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    Gamma-ray Bursts



    What are Gamma-ray Bursts?

    Gamma-ray Bursts (GRBs) are short-lived and luminous flashes of Gamma-rays from space. They can last from a few milliseconds to several minutes and come from random directions of the sky. They originate from sources at cosmological distance, and if they were isotropic emitters their large observed gamma-ray fluences, ranging from 10-4 ergs cm-2 to 10-7 ergs cm-2, would make them the most luminous objects in the sky. However, there are evidences to believe they are narrowly beamed making them comparable to Supernovae in the total energy release.

    GRBs have originally been detected in the late '60s by the Vela satellites, a series of U.S. Airforce observatories equipped with X-ray, gamma-ray and neutron detectors to monitor nuclear testing by nations on earth.
    The first dedicated observatory was the Burst And Transient Source Experiment (BATSE), on board the Compton Gamma Ray Observatory (CGRO). BATSE detected roughly a GRB per day for nine years, starting from 1991, in the 30 keV - 2 MeV energy range. The large BATSE dataset shows two main GRB classes: long (duration > 2 s) and short (duration < 2 s) GRBs.
    Astronomers think that long and short duration GRBs are due to fundamentally different progenitors and physical processes. The long GRBs are believed to be associated with a special type of Supernovae, called Hypernovae, marking the deaths of a peculiar class of fast rotating massive stars (possibly Wolf-Rayet stars) through an anisotropic core collapse process, leading to the formation of a black hole (BH) or a neutron star (NS) and followed by jet-collimated plasma ejection at ultra-relativistic velocity. The short GRBs may be associated with the coalescence of a binary system of compact objects (NS or BH), but no general consensus have been reached yet.

    The GRBs are followed by an afterglow, a long lasting and fading emission detectable in the X-ray, optical and radio bands. This radiation is believed to originate from synchrotron emission of the ultra-relativistic electrons accelerated at the forward shock between the head of the jet and the external medium that surrounds the explosion site. This shock is expanding at ultra-relativistic speed and decelerates while sweeping a growing amount of external matter. The overall afterglow behaviour resembles an expanding Supernova remnant, but is quite faster in the observer reference time.

    The more accurate localization of the source obtained with afterglow detection enabled the identification of host galaxies and the determination of the redshift from absorption/emission lines in the optical spectra of the afterglow/host galaxy.
    Galaxies associated to long GRBs are usually blue dwarf star forming galaxies, and the study of the afterglow positions relative to their hosts show that long GRBs are likey coming from star forming regions, i.e. young massive stars. Moreover, for a few nearby long GRBs, firm associations with nearly simultaneous Type Ib/Ic Supernovae have been found, suggesting that long GRBs may be the high energy counterparts of these astrophysical events.
    Galaxies associated to short GRBs are either early-type galaxies with low star formation rates, or galaxies with current star formation. However, the GRB explosion site is usually far from the centre of the galaxy and out of star forming regions, and no associated Supernova has been detected yet down to very strict limits. This supports the suggestion that short GRBs are not related to the death of massive stars.

    The IASF-Palermo has been actively partecipating the observational study of GRBs and their X-ray afterglows since 1997 thanks to its key role in the Beppo SAX and Swift missions.

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    Gamma-ray Bursts in the Beppo SAX Era

    Until 1997, no GRB had been seen in wavelengths longer than Gamma-rays and no quiescent counterpart had been found because the Gamma-ray detectors had poor localization capability. This greatly hampered the study of GRBs and the identification of their progenitors. The discovery of X-ray afterglows was a result of Beppo SAX thanks to its repointing strategy (Costa, E., et al. 1997, IAU Circ. 6533).

    First afterglow Beppo SAX observations of GRB 970228. Left panel: First SAX X-ray afterglow. Beppo SAX observation of the burst February 28th. Right panel: the same region of the sky on March 3rd, showing the burst object has faded.

    The detection of optical counterparts came shortly after thanks to the collaboration with ground based optical telescopes (van Paradijs et al. 1997, Nature, 386, 686).
    Since then, the IASF Palermo team contributed to many publications on the Beppo SAX GRBs and X-ray/optical afterglows (see Publications).
    During its six years of active life Beppo SAX discovered more than 50 GRBs, but any of them was short. Moreover, it could never repoint to a GRB with the Narrow Fields Instruments faster than a few hours. Then, Beppo SAX observations could not answer any question about existence and properties of afterglows of short GRBs, and could not give satisfactory insights on the transition from prompt Gamma-ray emission to softer afterglow emission.

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    Gamma-ray Bursts in the Swift Era

    The launch of the Swift satellite, dedicated to multiwavelength obervation of GRBs, opened a new era for the study of these sources. In its initial 4 years of activity Swift detected about 400 GRBs, 25% of them with measured redshift. This is particularly impressive when compared to the pre-Swift sample of GRBs with known redshift, amounting to about 40 sources detected since 1997. Moreover, the average redshift of Swift-detected GRBs is larger than 2, as opposed to average redshift of the pre-Swift GRB sample, which is ~1. Swift is more sensitive than other missions to distant sources.
    The Swift very fast re-pointing capability allowed for the first time continuous observation of the whole event, from the the prompt Gamma-ray emission to the afterglow. It has also been crucial in the detection of the afterglows of short GRBs.
    Main scientific results achieved by the Swift team thanks to active participation of IASF Palermo staff are illustrated below.

    The Swift GRB canonical light curve

    Since the very beginning, Swift-XRT observations showed that some X-ray afterglows are characterised by a surprisingly rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours (Tagliaferri et al. 2005, Natur, 436, 985 ; Goad et al. 2006, A&A, 449, 89). In a few months from the Swift launch a precise definition of what is now called the Swift GRB canonical light curve was obtained (Nousek et al. 2006, ApJ, 642, 389; Mangano et al. 2006, NCimB, 121, 1297). It was found that the early X-ray afterglows light curves broadly consist of three distinct power-law segments: an initial very steep decay (~t with α roughly between 3 and 5), followed by a very shallow decay (with α between 0.5 and 1), and finally a somewhat steeper decay (with α btweeen 1 and 1.5). These power-law segments are separated by two corresponding break times, the former often earlier than 103 s, the latter usually between 103 s and a few 104 s. On top of this canonical behavior, many events have superimposed X-ray flares, sometimes occurring even after one day from the explosion (Burrows et al. 2005, Sci, 309, 1833; Romano et al. 2006, A&A, 450, 59; Falcone et al. 2006, ApJ, 641, 1010; Burrows et al. 2007, RSPTA, 365, 1213; Perri et al. 2007, A&A, 471, 83; Chincarini et al. 2007, ApJ, 671, 1903; Falcone et al. 2007, ApJ, 671, 1921 ).

    Canonical light curve
    0.2-10 keV luminosity rest frame light curves of a sub sample of GRBs detected by Swift with well established redshift. Each light curve consists of the BAT light curve of the GRB prompt extrapolated to the XRT energy range and the XRT light curve, both referred to the time measured in the source rest frame as described in Mangano et al. 2006, NCimB, 121, 1297. The three phases of the canonical Swift GRB light curve can be easily recognized.



    First flare 0.2-10 keV light curve of the first X-ray flare detected by Swift in the afterglow og GRB 050406. From Romano et al. 2006, A&A, 450, 59.


    The flares are believed to be most likely caused by internal shocks due to long-lasting sporadic activity of the central engine, up to several hours after the GRB, i.e. the same origin of the peaks observed in Gamma-rays during the prompt emission phase.
    The initial steep decay of XRT light curves is consistent with it being the tail of the prompt emission, from photons that are radiated at large angles relative to our line of sight. The first break in the XRT light curve takes place when the afterglow emission becomes dominant compared to the GRB tail. Actually, another observed property of Swift detected GRBs is that BAT/XRT light curves match very well (O'Brien et al. 2006, ApJ, 647, 1213), implying the existence of an overlap between Gamma-ray prompt emission and X-ray afterglow.
    The intermediate shallow flux decay is likely caused by continuous energy injection into the external shock, possibly due to late activity of the central engine. When this energy injection stops, a second break is then observed in the light curve.

    Short GRBs

    One of the highlights of the Swift mission was the first detection of the X-ray afterglow of a short GRB (Gehrels et al. 2005, Natur, 437, 851), quickly followed by the first detection of optical afterglow, first redshift measure and first host galaxy identification. It has been found that short GRBs afterglows are similar to long GRBs, and may show superimposed X-ray flares too (e.g. La Parola et al. 2006, A&A, 454, 753). But Swift XRT light curves of short GRBs (e.g. GRB 050724, Campana et al. 2006, A&A, 454, 113) also confirmed the presence of extended emission, i.e. softer and fainter emission lasting tens of seconds after the initial spikelike emission comprising the burst, already noted in the BASTE short GRB sample (Lazzati, Ramirez-Ruiz & Ghisellini 2001, A&A, 379, L39; Norris & Bonnell 2006, ApJ, 643, 266).

    The GRB location and its distance from the host galaxy center (offset) are an indirect evidence of the progenitor system, and the statistical study of GRB offsets provide a clean observational test to distinguish between different progenitor populations (e.g. Bloom et al. 2002, AJ, 123, 1111). We led a comprehensive analysis of the prompt and afterglow properties of a large sample of well-localized short GRBs, and examine their correlation with the burst environment (Troja et al. 2008, MNRAS, 385, L10). We showed that the subclass of short GRBs with extended-duration soft emission components lie very close to their hosts, while the class of short GRBs show a wider spatial dispersion. Although other explanations are still possible, we argued that the observed trend in the offset distribution and, in particular, its correlation with the prolonged activity observed in some short GRBs suggest for two types of progenitor systems. NS-BH mergers could naturally account for the first group of GRBs (low offset/extended emission), and NS-NS mergers could explain the majority of short GRBs and their wide offset distribution.

    short GRB progenitors Left panel: projected physical offsets as afunction of the burst duration in the Gamma-ray band. The vertical dashed line marks the canonical temporal division between long and short hard bursts. The horizontal dot-dashed line reports the median offset for a sample of long GRBs with known redshift (from Bloom et al. 2002). Right panel: Offsets histogram for the same sample of long GRBs. From Troja et al. 2008, MNRAS, 385, L10.



    The GRB central engine

    Central engine Combined BAT, XRT and UVOT lightcurves of GRB 070110. From Troja et al. 2007, ApJ, 665, 599 .

    In the very anomalous case of the X-ray afterglow of GRB 070110 (Troja et al. 2007, ApJ, 665, 599 ) the abrupt drop of the X-ray flux at ~6 hr after the burst, at the end of the shallow decay phase, rules out an external shock as the origin of the emitted radiation during the shallow decay, and implies long-lasting continuous activity of the central engine. The very different behavior of the optical afterglow, showing a shallow smooth decay still fairly bright at later times, suggests a different origin for the optical emission. We argued that the engine powering the plateau could be a spinning-down pulsar, which has a constant luminosity lasting for an extended period of time. The duration of the plateau depends on the unknown pulsar parameters, but given a reasonable radiation efficiency, the luminosity (~1048 erg/s) and the duration (~20 ks in the observer frame) of the plateau of GRB 070110 are consistent with the parameters of a new-born magnetized millisecond pulsar as the GRB central engine.

    SN Shock break-out

    Although the link between long Gamma-ray bursts (GRBs) and Supernovae was already been established before the Swift launch, until the explosion of GRB 060218 (Campana et al. 2006, Natur, 442, 1008) there had been no observations of the beginning of a Supernova explosion and its intimate link to a GRB. In particular, we did not know how the jet that defines a GRB emerges from the star's surface, nor how a GRB progenitor explodes. GRB 060218 was connected to supernova SN 2006aj. We found that in addition to the classical nonthermal emission, it showed a thermal component in its X-ray spectrum, which cooled and shifted into the optical/ultraviolet band as time passed. We interpreted these features as arising from the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the progenitor. Then, in the case of GRB 060218 we caught a Supernova in the act of exploding, directly observing the shock break-out, and found indications that the GRB progenitor was a Wolf-Rayet star (Campana et al. 2008, ApJ, 683, L9).

    Supernova Upper panel: 0.3-10 keV XRT light curve of GRB 060218 (open black circles) together with ontribution to the flux by the blackbody component (open black squares). Lower panel: the UVOT light curve. Filled circles of different colors represent different UVOT filters: red - V (centered at 544 nm); green - B (439 nm), blue - U (345 nm), light blue - UVW1 (251 nm); magenta - UVM1 (217 nm) and yellow - UVW2 (188 nm). From Campana et al. 2008, ApJ, 683, L9.

    Peculiar GRBs

    GRB 060614 is a remarkable nearby GRB (z=0.125) observed by Swift with puzzling properties, which challenge current progenitor models and their correspondence to the observed long/short classes. GRB 060614 lacks any associated bright Supernova down to very strict limits (Della Valle et al. 2006, Natur, 444, 1050) and presents vanishing spectral lags typical of short GRBs, strikingly at odds with the long (102 s) duration of the Gamma-ray event. The Gamma-ray light curve may resemble a short GRB with extended emission like GRB 050724. For these reasons it has been suggested that GRB 060614 belongs to a new class of GRBs (Gehrels et al. 2006, Natur, 444, 1044). However, we showed that GRB 060614 presents optical, UV and X-ray afterglows in remarkable agreement with standard jetted fireball models (Mangano et al. 2007, A&A, 470, 105). In particular, spectral analysis of BAT and XRT data in the overlap time interval and after, show that the peak energy of the burst dacays and crosses the XRT energy band within 500 s from the trigger. Moreover, the XRT early light curve shows an exponential decay instead of the well known steep power-law decay. This behaviour is difficult to be interpreted as due to the tail of the GRB in the framework of the high latitude emission model.

    TypeIII Upper panel: XRT light curve of GRB 060614 converted to flux in the 0.2-10 keV energy range shown together with the BAT light curve extrapolated to the same energy range.
    Lower panel: plot of the photon index of BAT and XRT spectra as a function of time. From Mangano et al. 2007, A&A, 470, 105.

    Very high redshift GRBs

    GRB 050904 (Cusumano et al. 2006, Natur, 440, 164) was a long, multi-peaked, bright GRB with strong variability during its entire evolution. In The light curve observed by the XRT is characterized by the presence of a long flaring activity lasting up to 1 - 2 hr after the burst onset in the burst rest frame, with no evidence of a smooth power-law decay following the prompt emission as seen in other GRBs. However, in Cusumano et al. 2007, A&A, 462, 73 we showed that the BAT tail extrapolated to the XRT band joins the XRT early light curve and the overall behavior resembles that of a very long GRB prompt. The spectral energy distribution softens with time, with the photon index decreasing from -1.2 during the BAT observation to -1.9 at the end of the XRT observation.

    Very high redshift
    The 0.2-10 keV light curve of GRB 050904 as observed by the BAT and XRT. From Cusumano et al. 2007, A&A, 462, 73.


    GRB 050904 is also very far, with z=6.3. This means that this explosion happened 12.8 billion years ago, corresponding to a time when the Universe was just 890 million years old, close to the reionization era. In Campana et al. 2007, ApJ, 654, L17 we showed that the progenitor of GRB 050904 was a massive star embedded in a dense metal-enriched molecular cloud, then, from the cosmological point of view, the Swift observations of GRB 050904 prove that metal-enrichment had already taken place at redshifts larger than 6. It is possible that the peculiarities observed in GRB 050904 could be due to its origin from a population III star.
    Presently, there are only 2 GRBs known to be farthest than GRB 050904: GRB 080913 at z=6.7, (Greiner et al 2009, ApJ, 693, 1610) and GRB 090423 at z=8.2 (Salvaterra et al. 2009 Nature, 461, 1258). Recently, evidence for a photometric redshift of z=9.4 have been found for GRB 090429B (Cucchiara et al. 2011 ApJ,736, 7).

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    IASF-Palermo Team & useful links

    Team members:

    * Giancarlo Cusumano
    * Valentina La Parola
    * Vanessa Mangano (@PSU since 2013)
    * Alessandro Maselli
    * Teresa Mineo
    * Patrizia Romano
    * Luciano Nicastro
    (@IASF Bologna since 2005)
    * Boris Sbarufatti
    (@INAF OAB since 2011)

    Useful links:

    * Swift GRBs Real-time Skymap
    * Swift GRB Table and lookup
    * GRBs localized with Gamma-ray or X-ray astronomical missions


    Contact person at IASF-Palermo:

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    Publications

    For a complete list of refereed papers on Gamma-ray Bursts published by the IASF Palermo team since 1997 click this LINK.

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    Last Modification: Wednesday, January 15 2014
    Edit by Vanessa Mangano


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