to search for a signal and account for the residual cosmic-ray background after γ/hadron separation. We find no excess of dark matter signals, and thus place some of the strongest γ-ray constraints on the lifetime of heavy dark matter particles with mass between 105 and 109 GeV. Our results with LHAASO are robust, and have important implications for dark matter interpretations of the diffuse astrophysical high-energy neutrino emission.

The extension of the cosmic-ray spectrum beyond 1 petaelectronvolt (PeV; 1015 electronvolts) indicates the existence of the so-called PeVatrons—cosmic-ray factories that accelerate particles to PeV energies. We need to locate and identify such objects to find the origin of Galactic cosmic rays1. The principal signature of both electron and proton PeVatrons is ultrahigh-energy (exceeding 100 TeV) γ radiation. Evidence of the presence of a proton PeVatron has been found in the Galactic Centre, according to the detection of a hard-spectrum radiation extending to 0.04 PeV (ref. 2). Although γ-rays with energies slightly higher than 0.1 PeV have been reported from a few objects in the Galactic plane3–6, unbiased identification and in-depth exploration of PeVatrons requires detection of γ-rays with energies well above 0.1 PeV. Here we report the detection of more than 530 photons at energies above 100 teraelectronvolts and up to 1.4 PeV from 12 ultrahigh-energy γ-ray sources with a statistical significance greater than seven standard deviations. Despite having several potential counterparts in their proximity, including pulsar wind nebulae, supernova remnants and star-forming regions, the PeVatrons responsible for the ultrahigh-energy γ-rays have not yet been firmly localized and identified (except for the Crab Nebula), leaving open the origin of these extreme accelerators. Observations of γ-rays with energies up to 1.4 PeV find that 12 sources in the Galaxy are PeVatrons, one of which is the Crab Nebula.

Habitability of an exoplanet is believed to be profoundly affected by activities of the host stars, although the related coronal mass ejections (CMEs) are still rarely detected in solar-like and late-type stars. We here report an observational study on flares of two M-dwarfs triggered by the high-cadence survey performed by the Ground Wide-angle Camera system. In both events, the fast, time-resolved spectroscopy enables us to identify symmetric broad H$\alpha$ emission with not only a nearly zero bulk velocity, but also a large projected maximum velocity as high as $\sim700-800\ \mathrm{km\ s^{-1}}$. This broadening could be resulted from either Stark (pressure) effect or a flaring-associated CME at stellar limb. In the context of the CME scenario, the CME mass is estimated to be $\sim4\times10^{18}$ g and $2\times10^{19}$ g. In addition, our spectral analysis reveals a temporal variation of the line center of the narrow H$\alpha$ emission in both events. The variation amplitudes are at tens of $\mathrm{km\ s^{-1}}$, which could be ascribed to the chromospheric evaporation in one event, and to a binary scenario in the other one. With the total flaring energy determined from our photometric monitor, we show a reinforced trend in which larger the flaring energy, higher the CME mass is., Comment: 15 pages, 10 figures and 3 tables. Accepted by ApJ. arXiv admin note: text overlap with arXiv:2106.04774

We report the discovery of a new unidentified extended $\gamma$-ray source in the Galactic plane named LHAASO J0341+5258 with a pre-trial significance of 8.2 standard deviations above 25 TeV. The best fit position is R.A.$=55.34^{\circ}\pm0.11^{\circ}$ and Dec$=52.97^{\circ}\pm0.07^{\circ}$. The angular size of LHAASO J0341+5258 is $0.29^\circ \pm 0.06^\circ_{stat} \pm0.02^\circ_{sys}$. The flux above 25 TeV is about $20\%$ of the flux of Crab Nebula. Although a power-law fit of the spectrum from 10 TeV to 200 TeV with the photon index $\alpha=2.98 \pm 0.19_{stat} \pm 0.02_{sys}$ is not excluded, the LHAASO data together with the flux upper limit at 10 GeV set by the Fermi LAT observation, indicate a noticeable steepening of an initially hard power-law spectrum %($\alpha \leq 1.75$) spectrum with a cutoff at $\approx 50$ TeV. We briefly discuss the origin of UHE gamma-rays. The lack of an energetic pulsar and a young SNR inside or in the vicinity of LHAASO J0341+5258 challenge, but do not exclude both the leptonic and hadronic scenarios of gamma-ray production., Comment: Accepted by APJL

Gamma-ray bursts (GRBs) have been phenomenologically classified into long and short populations based on the observed bimodal distribution of duration1. Multi-wavelength and multi-messenger observations in recent years have revealed that in general long GRBs originate from massive star core collapse events2, whereas short GRBs originate from binary neutron star mergers3. It has been known that the duration criterion is sometimes unreliable, and multi-wavelength criteria are needed to identify the physical origin of a particular GRB4. Some apparently long GRBs have been suggested to have a neutron star merger origin5, whereas some apparently short GRBs have been attributed to genuinely long GRBs6 whose short, bright emission is slightly above the detector’s sensitivity threshold. Here, we report the comprehensive analysis of the multi-wavelength data of the short, bright GRB 200826A. Characterized by a sharp pulse, this burst shows a duration of 1 second and no evidence of an underlying longer-duration event. Its other observational properties such as its spectral behaviours, total energy and host galaxy offset are, however, inconsistent with those of other short GRBs believed to originate from binary neutron star mergers. Rather, these properties resemble those of long GRBs. This burst confirms the existence of short-duration GRBs with stellar core-collapse origin4, and presents some challenges to the existing models. © 2021, The Author(s), under exclusive licence to Springer Nature Limited., B.-B.Z. acknowledges support by the National Key Research and Development Programs of China (2018YFA0404204), the National Natural Science Foundation of China (grant nos. 11833003 and U2038105) and the Innovative and Entrepreneurial Talent Program in Jiangsu. Y.-Z.M. is supported by the National Postdoctoral Program for Innovative Talents (grant no. BX20200164). This work was supported in part by the Natural Science Foundation of China (grant nos. U1831135 (X.-H.Z.), 11922301 (H.-J.L.), 12041306 (Y.L.) and U1938201 (X.-G.W.)), the Guangxi Science Foundation (2017GXNSFFA198008 (H.-J.L.), 2017AD22006 (X.-G.W.) and 2016GXNSFFA380006 (X.-G.W.)) and the Bagui Young Scholars Program (H.-J.L.). Part of this work is based on observations made with the GTC installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, on the island of La Palma. We also acknowledge the use of public data from the Fermi Science Support Center., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

We report the discovery of a UHE gamma-ray source, LHAASO J2108+5157, by analyzing the LHAASO-KM2A data of 308.33 live days. Significant excess of gamma-ray induced showers is observed in both energy bands of 25-100 TeV and $\gt$100 TeV with 9.5 sigma and 8.5 sigma, respectively. This source is not significantly favored as an extensive source with the angular extension smaller than the point-spread function of KM2A. The measured energy spectrum from 20 to 200 TeV can be approximately described by a power-law function with an index of -2.83$\pm$ 0.18stat. A harder spectrum is demanded at lower energies considering the flux upper limit set by Fermi-LAT observations. The position of the gamma-ray emission is correlated with a giant molecular cloud, which favors a hadronic origin. No obvious counterparts have been found, deeper multiwavelength observations will help to shed new light on this intriguing UHE source., Comment: 12 pages, 7 figures, accepted by ApJL

The Water Cherenkov Detector Array (WCDA) is a major component of the Large High Altitude Air Shower Array Observatory (LHAASO), a new generation cosmic-ray experiment with unprecedented sensitivity, currently under construction. The WCDA is aimed at the study of TeV $\gamma$-rays. In order to evaluate the prospects of searching for TeV $\gamma$-ray sources with the WCDA, we present in this paper a projection for the one-year sensitivity of the WCDA to TeV $\gamma$-ray sources from TeVCat (footnote: http://tevcat.uchicago.edu) using an all-sky approach. Out of 128 TeVCat sources observable to the WCDA up to a zenith angle of $45^\circ$, we estimate that 42 would be detectable for one year of observations at a median energy of 1 TeV. Most of them are Galactic sources, and the extragalactic sources are Active Galactic Nuclei (AGN)., Comment: 10 pages,10 figures, it is to be published in Chinese Physics C

Full list of authors: Lin, L.; Zhang, C. F.; Wang, P.; Gao, H.; Guan, X.; Han, J. L.; Jiang, J. C.; Jiang, P.; Lee, K. J.; Li, D.; Men, Y. P.; Miao, C. C.; Niu, C. H.; Niu, J. R.; Sun, C.; Wang, B. J.; Wang, Z. L.; Xu, H.; Xu, J. L.; Xu, J. W.; Yang, Y. H.; Yang, Y. P.; Yu, W.; Zhang, B.; Zhang, B. -B.; Zhou, D. J.; Zhu, W. W.; Castro-Tirado, A. J.; Dai, Z. G.; Ge, M. Y.; Hu, Y. D.; Li, C. K.; Li, Y.; Li, Z.; Liang, E. W.; Jia, S. M.; Querel, R.; Shao, L.; Wang, F. Y.; Wang, X. G.; Wu, X. F.; Xiong, S. L.; Xu, R. X.; Yang, Y. -S.; Zhang, G. Q.; Zhang, S. N.; Zheng, T. C.; Zou, J. -H., Fast radio bursts (FRBs) are millisecond-duration radio transients of unknown physical origin observed at extragalactic distances1–3. It has long been speculated that magnetars are the engine powering repeating bursts from FRB sources4–13, but no convincing evidence has been collected so far14. Recently, the Galactic magnetar SRG 1935+2154 entered an active phase by emitting intense soft γ-ray bursts15. One FRB-like event with two peaks (FRB 200428) and a luminosity slightly lower than the faintest extragalactic FRBs was detected from the source, in association with a soft γ-ray/hard-X-ray flare18–21. Here we report an eight-hour targeted radio observational campaign comprising four sessions and assisted by multi-wavelength (optical and hard-X-ray) data. During the third session, 29 soft-γ-ray repeater (SGR) bursts were detected in γ-ray energies. Throughout the observing period, we detected no single dispersed pulsed emission coincident with the arrivals of SGR bursts, but unfortunately we were not observing when the FRB was detected. The non-detection places a fluence upper limit that is eight orders of magnitude lower than the fluence of FRB 200428. Our results suggest that FRB–SGR burst associations are rare. FRBs may be highly relativistic and geometrically beamed, or FRB-like events associated with SGR bursts may have narrow spectra and characteristic frequencies outside the observed band. It is also possible that the physical conditions required to achieve coherent radiation in SGR bursts are difficult to satisfy, and that only under extreme conditions could an FRB be associated with an SGR burst. © 2020, The Author(s), under exclusive licence to Springer Nature Limited., This work is supported by the Natural Science Foundation of China (grants 11988101, 11673002, 11703002, 11543004, 11722324, 11690024, 11633001, 11920101003, 11833003, 11722324, 11633001, 11690024, 11573014, 11725314, 11690024, 11743002, 11873067, 11533003, 11673006, U1938201, 11725313, 11721303, 1172130, U15311243, U1831207, U1838201, U1838202, U1838113, U1938109), National Key Research and Development Programs of China (grants 2018YFA0404204, 2017YFA0402600), the Program for Innovative Talents and Entrepreneur in Jiangsu, the KIAA-CAS Fellowship, the China Postdoctoral Science Foundation (grants 2018M631242, 2016YFA0400800, 2018YFA0400802, E01S11BQ10, XDB23010200,XDB2304040, QYZDY-SSW-SLH008), the International Partnership Program of Chinese Academy of Sciences (grant number 114A11KYSB20160008), the Cultivation Project for FAST Scientific Payoff and Research Achievement of CAMS-CAS, the Max-Planck Partner Group, the Spanish Science Ministry “Centro de Excelencia Severo Ochoa” Program (grant SEV-2017-0709), the Junta de Andalucía (Project P07-TIC-03094) and the Spanish Ministry Projects AYA2012-39727-C03-01, AYA2015-71718R and PID2019-109974RB-I00. We thank E. Fernández-García (IAA-CSIC), I. M. Carrasco-García and C. Pérez del Pulgar (UMA) and the rest of the BOOTES Team for making the reported BOOTES Network observations possible. This work made use of data from FAST—a Chinese national mega-science facility, built and operated by the National Astronomical Observatories, Chinese Academy of Sciences. We also acknowledge the use of public data from the Fermi Science Support Center (FSSC).

We report the detection and follow-up of a superstellar flare GWAC 181229A with an amplitude of ΔR ∼ 9.5 mag on an M9-type star by SVOM/GWAC and the dedicated follow-up telescopes. The estimated bolometric energy E bol is (5.56–9.25) × 1034 erg, which makes the event one of the most powerful flares seen on ultracool stars. The magnetic strength is inferred to be 3.6–4.7 kG. Thanks to sampling with a cadence of 15 s, a new component near the peak time with a very steep decay is detected in the R-band light curve, followed by the two-component flare template given by Davenport et al. An effective temperature of 5340 ± 40 K is measured by fitting a blackbody shape to the spectrum in the shallower phase during the flare. The filling factors of the flare are estimated to be ∼30% and 19% at the peak time and at 54 minutes after the first detection. The detection of this particular event with large amplitude, huge emitted energy, and a new component demonstrates that high-cadence sky monitoring cooperation with fast follow-up observations is very important for understanding the violent magnetic activity.

A sub-array of the Large High Altitude Air Shower Observatory (LHAASO), KM2A is mainly designed to observe a large fraction of the northern sky to hunt for γ-ray sources at energies above 10 TeV. Even though the detector construction is still underway, half of the KM2A array has been operating stably since the end of 2019. In this paper, we present the KM2A data analysis pipeline and the first observation of the Crab Nebula, a standard candle in very high energy γ-ray astronomy. We detect γ-ray signals from the Crab Nebula in both energy ranges of 10 100 TeV and 100 TeV with high significance, by analyzing the KM2A data of 136 live days between December 2019 and May 2020. With the observations, we test the detector performance, including angular resolution, pointing accuracy and cosmic-ray background rejection power. The energy spectrum of the Crab Nebula in the energy range 10-250 TeV fits well with a single power-law function dN/dE = (1.13 0.05 0.08 ) 10 (E/20 TeV) cm s TeV . It is consistent with previous measurements by other experiments. This opens a new window of γ-ray astronomy above 0.1 PeV through which new ultrahigh-energy γ-ray phenomena, such as cosmic PeVatrons, might be discovered.

Zhang, B.B. et. al., Double neutron star (DNS) merger events are promising candidates of short gamma-ray burst (sGRB) progenitors as well as high-frequency gravitational wave (GW) emitters. On August 17, 2017, such a coinciding event was detected by both the LIGO-Virgo gravitational wave detector network as GW170817 and Gamma-Ray Monitor on board NASA's Fermi Space Telescope as GRB 170817A. Here, we show that the fluence and spectral peak energy of this sGRB fall into the lower portion of the distributions of known sGRBs. Its peak isotropic luminosity is abnormally low. The estimated event rate density above this luminosity is at least 190 Gpc yr, which is close to but still below the DNS merger event rate density. This event likely originates from a structured jet viewed from a large viewing angle. There are similar faint soft GRBs in the Fermi archival data, a small fraction of which might belong to this new population of nearby, low-luminosity sGRBs., We acknowledge support from National Basic Research Program of China (973 Program, Grant No. 2014CB845800). B.-B.Z. and A.J.C.T. acknowledge support from the Spanish Ministry Projects AYA 2012-39727-C03-01 and AYA2015-71718-R. B.-B.Z. acknowledges the support from the National Thousand Young Talents program of China. This work is also supported by the Strategic Pioneer Program on Space Science, Chinese Academy of Sciences (Grant No. XDA15052100). B.Z., H.S., H.G., and X.F.W. are supported by the Strategic Priority Research Program of the Chinese Academy of Sciences >Multi-waveband Gravitational Wave Universe> (Grant No. XDB23040000). H.G. acknowledges the National Natural Science Foundation of China under grants Nos. 11722324, 11603003, 11633001, and 11690024. H.J.L. and E. W. L. acknowledge the National Natural Science Foundation of China under grant 11603006 and 11533003. Y.D.H. acknowledges the support by China Scholarships Council (CSC) under the Grant No. 201406660015. X.-F.W. acknowledges the National Natural Science Foundation of China under grant 11625312. L.S. acknowledges the support by the Joint NSFC-ISF Research Program (No. 11361140349), jointly funded by the National Natural Science Foundation of China and the Israel Science Foundation. X.F. was supported by Natural Science Foundation of China under Grants Nos. 11725314, 11673068, and 11433009.

The durations (T90) of 315 GRBs detected with Fermi/GBM (8-1000 keV) by 2011 September are calculated using the Bayesian Block method. We compare the T90 distributions between this sample and that observed with previous/current GRB missions. We show that T90 is energy-band dependent and the observed bimodal T90 distribution would be due to the instrumental selection effect.

We present a systematic temporal and spectral study of all Swift-XRT observations of GRB afterglows discovered between 2005 January and 2007 December. After constructing and fitting all light curves and spectra to power-law models, we classify the components of each afterglow in terms of the canonical X-ray afterglow and test them against the closure relations of the forward shock models for a variety of parameter combinations. The closure relations are used to identify potential jet breaks with characteristics including the uniform jet model with and without lateral spreading and energy injection, and a power-law structured jet model, all with a range of parameters. With this technique, we survey the X-ray afterglows with strong evidence for jet breaks (~12% of our sample), and reveal cases of potential jet breaks that do not appear plainly from the light curve alone (another ~30%), leading to insight into the missing jet break problem. Those X-ray light curves that do not show breaks or have breaks that are not consistent with one of the jet models are explored to place limits on the times of unseen jet breaks. The distribution of jet break times ranges from a few hours to a few weeks with a median of ~1 day, similar to what was found pre-Swift. On average Swift GRBs have lower isotropic equivalent gamma-ray energies, which in turn results in lower collimation corrected gamma-ray energies than those of pre-Swift GRBs. Finally, we explore the implications for GRB jet geometry and energetics.

The external forward shock (EFS) models have been the standard paradigm to interpret the broad-band afterglow data of gamma-ray bursts (GRBs). One prediction of the models is that some afterglow temporal breaks at different energy bands should be achromatic. Observations in the Swift era have revealed chromatic afterglow behaviors at least in some GRBs, casting doubts on the EFS origin of GRB afterglows. In this paper, we perform a systematic study to address the question: how bad/good are the external forward shock models? Our sample includes 85 GRBs well-monitored X-ray and optical lightcurves. Based on how well the data abide by the EFS models, we categorize them as: Gold sample: (Grade I and II) include 45/85 GRBs. They show evidence of, or are consistent with having, an achromatic break. The temporal/spectral behaviors in each afterglow segment are consistent with the predictions (closure relations) of the EFS models. Silver sample: (Grade III and IV) include 37/85 GRBs. They are also consistent with having an achromatic break, even though one or more afterglow segments do not comply with the closure relations. Bad sample: (Grade V), 3/85 shows direct evidence of chromatic behaviors, suggesting that the EFS models are inconsistent with the data. These are included in the Bad sample. We further perform statistical analyses of various observational properties ($\alpha$, $\beta$, $t_b$ and model parameters (energy injection index q, p, $\theta_j$, $\eta_\gamma$, etc) of the GRBs in the Gold Sample, and derive constraints on the magnetization parameter $\epsilon_B$ in the EFS. Overall, we conclude that the simplest EFS models can account for the multi-wavelength afterglow data of at least half of the GRBs. When more advanced modeling (e.g., long-lasting reverse shock, structured jets) is invoked, up to $>90 \%$ of the afterglows may be interpreted within the framework of the ESF models., Comment: 69 pages,134 figures, 7 tables, ApJS, in press

We study the peak energy ($E_{\rm{p}}$) distribution of the $\nu F_{\nu}$ spectra of gamma-ray bursts (GRBs) and X-ray flashes (XRFs) with a sample of 57 bursts observed by {\em High Energy Transient Explorer 2} ({\em HETE-2}) French Gamma Telescope and discuss its implications for the jet structure models. Combining the observed $E_{\rm{p}}$ distribution of {\em HETE--2} GRBs/XRFs with that of BATSE GRBs, we find that the observed $E_{\rm{p}}$ distribution of GRBs/XRFs is a bimodal one with peaks of $\lesssim 30$ keV and $\sim 160-250$ keV. According to the recently-discovered equivalent-isotropic energy-$E_{\rm{p}}$ relationship, such a bimodal distribution implies a two-component structure of GRB/XRF jets. A simple simulation analysis shows that this structured jet model does roughly reproduce a bimodal distribution with peaks of $\sim 15$ and $\sim 200$ keV. We argue that future observations of the peak of $\sim 15$ keV in the $E_{\rm{p}}$ distribution would be evidence supporting this model. {\em Swift}, which covers an energy band of 0.2--150 keV, is expected to provide a key test for our results., Comment: 4 pages plus 2 figures, emulateapj style, ApJ Letters, 608, in press

The High Energy cosmic-Radiation Detection (HERD) facility is one of several space astronomy payloads of the cosmic lighthouse program onboard China's Space Station, which is planned for operation starting around 2020 for about 10 years. The main scientific objectives of HERD are indirect dark matter search, precise cosmic ray spectrum and composition measurements up to the knee energy, and high energy gamma-ray monitoring and survey. HERD is composed of a 3-D cubic calorimeter (CALO) surrounded by microstrip silicon trackers (STKs) from five sides except the bottom. CALO is made of about 10$^4$ cubes of LYSO crystals, corresponding to about 55 radiation lengths and 3 nuclear interaction lengths, respectively. The top STK microstrips of seven X-Y layers are sandwiched with tungsten converters to make precise directional measurements of incoming electrons and gamma-rays. In the baseline design, each of the four side SKTs is made of only three layers microstrips. All STKs will also be used for measuring the charge and incoming directions of cosmic rays, as well as identifying back scattered tracks. With this design, HERD can achieve the following performance: energy resolution of 1\% for electrons and gamma-rays beyond 100 GeV, 20\% for protons from 100 GeV to 1 PeV; electron/proton separation power better than $10^{-5}$; effective geometrical factors of $>$3 ${\rm m}^{2}{\rm sr}$ for electron and diffuse gamma-rays, $>$2 $ {\rm m}^{2}{\rm sr}$ for cosmic ray nuclei. R\&D is under way for reading out the LYSO signals with optical fiber coupled to image intensified CCD and the prototype of one layer of CALO., 9 pages, 6 figures, 2 tables. Oral talk presented at SPIE Astronomical Telescopes and Instrumentation, June, 2014, Montreal

The broadband SEDs of four gamma-ray NLS1s are compiled and explained with the leptonic model. It is found that their characteristics and fitting parameters of the observed SEDs are more like FSRQs than BL Lacs., Comment: 2 pages; 1 figure; to appear in the proceedings of the IAU Symposium 290, "Feeding compact objects: accretion on all scales", Beijing, 20-24 Aug 2012

We report on the detection by Swift of GRB 080913, and subsequent optical/near-infrared follow-up observations by GROND which led to the discovery of its optical/NIR afterglow and the recognition of its high-z nature via the detection of a spectral break between the i' and z' bands. Spectroscopy obtained at the ESO-VLT revealed a continuum extending down to lambda = 9400 A, and zero flux for 7500 A < lambda, Comment: 14 pages, 7 figures, ApJ (subm)

We present a systematic survey of the temporal and spectral properties of all GRB X-ray afterglows observed by Swift-XRT between January 2005 and July 2007. We have constructed a catalog of all light curves and spectra and investigate the physical origin of each afterglow segment in the framework of the forward shock models by comparing the data with the closure relations. We search for possible jet-like breaks in the lightcurves and try to explain some of the "missing" X-ray jet breaks in the lightcurves., Comment: 4 pages, 4 figures, contributed talk, submitted to the proceedings of Gamma Ray Bursts 2007, Santa Fe, New Mexico, November 5-9 2007

Very early observations with the Swift satellite of gamma-ray burst (GRB) afterglows reveal that the optical component is not detected in a large number of cases. This is in contrast to the bright optical flashes previously discovered in some GRBs (e.g. GRB 990123 and GRB 021211). Comparisons of the X-ray afterglow flux to the optical afterglow flux and prompt gamma-ray fluence is used to quantify the seemingly deficient optical, and in some cases X-ray, light at these early epochs. This comparison reveals that some of these bursts appear to have higher than normal gamma-ray efficiencies. We discuss possible mechanisms and their feasibility for explaining the apparent lack of early optical emission. The mechanisms considered include: foreground extinction, circumburst absorption, Ly-alpha blanketing and absorption due to high redshift, low density environments, rapid temporal decay, and intrinsic weakness of the reverse shock. Of these, foreground extinction, circumburst absorption, and high redshift provide the best explanations for most of the non-detections in our sample. There is tentative evidence of suppression of the strong reverse shock emission. This could be because of a Poynting-flux-dominated flow or a pure non-relativistic hydrodynamical reverse shock., 22 pages, 5 figures. Accepted for publication in ApJ

Three different methods of measuring cosmology with gamma-ray bursts (GRBs) have been proposed since a relation between the $\gamma$-ray energy $E_{\gamma} $ of a GRB jet and the peak energy $E_p$ of the $\nu F_{\nu}$ spectrum in the burst frame was reported by Ghirlanda and coauthors. In Method I, to calculate the probability for a favored cosmology, only the contribution of the $E_\gamma-E_p$ relation that is already best fitted for this cosmology is considered. We apply this method to a sample of 17 GRBs, and obtain the mass density $\Omega_M=0.15^{+0.45}_{-0.13}$ ($1\sigma$) for a flat $\Lambda$CDM universe. In Method II, to calculate the probability for some certain cosmology, contributions of all the possible $E_\gamma-E_p$ relations that are best fitted for their corresponding cosmologies are taken into account. With this method, we find a constraint on the mass density $0.14, Comment: 26 pages including 6 figures, different methods compared, accepted for publication in ApJ

Based on the standard energy reservoir of gamma-ray burst (GRB) jets and the relationship between peak energy ($E_p$) of $\nu F_{\nu}$ spectrum and the equivalent-isotropic energy ($E_{\rm{iso}}$), we test the GRB probability distributions as a function of viewing angle ($\theta$) and of viewing angle together with redshift $z$, $P(\theta)$ and $P(\theta,z)$, predicted by the universal-structured jet (USJ) model with simulations on observational bases for a detection threshold of $S>4 \times 10^{-5}$ erg cm$^{-2}$, where $S$ is the total fluence in an energy band $>20$ keV. We simulate a sample including $10^{6}$ GRBs, and then derive $P(\theta)$ and $P(\theta,z)$. We show that both simulated and theoretical $P(\theta)$ and $P(\theta,z)$ are consistent with each other. This consistency seems to be regarded as support for the USJ model. A small difference between the theoretical and simulated results might be due to the observed $E_p$ distribution used in our simulations, which perhaps suffers a little of biases for $E_p, Comment: 6 pages including 4 figures, MNRAS in press

We derive the optical afterglow luminosity distributions at different epoches for gamma-ray bursts (GRBs) from a sample of 146 GRBs that have a well-sampled optical afterglow lightcurve, then explore the luminosity function of GRB optical afterglows using the Monte Carlo simulation. We show that an intrinsic broken power-law luminosity function can well reproduced the observed magnitude distributions.

GRB 100418A is a long gamma-ray burst (GRB) at redshift z = 0.6235 discovered with the Swift Gamma-ray Burst Explorer with unusual optical and X-ray light curves. After an initial short-lived, rapid decline in X-rays, the optical and X-ray light curves observed with Swift are approximately flat or rising slightly out to at least similar to 7 x 10(3) s after the trigger, peak at similar to 5 x 10(4) s, and then follow an approximately power-law decay. Such a long optical plateau and late peaking is rarely seen in GRB afterglows. Observations with Rapid Eye Mount during a gap in the Swift coverage indicate a bright optical flare at similar to 2.5 x 10(4) s. The long plateau phase of the afterglow is interpreted using either a model with continuous injection of energy into the forward shock of the burst or a model in which the jet of the burst is viewed off-axis. In both models the isotropic kinetic energy in the late afterglow after the plateau phase is >= 10(2) times the 10(51) erg of the prompt isotropic gamma-ray energy release. The energy injection model is favored because the off-axis jet model would require the intrinsic T-90 for the GRB jet viewed on-axis to be very short, similar to 10 ms, and the intrinsic isotropic gamma-ray energy release and the true jet energy to be much higher than the typical values of known short GRBs. The non-detection of a jet break up to t similar to 2 x 10(6) s indicates a jet half-opening angle of at least similar to 14 degrees, and a relatively high-collimation-corrected jet energy of E-jet >= 10(52) erg.