The discovery of a statistically significant clustering in the distribution of gamma-ray bursts (GRBs) has recently been reported. Given that the cluster has a characteristic size of 2000-3000Mpc and a redshift between 1.6<=z<=2.1, it has been claimed that this structure is incompatible with the cosmological principle of homogeneity and isotropy of our Universe. In this paper, we study the homogeneity of the GRB distribution using a subsample of the Greiner GRB catalogue, which contains 314 objects with redshift 0<z<2.5 (244 of them discovered by the Swift GRB mission). We try to reconcile the dilemma between the new observations and the current theory of structure formation and growth. To test the results against the possible biases in redshift determination and the incompleteness of the Greiner sample, we also apply our analysis to the 244 GRBs discovered by Swift and the subsample presented by the Swift Gamma-Ray Burst Host Galaxy Legacy Survey (SHOALS). The real space two-point correlation function (2PCF) of GRBs, {ksi}(r), is calculated using a Landy-Szalay estimator. We perform a standard least-{Chi}^2^ fit to the measured 2PCFs of GRBs. We use the best-fit 2PCF to deduce a recently defined homogeneity scale. The homogeneity scale, R_H_, is defined as the comoving radius of the sphere inside which the number of GRBs N(<r) is proportional to r^3^ within 1%, or equivalently above which the correlation dimension of the sample D^2^ is within 1% of D^2^=3.
We discuss INTEGRAL's ability to detect a high redshift population of Gamma-Ray Bursts (GRBs) in comparison to other high-energy missions. Emphasis is placed on the study of the relative capabilities of IBIS on board INTEGRAL with respect to SWIFT and HETE 2 in detecting a high redshift population of GRBs. We conclude that, if the GRB rate is proportional to the star formation rate, INTEGRAL's ability to study GRBs are complementary to the ones of missions like SWIFT and HETE 2, devoted to prompt localisations of GRBs. Whereas SWIFT and HETE 2 would detect a higher number of GRBs than INTEGRAL, IBIS might be able to detect high redshift (z>~7) GRBs, unreachable by SWIFT and HETE 2. We discuss the relevance of performing near-infrared (NIR) observations of the INTEGRAL GRBs and the strategy that large-class telescopes might follow.
We have searched for intermediate-scale anisotropy in the arrival directions of ultrahigh-energy cosmic rays with energies above 57 EeV in the northern sky using data collected over a 5 yr period by the surface detector of the Telescope Array experiment. We report on a cluster of events that we call the hotspot, found by oversampling using 20{deg} radius circles. The hotspot has a Li-Ma statistical significance of 5.1{sigma}, and is centered at RA = 146.{deg}7, DE = 43.{deg}2. The position of the hotspot is about 19{deg} off of the supergalactic plane. The probability of a cluster of events of 5.1{sigma} significance, appearing by chance in an isotropic cosmic-ray sky, is estimated to be 3.7x10^-4^ (3.4{sigma}).
The nearby Large Magellanic Cloud (LMC) provides a rare opportunity of a spatially resolved view of an external star-forming galaxy in {gamma}-rays. The LMC was detected at 0.1-100GeV as an extended source with CGRO/EGRET and using early observations with the Fermi-LAT. The emission was found to correlate with massive star-forming regions and to be particularly bright towards 30 Doradus. Studies of the origin and transport of cosmic rays (CRs) in the Milky Way are frequently hampered by line-of-sight confusion and poor distance determination. The LMC offers a complementary way to address these questions by revealing whether and how the {gamma}-ray emission is connected to specific objects, populations of objects, and structures in the galaxy. We revisited the {gamma}-ray emission from the LMC using about 73 months of Fermi-LAT P7REP data in the 0.2-100GeV range. We developed a complete spatial and spectral model of the LMC emission, for which we tested several approaches: a simple geometrical description, template-fitting, and a physically driven model for CR-induced interstellar emission.
The Fermi Gamma-ray Burst Monitor (GBM) has detected over 1400 gamma-ray bursts (GRBs) since it began science operations in 2008 July. We use a subset of over 300 GRBs localized by instruments such as Swift, the Fermi Large Area Telescope, INTEGRAL, and MAXI, or through triangulations from the InterPlanetary Network, to analyze the accuracy of GBM GRB localizations. We find that the reported statistical uncertainties on GBM localizations, which can be as small as 1{deg}, underestimate the distance of the GBM positions to the true GRB locations and we attribute this to systematic uncertainties. The distribution of systematic uncertainties is well represented (68% confidence level) by a 3.{deg}7 Gaussian with a non-Gaussian tail that contains about 10% of GBM-detected GRBs and extends to approximately 14{deg}. A more complex model suggests that there is a dependence of the systematic uncertainty on the position of the GRB in spacecraft coordinates, with GRBs in the quadrants on the Y axis better localized than those on the X axis.
Gamma-ray bursts (GRBs) are classified into long and short categories based on their durations. Broadband studies suggest that these two categories of objects roughly correspond to two different classes of progenitor systems, i.e., compact star mergers (Type I) versus massive star core collapse (Type II). However, the duration criterion sometimes leads to mis-identification of the progenitor systems. We perform a comprehensive multi-wavelength comparative study between duration-defined long GRBs and short GRBs as well as the so-called "consensus" long GRBs and short GRBs (which are believed to be more closely related to the two types of progenitor systems). The parameters we study include two parts: the prompt emission properties including duration (T_90_), spectral peak energy (E_p_), low energy photon index ({alpha}), isotropic {gamma}-ray energy (E_{gamma},iso_), isotropic peak luminosity (L_p,iso_), and the amplitude parameters (f and f_eff_); and the host galaxy properties including stellar mass (M_*_), star formation rate, metallicity ([X/H]), half light radius (R_50_), angular and physical (R_off_) offset of the afterglow from the center of the host galaxy, the normalized offset (r_off_=R_off_/R_50_), and the brightness fraction F_light_. For most parameters, we find interesting overlapping properties between the two populations in both one-dimensional (1D) and 2D distribution plots. The three best parameters for the purpose of classification are T_90_, f_eff_, and F_light_. However, no single parameter alone is good enough to place a particular burst into the right physical category, suggesting the need for multiple criteria for physical classification.
Magnetars are an extreme, highly magnetized class of isolated neutron stars whose large X-ray luminosity is believed to be driven by their high magnetic field. Study for the first time the possible very high energy {gamma}-ray emission above 100GeV from magnetars, observing the sources 4U 0142+61 and 1E 2259+586.
Known TeV sources detected by major,Cerenkov telescopes are investigated to identify possible MeV-GeV {gamma}-ray counterparts. A systematic study of the known sources in the web-based TeVCat catalog has been performed to search for possible {gamma}-ray counterparts on the AGILE data collected during the first period of operations in observing pointing mode. For each TeV source, a search for a possible {gamma}-ray counterpart that is based on a multi-source maximum likelihood algorithm is performed on the AGILE data taken with the GRID instrument from July 2007 to October 2009. In the case of high-significance detection, the average {gamma}-ray flux is estimated. For cases of low-significance detection the 95% confidence level (CL) flux upper limit is given. 52TeV sources out of 152 (corresponding to ~34% of the analysed sample) show a significant excess in the AGILE data covering the pointing observation period. This analysis found 26 new AGILE sources with respect to the AGILE reference catalogs, 15 of which are galactic, 7 are extragalactic and 4 are unidentified. Detailed tables with all available information on the analysed sources are presented.
Using LOFAR, we have performed a very-low-frequency (115-155MHz) radio survey for millisecond pulsars (MSPs). The survey targeted 52 unidentified Fermi {gamma}-ray sources. Employing a combination of coherent and incoherent dedispersion, we have mitigated the dispersive effects of the interstellar medium while maintaining sensitivity to fast-spinning pulsars. Toward 3FGLJ1553.1+5437 we have found PSRJ1552+5437, the first MSP to be discovered (through its pulsations) at a radio frequency <200MHz. PSR J1552+5437 is an isolated MSP with a 2.43ms spin period and a dispersion measure of 22.9pc/cm^3^. The pulsar has a very steep radio spectral index ({alpha}-2.8+/-0.4). We obtain a phase-connected timing solution combining the 0.74yr of radio observations with {gamma}-ray photon arrival times covering 7.5yr of Fermi observations. We find that the radio and {gamma}-ray pulse profiles of PSR J1552+5437 appear to be nearly aligned. The very steep spectrum of PSR J1552+5437, along with other recent discoveries, hints at a population of radio MSPs that have been missed in surveys using higher observing frequencies. Detecting such steep spectrum sources is important for mapping the population of MSPs down to the shortest spin periods, understanding their emission in comparison to slow pulsars, and quantifying the prospects for future surveys with low-frequency radio telescopes like SKA-Low and its precursors.
We present 12mm Mopra observations of dense molecular gas towards the W28 supernova remnant (SNR) field. The focus is on the dense molecular gas towards the TeV gamma-ray sources detected by the HESS telescopes, which likely trace the cosmic rays from W28 and possibly other sources in the region. Using the NH_3_ inversion transitions we reveal several dense cores inside the molecular clouds,
