Observations Of Gamma-ray Bursts Research Articles

Shock dissipation in magnetically dominated impulsive flows

The idea that cosmic relativistic jets are magnetically driven Poynting-dominated flows has many attractive features but also some problems. One of them is the low efficiency of shock dissipation in highly magnetized plasma. Indeed, the observations of gamma-ray bursts (GRBs) and their afterglow emission indicate very high radiative efficiency of relativistic jets associated with these phenomena. We have revisited the issue of shock dissipation and emission and its implications for the internal shock model of the prompt GRB emission and studied it in the context of impulsive Poynting-dominated flows. Our results show that unless the magnetization of GRB jets is extremely high, σ > 100 in the prompt emission zone, the magnetic model may still be compatible with the observations. First, for σ≃ 1 the dissipation efficiency of fast magnetosonic shock is still quite high, ∼30 per cent. Secondly, the main effect of reduced dissipation efficiency is merely an increase in the size of the dissipation zone, and even for highly magnetized GRB jets, this size may remain below the external shock radius, provided the central engine can emit magnetic shells on the time-scale well below the typical observed variability scale of 1 s. Our analytical and numerical results suggest that strong interaction between shells begins not during the coasting phase but well before it. As the result, the impulsive jet in the dissipation zone is best described not as a collection of shells but as a continuous highly magnetized flow with a high amplitude magnetosonic wave component. How exactly the dissipated wave energy is distributed between the radiation and the bulk kinetic energy of radial jets depends on the relative rates of radiative and adiabatic cooling. In the fast radiative cooling regime, the corresponding radiative efficiency can be as high as the wave contribution to their energy budget, independently of the magnetization. Moreover, after leaving the zone of prompt emission, the jet may still remain Poynting dominated, leading to weaker emission from the reverse shock compared to non-magnetic models. Energetically subdominant weakly magnetized ‘clouds’ in otherwise strongly magnetized jets may significantly increase the overall efficiency of the shock dissipation.

A RADIO-SELECTED SAMPLE OF GAMMA-RAY BURST AFTERGLOWS

We present a catalog of radio afterglow observations of gamma-ray bursts (GRBs) over a 14 year period from 1997 to 2011. Our sample of 304 afterglows consists of 2995 flux density measurements (including upper limits) at frequencies between 0.6 GHz and 660 GHz, with the majority of data taken at 8.5 GHz frequency band (1539 measurements). We use this dataset to carry out a statistical analysis of the radio-selected sample. The detection rate of radio afterglows has stayed unchanged almost at 31% before and after the launch of the {\em Swift} satellite. The canonical long-duration GRB radio light curve at 8.5 GHz peaks at 3-6 days in the source rest frame, with a median peak luminosity of $10^{31}$ erg s$^{-1}$ Hz$^{-1}$. The peak radio luminosities for short-hard bursts, X-ray flashes and the supernova-GRB classes are an order of magnitude or more fainter than this value. There are clear relationships between the detectability of a radio afterglow and the fluence or energy of a GRB, and the X-ray or optical brightness of the afterglow. However, we find few significant correlations between these same GRB and afterglow properties and the peak radio flux density. We also produce synthetic light curves at centimeter (cm) and millimeter (mm) bands using a range of blastwave and microphysics parameters derived from multiwavelength afterglow modeling, and we use them to compare to the radio sample. Finding agreement, we extrapolate this behavior to predict the cm and mm behavior of GRBs observed by the Expanded Very Large Array and the Atacama Large Millimeter Array.

Diversity of multiwavelength emission bumps in the GRB 100219A afterglow

Context. Multi-wavelength observations of gamma-ray burst (GRB) afterglows provide important information about the activity of their central engines and their environments. In particular, the short timescale variability, such as bumps and/or rebrightening features visible in the multi-wavelength light curves, is still poorly understood. Aims. We analyze the multi-wavelength observations of the GRB100219A afterglow at redshift 4.7. In particular, we attempt to identify the physical origin of the late achromatic flares/bumps detected in the X-ray and optical bands. Methods. We present ground-based optical photometric data and Swift X-ray observations on GRB100219A. We analyzed the temporal behavior of the X-ray and optical light curves, as well as the X-ray spectra. Results. The early flares in the X-ray and optical light curves peak simultaneously at about 1000 s after the burst trigger, while late achromatic bumps in the X-ray and optical bands appear at about 20000 s after the burst trigger. These are uncommon features in the afterglow phenomenology. Considering the temporal and spectral properties, we argue that both optical and X-ray emissions come from the same mechanism. The late flares/bumps may be produced by late internal shocks from long-lasting activity of the central engine. An off-axis origin for a structured jet model is also discussed to interpret the bump shapes. The early optical bump can be interpreted as the afterglow onset, while the early X-ray flare could be caused by the internal activity. GRB 100219A exploded in a dense environment as revealed by the strong attenuation of X-ray emission and the optical-to-X-ray spectral energy distribution.

HIGH-ENERGY EMISSION FROM GAMMA-RAY BURSTS

The theoretical consequences of the recent observations of gamma-ray bursts at high-energy by Fermi are discussed, with a focus on the internal-external shock model.

Gamma-ray burst observations by Fermi Large Area Telescope revisited: new candidates found

ABSTRACT We search the Fermi Large Area Telescope (LAT) photon data base for an extended gamma-ray emission which could be associated with any of the 581 previously detected gamma-ray bursts (GRBs) visible to the Fermi-LAT. For this purpose, we compare the number of photons with energies E > 100 MeV and E > 1 GeV which arrived in the first 1500 s after the burst from the same region, to the expected background. We require that the expected number of false detections does not exceed 0.05 for the entire search and find the high-energy emission in 19 bursts, four of which (GRB 081009, GRB 090720B, GRB 100911 and GRB 100728A) were previously unreported. The first three are detected at energies above 100 MeV, while the last one shows a statistically significant signal only above 1 GeV.

High Energy Neutrino Emissions from GRBs: Predictions and Issues

Gamma-ray bursts (GRBs) are believed to be cosmic ray accelerators and high energy neutrino emitters. Within the framework of the standard GRB fireball model, neutrinos of a wide range of energy are produced from different emission sites. The published predictions of GRB neutrino emission are reviewed, especially in view of the latest GRB observations with Swift. Issues regarding the uncertainties in those predictions are discussed.

Probing the ambient medium of GRB 090618 with XMM-Newton observations

Long Gamma-ray Bursts (GRBs) signal the death of massive stars. The afterglow emission can be used to probe the progenitor ambient through a detailed study of the absorption pattern imprinted by the circumburst material as well as the host galaxy interstellar medium on the continuum spectrum. This has been done at optical wavelengths with impressive results. Similar studies can in principle be carried out in the X-ray band, allowing us to shed light on the material metallicity, composition and distance of the absorber. We start exploiting this route through high resolution spectroscopy XMM-Newton observations of GRB 090618. We find a high metallicity absorbing medium (Z> 0.2 Zsun) with possible enhancements of S and Ne with respect to the other elements (improving the fit at a level of >3.4 sigma). Including the metallicity effects on the X-ray column density determination, the X-ray and optical evaluations of the absorption are in agreement for a Small Magellanic Cloud extinction curve.

The AGILE observations of the hard and bright GRB 100724B

The observation of Gamma Ray Bursts (GRBs) in the gamma-ray band has been advanced by the AGILE and Fermi satellites after the era of the Compton Gamma-Ray Observatory. AGILE and Fermi are showing that the GeV-bright GRBs share a set of common features, particularly the high fluence from the keV up to the GeV energy bands, the high value of the minimum Lorentz factor, the presence of an extended emission of gamma-rays, often delayed with respect to lower energies, and finally the possible presence of multiple spectral components. GRB 100724B, localised in a joint effort by Fermi and the InterPlanetary Newtork, is the brightest burst detected in gamma-rays so far by AGILE. Characteristic features of GRB 100724B are the simultaneous emissions at MeV and GeV, without delayed onset nor time lag as shown by the analysis of the cross correlation function, and the significant spectral evolution in hard X-rays over the event duration. In this paper we show the analysis of the AGILE data of GRB 100724B and we discuss its features in the context of the bursts observed so far in gamma-rays and the recently proposed models.

Determining interstellar dust properties with scattered X-ray halo

X-ray photons, emitted by X-ray sources, are absorbed and scattered by dust grains when they travel through interstellar medium. The scattered photons within small angles result in a diffuse X-ray “halo”. Therefore, the scattered X-ray photons carry information about various properties of the dust. We study the light curves of the dust-scattered X-ray halo of Cyg X-1. Significant time lags are found between the light curves of the point source and its halo. This time lag increases with the angular distance. This can be caused by a dust concentration at 1.75 kpc from the Earth along the line of sight (LOS), with a width of ΔL = 33 pc. The time lag of Cyg X-3 also reveals a dust concentration between the point source and the Earth, and the distance of Cyg X-3 is estimated to be about 7.2 kpc. The origin of these dust concentrations is still not clearly known. Along with recent results on the dust halos of Gamma Ray Bursts (GRBs), we conclude that the dust distribution is quite nonuniform. We also study dust properties with an expanding X-ray ring from the observation of a distant GRB. The distribution of the halo photons can be used to determine the dust-radius model. Our preliminary results show that dust grain geometries may vary significantly across the Galaxy. We also propose to use the scattered X-ray halo to study the composition of the dust grains. The applicability of this method needs to be tested with advanced instruments in the future.

GRB 071227: ANOTHER DISGUISED SHORT BURST

Observations of Gamma-ray Bursts (GRBs) put forward in the recent years have revealed, with increasing evidence, that the historical classification between long and short bursts has to be revised. Within the Fireshell scenario, both short and long bursts are canonical bursts, consisting of two different phases. First, a Proper-GRB (P-GRB), that is the emission of photons at the transparency of the fireshell. Then, the Extended Afterglow, multiwavelength emission due to the interacion of the baryonic remnants of the fireshell with the CircumBurst Medium (CBM). We discriminate between long and short bursts by the amount of energy stored in the first phase with respect to the second one. Within the Fireshell scenario, we have introduced a third intermediate class: the disguised GRBs. They appear like short bursts, because their morphology is characterized by a first, short, hard episode and a following deflated tail, but this last part — coincident with the peak of the afterglow — is energetically predominant. The origin of this peculiar kind of sources is inferred to a very low average density of the environment (of the order of 10-3). After GRB 970228 and GRB 060614, we find in GRB 071227 a third example of disguised burst.

Is GeV emission from Gamma-Ray Bursts of external shock origin?

Recent observations of Gamma-Ray Bursts (GRBs) by the Fermi Large Area Telescope (LAT) revealed a power-law decay feature of the high-energy emission (above 100 MeV), which led to the suggestion that it originates from an external shock. We analyse four GRBs (080916C, 090510, 090902B and 090926A) jointly detected by Fermi LAT and Gamma-ray Burst Monitor (GBM), which have high-quality light curves in both instrument energy bands. Using the MeV prompt emission (GBM) data, we can record the energy output from the central engine as a function of time. Assuming a constant radiative efficiency, we are able to track energy accumulation in the external shock using our internal/external shell model code. By solving for the early evolution of both an adiabatic and a radiative blastwave, we calculate the high-energy emission light curve in the LAT band and compare it with the observed one for each burst. The late time LAT light curves after T90 can be well fitted by the model. However, due to continuous energy injection into the blastwave during the prompt emission phase, the early external shock emission cannot account for the observed GeV flux level. The high-energy emission during the prompt phase (before T90) is most likely a superposition of a gradually enhancing external shock component and a dominant emission component that is of an internal origin.

Study of very early phase GRB afterglows with MITSuME

AbstractWe review the results of very early phase optical follow-up observations of recent gamma-ray bursts (GRBs) with the multi-color optical telescopes “MITSuME”. The MITSuME telescopes were designed to perform “real time” and “automatic” follow-up observations prompted by the GCN alerts via the Internet. The rapidly slewing equatorial mounts allow MITSuME to start photometric observations within 100 seconds after the trigger for several GRBs. In particular, we detected a brightening just after the trigger for two GRBs. These phenomena could be interpreted as the “on-set” of afterglow. In this paper we summarize these optical observations with a brief interpretation.

The soft X-ray landscape of gamma-ray bursts: thermal components

AbstractThe repository of GRB (gamma-ray burst) observations made by the Swift X-ray Telescope, now consisting of over 650 bursts, is a valuable and unique resource for the study of GRB X-ray emission. The observed soft X-ray spectrum typically arises from an underlying power law continuum, absorbed by gas along the line-of-sight. However, particularly at early times in a burst's evolution the continuum emission is not always understood and may comprise multiple components including thermal emission unexpected in the standard model. A thermal X-ray component has been discovered in two very unusual GRBs, perhaps suggesting an association only with this subset of events. However, evidence exists for thermal emission from more typical examples and here we present a new discovery of one such case and describe a systematic search for thermal components among all early GRB X-ray spectra.

Observing the prompt emission of GRBs

Gamma-ray bursts (GRBs) were first detected thanks to their prompt emission, which was the only information available for decades. In 2010, while the high-energy prompt emission remains the main tool for the detection and the first localization of GRB sources, our understanding of this crucial phase of GRBs has made great progress. We discuss some recent advances in this field, like the occasional detection of the prompt emission at all wavelengths, from optical to GeV; the existence of sub-luminous GRBs; the attempts to standardize GRBs; and the possible detection of polarization in two very bright GRBs. Despite these advances, tantalizing observational and theoretical challenges still exist, concerning the detection of the faintest GRBs, the panchromatic observation of GRBs from their very beginning, the origin of the prompt emission, or the understanding of the physics at work during this phase. Significant progress on this last topic is expected with SVOM thanks to the observation of dozens of GRBs from optical to MeV during the burst itself, and the measure of the redshift for the majority of them. SVOM will also change our view of the prompt GRB phase in another way. Within a few years, the sensitivity of sky surveys at optical and radio frequencies, and outside the electromagnetic domain in gravitational waves or neutrinos, will allow them to detect several new types of transient signals, and SVOM will be uniquely suited to identify which of these transients are associated with GRBs. This radically novel look at GRBs may elucidate the complex physics producing these bright flashes.

The Fermi view of gamma-ray bursts

Since its successful launch in June 2008, the {\it Fermi} Gamma-ray Space Telescope has made important breakthroughs in the understanding of the Gamma-Ray Burst (GRB) phenomemon. The combination of the GBM and the LAT instruments onboard the {\it Fermi} observatory has provided a wealth of information from its observations of GRBs over seven decades in energy. We present brief descriptions of the {\it Fermi} instruments and their capabilities for GRB science, and report highlights from {\it Fermi} observations of high-energy prompt and extended GRB emission. The main physical implications of these results are discussed, along with open questions regarding GRB modelling. We emphasize future synergies with ground-based \v{C}erenkov telescopes at the time of the SVOM mission.

Studying the SN‐GRB connection with X‐shooter: The GRB 100316D / SN 2010bh case

AbstractDuring the last ten years, observations of long‐duration gamma‐ray bursts brought to the conclusion that at least a fraction of them is associated with bright supernovae of type Ib/c. In this talk, after a short review on the previously observed GRB‐SN connection cases, we present the recent case of GRB 100316/SN 2010bh. In particular, during the observational campaign of SN 2010bh, a pivotal role was played by VLT/X‐shooter, sampling with unique high quality data the spectral energy distribution of the early evolution phases from the UV to the K band (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

WIDGET: System Performance and GRB Prompt Optical Observations

The WIDeField telescope for Gamma-ray burst Early Timing (WIDGET) is used for a fully automated, ultra-wide-field survey aimed at detecting the prompt optical emission associated with Gamma-ray Bursts (GRBs). WIDGET surveys the HETE-2 and Swift/BAT pointing directions, covering a total field of view of 62$^{\circ}$$\times$ 62$^{\circ}$ every 10 secounds using a unfiltered system. This monitoring survey allows the exploration of optical emission before the $\gamma $-ray trigger. The unfiltered magnitude is well converted to the SDSS $r'$ system at a 0.1 mag level. Since 2004, WIDGET has made a total of ten simultaneous and one pre-trigger GRB observations. The efficiency of synchronized observations with HETE-2 is four-times better than that of Swift. There has been no bright optical emission similar to that from GRB 080319B. A statistical analysis implies that GRB 080319B is a rare event. This paper summarizes the design and operation of the WIDGET system and the simultaneous GRB observations obtained with this instrument.

Instruments of RT-2 experiment onboard CORONAS-PHOTON and their test and evaluation I: ground calibration of RT-2/S and RT-2/G

Phoswich detectors (RT-2/S & RT-2/G) are major scientific payloads of the RT-2 Experiment onboard the CORONAS-PHOTON mission, which was launched into a polar Low Earth Orbit of around 550 km on 2009 January 30. These RT-2 instruments are designed and developed to observe solar flares in hard X-rays and to understand the energy transport processes associated with these flares. Apart from this, these instruments are capable of observing Gamma Ray Bursts (GRBs) and Cosmic diffuse X-ray background (CDXRB). Both detectors consist of identical NaI(Tl) and CsI(Na) scintillation crystals in a Phoswich combination, having the same diameter (116 mm) but different thicknesses. The normal working energy range is from 15 keV to 150 keV, but may be extendable up to ~1 MeV. In this paper, we present the RT-2/S and RT-2/G instruments and discuss their testing and calibration results. We used different radio-active sources to calibrate both detectors. The radio-active source 57Co (122 keV) is used for onboard calibration of both instruments. During its lifetime (∼3–5 years), RT-2 is expected to cover the peak of the 24th solar cycle.

The unusual X-ray emission of the short Swift GRB 090515: evidence for the formation of a magnetar?

The majority of short gamma-ray bursts (SGRBs) are thought to originate from the merger of compact binary systems collapsing directly to form a black hole. However, it has been proposed that both SGRBs and long gamma-ray bursts (LGRBs) may, on rare occasions, form an unstable millisecond pulsar (magnetar) prior to final collapse. GRB 090515, detected by the Swift satellite was extremely short, with a T_90 of 0.036 +/- 0.016 s, and had a very low fluence of 2 x 10^-8 erg cm^-2 and faint optical afterglow. Despite this, the 0.3 - 10 keV flux in the first 200 s was the highest observed for a SGRB by the Swift X-ray Telescope (XRT). The X-ray light curve showed an unusual plateau and steep decay, becoming undetectable after ~500 s. This behaviour is similar to that observed in some long bursts proposed to have magnetars contributing to their emission. In this paper, we present the Swift observations of GRB 090515 and compare it to other gamma-ray bursts (GRBs) in the Swift sample. Additionally, we present optical observations from Gemini, which detected an afterglow of magnitude 26.4 +/- 0.1 at T+ 1.7 hours after the burst. We discuss potential causes of the unusual 0.3 - 10 keV emission and suggest it might be energy injection from an unstable millisecond pulsar. Using the duration and flux of the plateau of GRB 090515, we place constraints on the millisecond pulsar spin period and magnetic field.

Challenging gamma-ray burst models through the broadband dataset of GRB 060908

<i>Context. <i/>Multiwavelength observations of gamma-ray burst prompt and afterglow emission are a key tool to separate the various possible emission processes and scenarios proposed to interpret the complex gamma-ray burst phenomenology.<i>Aims. <i/>We collected a large dataset on GRB 060908 in order to carry out a comprehensive analysis of the prompt emission as well as the early and late afterglow.<i>Methods. <i/>Data from <i>Swift<i/>-BAT, -XRT and -UVOT together with data from a number of different ground-based optical/near-infrared and millimeter telescopes allowed us to follow the afterglow evolution after about a minute from the high-energy event down to the host galaxy limit. We discuss the physical parameters required to model these emissions.<i>Results. <i/>The prompt emission of GRB 060908 was characterised by two main periods of activity, spaced by a few seconds of low intensity, with a tight correlation between activity and spectral hardness. Observations of the afterglow began less than one minute after the high-energy event, when it was already in a decaying phase, and it was characterised by a rather flat optical/near-infrared spectrum which can be interpreted as due to a hard energy-distribution of the emitting electrons. On the other hand, the X-ray spectrum of the afterglow could be fit by a rather soft electron distribution.<i>Conclusions. <i/>GRB 060908 is a good example of a gamma-ray burst with a rich multi-wavelength set of observations. The availability of this dataset, built thanks to the joint efforts of many different teams, allowed us to carry out stringent tests for various interpretative scenarios, showing that a satisfactorily modelling of this event is challenging. In the future, similar efforts will enable us to obtain optical/near-infrared coverage comparable in quality and quantity to the X-ray data for more events, therefore opening new avenues to progress gamma-ray burst research.