Relationships between Relative Spectral Lags and Relative Widths of Gamma-ray Bursts

The phenomenon of gamma-ray burst (GRB) spectral lags is very common, but a definitive explanation has not yet been given. From a sample of 82 GRB pulses we find that the spectral lags are correlated with the pulse widths, however, there is no correlation be- tween the relative spectral lags and the relative pulse widths. We suspect that the correlations between spectral lags and pulse widths might be caused by the Lorentz factor of the GRBs concerned. Our analysis on the relative quantities suggests that the intrinsic spectral lag might reflect other aspect of pulses than the aspect associated with the dynamical time of shocks or that associated with the time delay due to the curvature effect.     

Spectral lag of gamma‐ray bursts caused by the intrinsic spectral evolution and the curvature effect

Assuming an intrinsic ‘Band’ shape spectrum and an intrinsic energy‐independent emission profile we have investigated the connection between the evolution of the rest‐frame spectral parameters and the spectral lags measured in gamma‐ray burst (GRB) pulses by using a pulse model. We first focus our attention on the evolution of the peak energy, E_0,p, and neglect the effect of the curvature effect. It is found that the evolution of E_0,p alone can produce the observed lags. When E_0,p varies from hard to soft only the positive lags can be observed. The negative lags would occur in the case of E_0,p varying from soft to hard. When the evolution of E_0,p and the low‐energy spectral index α₀ varying from soft to hard then to soft we can find the aforesaid two sorts of lags. We then examine the combined case of the spectral evolution and the curvature effect of fireball and find the observed spectral lags would increase. A sample including 15 single pulses whose spectral evolution follows hard to soft has been investigated. All the lags of these pulses are positive, which is in good agreement with our theoretical predictions. Our analysis shows that only the intrinsic spectral evolution can produce the spectral lags and the observed lags should be contributed by the intrinsic spectral evolution and the curvature effect. But it is still unclear what cause the spectral evolution (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Coherent Synchrotron Radiation of Gamma-Ray Bursts

By now there is no doubt that the gamma-ray bursts (GRB) have a cosmological origin. This allows to regard GRB as the most powerful known energy sources, ε∼ 10⁵⁴ erg (with a total number of gamma quanta N_γ∼ 10⁶⁰). A plausible mechanism of coherent synchrotron radiation (CSR) of relativistic electrons driven by a local magnetic field is studied in this paper. We consider relativistic electrons arising in the Compton scattering of a GRB in directions close to that of the ray from the source to a ground-based observer. The synchrotron pulses from Compton electrons located at different points on the line between the GRB source and the observer arrive at the observation point simultaneously. This simultaneity ensures the coherence of the detected radiation. Both molecular clouds in the host galaxy of the GRB and our own Galaxy, as well as the Earth atmosphere are assumed to be scatterers of the GRB radiation. Signals of each scatterer reach the Earth surface, and can be detected at radio wavelengths. We estimate the characteristics of this radiation. The comparison of GRB data with the corresponding information on CSR pulses offers a way to determine some global characteristics of the medium between the Earth and the GRB source.     

Important Property of GRB Pulse: Power-Law Indices of Time Properties on Energy

The dependence of pulse temporal properties (pulse width, pulse rise width and pulse decay width) on energy is power-law function. Some correlated relationships between the power-law indices of the pulse time properties on energy and the spectral lags, relative spectral lags, spectral parameters of band function, and photon flux using a well-separated long-duration γ-ray burst (GRB) pulse sample is demonstrated here. We argue that the curvature effect can explain the correlated properties.     

The <Emphasis Type="Italic">Ulysses</Emphasis> Catalog of Solar Hard X-Ray Flares

Ulysses was launched in October 1990, and its Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) has provided more than 13 years of uninterrupted observations of solar X-ray flare activity. Due to the large variation of the relative solar latitude and longitude of the spacecraft orbit with respect to the Earth, the perspective of the GRB instrument often differed significantly from that of X-ray instruments on Earth-orbiting satellites. During extended periods the GRB experiment made direct observations of flares on the hidden face of the Sun, providing a unique record of events not visible to other instruments. The small detector area of GRB and its optimization for very high counting rates minimized the effects of pulse pile-up. We interpret the spectra, time histories, and occurrence distribution patterns of GRB data in terms of “thermal feed-through”, the confusion of thermal soft X-rays and non-thermal hard X-rays. This effect is a systematic problem for scintillation-counter spectrometers observing the solar hard X-ray spectrum. This paper provides a definitive catalog of the Ulysses X-ray flare observations and discusses various features of this unique database. For the equivalent GOES range X2 – X25, we find a power-law fit for the (differential) occurrence frequency at >25 keV with slope −1.61±0.04, with no evidence for a downturn at the highest event magnitudes (for the relatively small sample of such events available in this study). If the nine most intense events are excluded because of concerns about the effects of pulse pile-up, the slope steepens to −1.75±0.08.     

Time-resolved spectral analysis of prompt emission from long gamma-ray bursts with GeV emission

We performed detailed time-resolved spectroscopy of bright long gammaray bursts(GRBs)which show significant GeV emissions(GRB 080916C,GRB090902B and GRB 090926A).In addition to the standard Band model,we also use a model consisting of a black body and a power law to fit the spectra.We find that for the latter model there are indications of an additional soft component in the spectra.While previous studies have shown that such models are required for GRB 090902B,here we find that a composite spectral model consisting of two blackbodies and a power law adequately fits the data of all the three bright GRBs.We investigate the evolution of the spectral parameters and find several interesting features that appear in all three GRBs,like(a)temperatures of the blackbodies are strongly correlated with each other,(b)fluxes in the black body components are strongly correlated with each other,(c)the temperatures of the black body trace the profile of the individual pulses of the GRBs,and(d)the characteristics of power law components like the spectral index and the delayed onset bear a close similarity to the emission characteristics in the GeV regions.We discuss the implications of these results and the possibility of identifying the radiation mechanisms during the prompt emission of GRBs.     

Observation of the Second LIGO/Virgo Event Connected with a Binary Neutron Star Merger S190425z in the Gamma-Ray Range

The results of observations of the gravitational-wave (GW) event S190425z recorded by the LIGO/Virgo detectors with the anti-coincidence shield (ACS) of the SPI gamma-ray spectrometer onboard the INTEGRAL observatory are presented. With a high probability (>99%) it was associated with a neutron star (NS) merger in a close binary system. This is only the second event of such a type in the history of gravitational-wave observations (after GW170817). A weak gamma-ray burst, GRB190425, consisting of two pulses ～0.5 and ～5.9 s after the NS merger in the event S190425z with an a priori significance of 3.5 and 4.4σ (taken together 5.5σ) was detected by SPI-ACS. Analysis of the SPI-ACS count rate history recorded on these days (a total of ～125 ks of observations) has shown that the rate of random occurrence of two close spikes with the characteristics of GRB190425 does not exceed 6.4 × 10^?5 s^?1 (i.e., such events occur by chance, on average, every ～4.3 hours). Note that the time profile of GRB190425 has much in common with the profile of GRB170817A accompanying the event GW170817, that both NS mergers were the nearest (≤150 Mpc) of all the events recorded by the LIGO/Virgo detectors, and that no significant excesses of the gamma-ray flux above the background were detected in any of ～30 black hole merger events recorded to date by these detectors. No bursts of hard radiation were detected in the field of view of the SPI and IBIS/ISGRI gamma-ray telescopes onboard INTEGRAL. This, along with the absence of detection of gamma-ray emission from GRB190425 by the GBM gamma-ray burst monitor of the Fermi observatory suggesting its occultation by the Earth, allows the localization region for the source of this GWevent to be reduced significantly. The parameters E_iso and E_p for GRB190425 are estimated and compared with those for GRB170817A.

     

Spectral hardness evolution characteristics of tracking gamma-ray burst pulses

Employing a sample presented by Kaneko et al. (2006) [Kaneko, Y. et al., 2006. ApJS 166, 298 (Paper I)] and Kocevski et al. (2003) [Kocevski, D. et al., 2003. ApJ 596, 389], we select 42 individual tracking pulses (here we defined tracking as the cases in which the hardness follows the same pattern as the flux or count rate time profile) within 36 gamma-ray bursts (GRBs) containing 527 time-resolved spectra and investigate the spectral hardness, E_peak (where E_peak is the maximum of the νF_ν spectrum), evolutionary characteristics. The evolution of these pulses follow soft-to-hard-to-soft (the phase of soft-to-hard and hard-to-soft are denoted by rise phase and decay phase, respectively) with time. It is found that the overall characteristics of E_peak of our selected sample are: (1) the E_peak evolution in the rise phase always start on the high state (the values of E_peak are always higher than 50 keV); (2) the spectra of rise phase clearly start at higher energy (the median of E_peak are about 300 keV), whereas the spectra of decay phase end at much lower energy (the median of E_peak are about 200 keV); (3) the spectra of rise phase are harder than that of the decay phase and the duration of rise phase are much shorter than that of decay phase as well. In other words, for a complete pulse the initial E_peak is higher than the final E_peak and the duration of initial phase (rise phase) are much shorter than the final phase (decay phase). This results are in good agreement with the predictions of [Lu, R.J. et al., 2007. ApJ 663, 1110] and current popular view on the production of GRBs. We argue that the spectral evolution of tracking pulses may be relate to both of kinematic and dynamic process even if we currently can not provide further evidences to distinguish which one is dominant. Moreover, our statistical results give some witnesses to constrain the current GRB model.     

Optical observations of GRB 050401 afterglow : a case for double-jet model

The afterglow of GRB 050401 presents several novel and interesting features. (i) An initially faster decay in optical band than in X-rays. (ii) A break in the X-ray light curve after ∼0.06 d with an unusual slope after the break. (iii)The X-ray afterglow does not show any spectral evolution across the break while the R -band light curve does not show any break. We have modelled the observed multiband evolution of the afterglow of GRB 050401 as originating in a two-component jet, and interpreting the break in X-ray light curve as due to lateral expansion of a narrow collimated outflow which dominates the X-ray emission. The optical emission is attributed to a wider jet component. Our model reproduces all the observed features of multiband afterglow of GRB 050401. We present optical observations of GRB 050401 using the 104-cm Sampurnanand Telescope at the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital. Results of the analysis of multiband data are presented and compared with GRB 030329, the first reported case of double jet.     

Catalog of short gamma-ray transients detected in the SPI/INTEGRAL experiment

We analyzed the data obtained by the SPI telescope onboard the INTEGRAL observatory to search for short transient events with a duration from 1 ms to a few tens of seconds. An algorithm for identifying gamma-ray events against the background of a large number of charged particle interactions with the detector has been developed. The classification of events was made. Apart from the events associated with cosmic gamma-ray bursts (GRBs) confirmed by other space experiments and the activity of known soft gamma repeaters (for example, SGR 1806-20), previously unreported GRBs have been found. GRB candidates and short gamma-ray events probably associated with the activity of known SGRs and AXPs have been selected. The spectral evolution of 28 bright GRBs from the catalog has been studied extensively. A new method for investigating the spectral evolution is proposed. The energy dependence of the spectral lag for bursts with a simple structure of their light curves and for individual pulses of multipulse events is shown to be described by a logarithmic function, lag ～ Alog(E). It has been established that the parameter A depends on the pulse duration, with the dependence being universal for all of the investigated GRBs. No negative spectral lags have been detected for bursts with a simple structure of their light curves.     

Cosmological rates of coalescing neutron stars and GRB

Assuming that gamma-ray bursts (GRB) originate from binary neutron star (NS) or black holes (BH) merging in distant galaxies, theoretical logN-logS distributions for gamma-ray bursts (GRB) are calculated using the compact binaries coalescence rates computed for model galaxies with different star formation histories. A flat cosmological model ( = 1) with different values of the cosmological constant is used. The calculated source evolution predicts a 5–10 times increase of the source statistics at count rates 3–10 times lower than the existing BATSE sensitivity limit. The most important parameter in fitting the 2nd BATSE catalogue is the initial redshift of star formation, which is found to bez _* = 2 — 5 depending on a poorly determined average spectral index of GRB.     

Gamma-Ray Bursts and magnetar-forming Supernovae

The connection between long Gamma Ray Bursts (GRBs) and Supernovae (SNe) have been established through the well observed cases. These events can be explained as the prompt collapse to a black hole (BH) of the core of a massive star (M≳40M _⊙). The energies of these GRB-SNe were much larger than those of typical SNe, thus these SNe are called Hypernovae (HNe). The case of SN 2006aj/GRB060218 appears different: the GRB was weak and soft, being called an X-Ray Flash (XRF); the SN is dimmer and has very weak oxygen lines. The explosion energy of SN 2006aj was smaller, as was the ejected mass. In our model, the progenitor star had a smaller mass than other GRB-SNe (M∼20M _⊙), suggesting that a neutron star (NS) rather than a BH was formed. If the nascent NS was strongly magnetized as a magnetar and rapidly spinning, it may launch a weak GRB or an XRF. The peculiar light curve of Type Ib SN 2005bf may also be powered by a magnetar. The blue-shifted nebular emission lines of 2005bf indicate the unipolar explosion possibly related to standing accretion shock instability (SASI) associated with a newly born NS.     

Statistical Properties of the Highest Pulses in Gamma-Ray Bursts

We study the statistical properties of the highest pulses within individual gamma-ray bursts (GRBs). A wavelet package analysis technique and a developed pulse-finding algorithm have been applied to identify the highest pulses from burst profiles observed by BATSE on board CGRO from 1991 April 21 to 1999 January 26. The statistical light curves of the highest pulses in four energy channels have been derived by an aligning method, which illustrate the temporal evolution of the pulse emission. Our result that narrower pulses go with higher energies is consistent with previous findings. By normalizing both the pulse durations and counts to unity, “characteristic” profiles of the highest pulses in the four channels are also derived. The four characteristic profiles are turned out to be almost the same, thus strongly support the previous conclusion that the temporal profiles in different energy channels are self-similar and the previous conjecture on GRB pulses, implying that the emission process is similar at different energies. The cosmological time dilation effect is examined by investigating the relationship between the pulse flux and pulse duration. An anti-correlation between the two was found, which agrees with the expectation of the cosmological time dilation effect. Also, the evolution of the pulse duration with the observational epoch is studied. The result shows that the pulse duration tends to be shorter in later epochs. This trend cannot be explained by the present theoretical models, and may represent a great challenge to current theories.     

A unified picture for the γ-ray and prompt optical emissions of GRB 990123

The prompt optical emission of GRB 990123 was uncorrelated to the γ-ray light curve and exhibited temporal properties similar to those of the steeply decaying, early X-ray emission observed by Swift at the end of many bursts. These facts suggest that the optical counterpart of GRB 990123 was the large-angle emission released during (the second pulse of) the burst. If the optical and γ-ray emissions of GRB 990123 have, indeed, the same origin then their properties require that (i) the optical counterpart was synchrotron emission and γ-rays arose from inverse-Compton scatterings (the 'synchrotron self-Compton model'), (ii) the peak energy of the optical-synchrotron component was at ∼20 eV and (iii) the burst emission was produced by a relativistic outflow moving at Lorentz factor  ≳450  and at a radius  ≳10¹⁵  cm, which is comparable to the outflow deceleration radius. Because the spectrum of GRB 990123 was optically thin above 2 keV, the magnetic field behind the shock must have decayed on a length-scale of  ≲1  per cent  of the thickness of the shocked gas, which corresponds to  10⁶–10⁷  plasma skin depths. Consistency of the optical counterpart decay rate and its spectral slope (or that of the burst, if they represent different spectral components) with the expectations for the large-angle burst emission represents the most direct test of the unifying picture proposed here for GRB 990123.     

Search for constraints on Lorentz violation parameters by studying the polarization of afterglows from high-energy gamma-ray bursts

The birefringence of electromagnetic radiation from gamma-ray burst (GRB) afterglows due to the Lorentz violation (LV) of spacetime has been calculated. The dependence of this effect on the radiation wavelength and redshift z in the ΛCDM model of the Universe has been established. It is shown that polarization tests for GRB afterglows can be invoked to measure the LV parameters. LV causes oscillations in the parameters of the afterglow spectrum that can be in the ultraviolet spectral range and will depend significantly on redshift z. This can facilitate the recording of LV for an electromagnetic field and can lower significantly the energy threshold of its detection.     

Using X‐shooter to measure the extinction in long GRB host galaxies from emission lines

The spectra of the host galaxies of gamma‐ray bursts at low redshift generally show strong hydrogen lines of the Balmer, Paschen and Brackett series, in addition to strong nebular metal lines. In special cases the hosts can be resolved in separate star forming regions, and spatially resolved spectroscopy can be obtained. Generally, the three strongest Balmer lines are used to derive the reddening experienced by the emission lines of the host gas, assuming a Milky Way extinction curve, case B recombination and a fixed electron temperature. We demonstrate how the wide wavelength range of X‐shooter, in combination with a rigorous calibration strategy, can be used to fit explicitly for R_V, T_e, and A_V simultaneously using a large number of H and He I recombination lines, explicitly corrected for stellar atmosphere absorption. This increases our understanding of extinction and absorption in starforming regions in GRB hosts. We use two GRB hosts as examples of the methods, outlining the advantages of using X‐shooter over other instruments (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the pair-instability supernovae and gamma-ray burst phenomenon

We present a model of a gamma-ray burst (GRB) related to “very massive” stars. In the framework of our model, the GRB phenomenon is a result of helium burning in degenerate conditions in a massive star (≳130 M _⊙), in which the thermal nuclear burning occurs in the deflagration regime and has a pulsating temporal pattern. The shock runs away from the burning (reaction) zone, which leads to the development of a coronal outflow (jet-like) structure. In our scenario the GRB observable prompt fast rise and decay part can be a result of photon propagation through the hot corona (Comptonization photosphere) of the star. On the other hand, the GRB afterglow is a cooling phase of the expanding and outflowing envelope. Presumably, the X-ray part of the GRB emergent spectrum is formed due to upscattering of soft photons of outer layers of the star off hot coronal electrons, and thus it should have a specific shape of the Comptonization spectrum.     

Thermal synchrotron radiation and gamma-ray burst spectra

The standard classical expressions for the thermal synchrotron (TS) radiation from an optically thin thermal plasma are shown to be inapplicable at photon energiesEkT since they neglect quantum effects. Quantum relationships are obtained for the TS spectral emissivity, opacity, and polarization. The quantum TS spectra are much softer atEkT than the classical ones. The TS radiation exhibits strong linear polarization in the classical domain, whereas the quantum effects reduce the polarization at highE. Expressions for the classical TS luminosity are obtained with quantum corrections which turn out to be significant for (B/B _c )(kT/mc ²)10^–2(B _c =4.41×10¹³ G).Fitting the gamma-ray burst (GRB) spectra by the classical TS law (see, e.g., Lianget al., 1983) is incorrect in cases wherekT is less than the maximum detected photon energy. The continua of the GRB spectra in the rangeE20 keV-2 MeV (Mazetset al., 1981a; Andreevet al., 1983) can be fitted satisfactorily by the quantum TS spectra. The results of this fitting may suggest the existence of temperatures much higher (up to 10 MeV), and of magnetic fields much lower (down to 10⁹ G) than those usually accepted. Under these conditions the thickness of the TS sources (10³–10⁴ cm) could be comparable with their transverse dimensions (in contrast to sources with ordinary temperatures and fields), if they lie within a few kpc. The quantum TS spectra are too soft to account for the hard components (up to tens of MeV) of the GRB spectra detected by the Solar Maximum Mission (Nolanet al., 1984), unless the temperatures are unreasonably high.A straightforward TS interpretation of the GRB spectra seems to be unrealistic. Most probably, the continuum radiation escapes from an optically thick, strongly magnetized, highly non-stationary, hot plasma near the surface of a neutron star.     

The first hours of the optical afterglow from the cosmic gamma-ray burst 030329

We describe the first results of our observations of the exceptionally bright optical afterglow from the cosmic gamma-ray burst (GRB) of March 29, 2003 (030329), with the 1.5-m Russian-Turkish telescope (RTT150) installed at the TUBITAK National Observatory (Turkey) at Mount Bakyrlytepe. RTT150 was one of the first medium-class telescopes pointed at the afterglow. The observations began as early as about six hours after the GRB. During the first five hours of our observations, the BV RI flux fell off exactly as a power law with the same slope ?1.19±0.01. Subsequently, in all of the BV RI bands, we observed the same increase in the power-law slope of the light curve to a value that was later recorded during the observations at observatories in the western hemisphere. The break in the power-law light curve occurs at t ? t ₀ ≈ 0.57 days (13.5 h) and lasts for about 0.2 days. Apart from this smooth decrease in the flux, the afterglow exhibited no flux variability. The upper limits on the variability are 10–1% on time scales of 0.1–1000 s, respectively. The BV RI spectral flux distribution during the first night of our observations closely corresponds to a power-law spectrum with a spectral index α=0.66±0.01. The change in the power-law slope of the light curve at the end of our observations is probably attributable to the deceleration of the ultrarelativistic jet to a gamma factor when its structural features begin to show up in the light curve. The radio, optical, and X-ray broadband spectrum is consistent with the assumption about the synchrotron radiation of the ultrarelativistic jet. This unique object continues to be observed with RTT150.