arXiv subjects

G. Tagliaferri

Publications and source records attributed to G. Tagliaferri.

At least 199 records · Page 11Linked to original sources

arXiv2008

Evidence for Luminosity Evolution of Long Gamma-ray Bursts in Swift Data

We compute the luminosity function (LF) and the formation rate of long gamma ray bursts (GRBs) by fitting the observed differential peak flux distribution obtained by the BATSE experiment in two different scenarios: i) the GRB luminosity evolves with redshift and ii) GRBs form preferentially in low-metallicity environments. In both cases, model predictions are consistent with the Swift number counts and with the number of detections at z>2.5 and z>3.5. To discriminate between the two evolutionary scenarios, we compare the model results with the number of luminous bursts (i.e. with isotropic peak luminosity in excess of 10^53 erg s^-1) detected by Swift in its first three years of mission. Our sample conservatively contains only bursts with good redshift determination and measured peak energy. We find that pure luminosity evolution models can account for the number of sure identifications. In the case of a pure density evolution scenario, models with Z_th>0.3 Zsun are ruled out with high confidence. For lower metallicity thresholds, the model results are still statistically consistent with available lower limits. However, many factors can increase the discrepancy between model results and data, indicating that some luminosity evolution in the GRB LF may be needed also for such low values of Z_th. Finally, using these new constraints, we derive robust upper limits on the bright-end of the GRB LF, showing that this cannot be steeper than ~2.6.

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arXiv2008

A new measurement of the cosmic X-ray background

We present a new measurement of the cosmic X-ray background (CXRB) in the 1.5-7 keV energy band, performed by exploiting the Swift X-ray telescope (XRT) data archive. We also present a CXRB spectral model in a wider energy band (1.5-200 keV), obtained by combining these data with the recently published Swift-BAT measurement. From the XRT archive we collect a complete sample of 126 high Galactic latitude gamma-ray burst (GRB) follow-up observations. This provides a total exposure of 7.5 Ms and a sky-coverage of 7 square degrees which represents a serendipitous survey, well suited for a direct measurement of the CXRB in the 1.5-10 keV interval. Our work is based on a complete characterization of the instrumental background and an accurate measurement of the stray-light contamination and vignetting calibration. We find that the CXRB spectrum in the 1.5-7 keV energy band can be equally well fitted by a single power-law with photon index Gamma=1.47+/-0.07 or a single power-law with photon index Gamma=1.41+/-0.06 and an exponential roll-off at 41 keV. The measured flux in the 2-10 keV energy band is 2.18+/-0.13 E-11 erg/(cm2 s deg2) in the 2-10 keV band. Combining Swift-XRT with Swift-BAT (15-200 keV) we find that, in the 1.5-200 keV band, the CXRB spectrum can be well described by two smoothly-joined power laws with the energy break at 29.0+/-0.5 keV corresponding to a nu F_nu peak located at 22.4+/-0.4 keV. Taking advantage of both the Swift high energy instruments (XRT and BAT), we produce an analytical description of the CXRB spectrum over a wide (1.5-200 keV) energy band. This model is marginally consistent with the HEAO1 measurement (~10% higher at energies higher than 20 keV, while it is significantly (30%) higher at low energies (2-10 keV).

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arXiv2008

Probing the complex environments of GRB host galaxies and intervening systems: high resolution spectroscopy of GRB050922C

The aim of this paper is to investigate the environment of gamma ray bursts (GRBs) and the interstellar matter of their host galaxies. We use to this purpose high resolution spectroscopic observations of the afterglow of GRB050922C, obtained with UVES/VLT 3.5 hours after the GRB event. We found that, as for most high resolution spectra of GRBs, the spectrum of the afterglow of GRB050922C is complex. At least seven components contribute to the main absorption system at z=2.1992. The detection of lines of neutral elements like MgI and the detection of fine-structure levels of the ions FeII, SiII and CII allows us to separate components in the GRB ISM along the line of sight. Moreover, in addition to the main system, we have analyzed the five intervening systems between z = 2.077 and z = 1.5664 identified along the GRB line of sight. GRB afterglow spectra are very complex, but full of information. This can be used to disentangle the contribution of the different parts of the GRB host galaxy and to study their properties. Our metallicity estimates agree with the scenario of GRBs exploding in low metallicity galaxies

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arXiv2008

The prompt, high resolution spectroscopic view of the "naked-eye" GRB080319B

GRB080319B reached 5th optical magnitude during the burst prompt emission. Thanks to the VLT/UVES rapid response mode, we observed its afterglow just 8m:30s after the GRB onset when the magnitude was R ~ 12. This allowed us to obtain the best signal-to-noise, high resolution spectrum of a GRB afterglow ever (S/N per resolution element ~ 50). The spectrum is rich of absorption features belonging to the main system at z=0.937, divided in at least six components spanning a total velocity range of 100 km/s. The VLT/UVES observations caught the absorbing gas in a highly excited state, producing the strongest Fe II fine structure lines ever observed in a GRB. A few hours later the optical depth of these lines was reduced by a factor of 4-20, and the optical/UV flux by a factor of ~ 60. This proves that the excitation of the observed fine structure lines is due to "pumping" by the GRB UV photons. A comparison of the observed ratio between the number of photons absorbed by the excited state and those in the Fe II ground state suggests that the six absorbers are ~ 2-6 kpc from the GRB site, with component I ~ 3 times closer to the GRB site than components III to VI. Component I is characterized also by the lack of Mg I absorption, unlike all other components. This may be due both to a closer distance and a lower density, suggesting a structured ISM in this galaxy complex.

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arXiv2008

The short GRB070707 afterglow and its very faint host galaxy

We present the results from an ESO/VLT campaign aimed at studying the afterglow properties of the short/hard gamma ray burst GRB 070707. Observations were carried out at ten different epochs from ~0.5 to ~80 days after the event. The optical flux decayed steeply with a power-law decay index greater than 3, later levelling off at R~27.3 mag; this is likely the emission level of the host galaxy, the faintest yet detected for a short GRB. Spectroscopic observations did not reveal any line features/edges that could unambiguously pinpoint the GRB redshift, but set a limit z < 3.6. In the range of allowed redshifts, the host has a low luminosity, comparable to that of long-duration GRBs. The existence of such faint host galaxies suggests caution when associating short GRBs with bright, offset galaxies, where the true host might just be too dim for detection. The steepness of the decay of the optical afterglow of GRB 070707 challenges external shock models for the optical afterglow of short/hard GRBs. We argue that this behaviour might results from prolonged activity of the central engine or require alternative scenarios.

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arXiv2008

The Luminosity Function of Long Gamma-Ray Bursts and their rate at z>6

We compute the luminosity function (LF) and the formation rate of long gamma ray bursts (GRBs) in three different scenarios: i) GRBs follow the cosmic star formation and their LF is constant in time; ii) GRBs follow the cosmic star formation but the LF varies with redshift; iii) GRBs form preferentially in low-metallicity environments. We then test model predictions against the Swift 3-year data, showing that scenario i) is robustly ruled out. Moreover, we show that the number of bright GRBs detected by Swift suggests that GRBs should have experienced some sort of luminosity evolution with redshift, being more luminous in the past. Finally we propose to use the observations of the afterglow spectrum of GRBs at z>5.5 to constrain the reionization history and we applied our method to the case of GRB 050904.

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arXiv2008

The Brera Multi-scale Wavelet Chandra Survey. I. Serendipitous source catalogue

We present the BMW-Chandra source catalogue drawn from essentially all Chandra ACIS-I pointed observations with an exposure time in excess of 10ks public as of March 2003 (136 observations). Using the wavelet detection algorithm developed by Lazzati et al. (1999) and Campana et al. (1999), which can characterise both point-like and extended sources, we identified 21325 sources. Among them, 16758 are serendipitous, i.e. not associated with the targets of the pointings, and do not require a non-automated analysis. This makes our catalogue the largest compilation of Chandra sources to date. The 0.5--10 keV absorption corrected fluxes of these sources range from ~3E-16 to 9E-12 erg cm^-2 s^-1 with a median of 7E-15 erg cm^-2 s^-1. The catalogue consists of count rates and relative errors in three energy bands (total, 0.5-7keV; soft, 0.5-2keV; and hard, 2-7keV), and source positions relative to the highest signal-to-noise detection among the three bands. The wavelet algorithm also provides an estimate of the extension of the source. We include information drawn from the headers of the original files, as well, and extracted source counts in four additional energy bands, SB1 (0.5-1keV), SB2 (1-2keV), HB1 (2-4keV), and HB2 (4-7keV). We computed the sky coverage for the full catalogue and for a subset at high Galactic latitude (|b|> 20deg). The complete catalogue provides a sky coverage in the soft band (0.5-2keV, S/N =3) of ~8 deg^2 at a limiting flux of 1E-13 erg cm^-2 s^-1, and ~2 deg^2 at a limiting flux of ~1E-15 erg cm^-2 s^-1.

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arXiv2008

XIAO: a soft X-ray telescope for the SVOM Mission

The study of Gamma-ray bursts (GRBs) is a key field to expand our understanding of several astrophysical and cosmological phenomena. SVOM is a Chinese-French Mission which will permit to detect and rapidly locate GRBs, in particular those at high redshift, and to study their multiwavelength emission. The SVOM satellite, to be launched in 2013, will carry wide field instruments operating in the X/gamma-ray band and narrow field optical and soft X-ray telescopes. Here we describe a small soft X-ray telescope (XIAO) proposed as an Italian contribution to the SVOM mission. Thanks to a grazing incidence X-ray telescope with effective area of ~120 cm^2 and a short focal length, coupled to a very compact, low noise, fast read out CCD camera, XIAO can substantially contribute to the overall SVOM capabilities for both GRB and non-GRB science.

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arXiv2008

Outliers from the mainstream: how a massive star can produce a gamma-ray burst

It is now recognized that long-duration Gamma-Ray Bursts (GRBs) are linked to the collapse of massive stars, based on the association between (low-redshift) GRBs and (type Ic) core-collapse supernovae (SNe). The census of massive stars and GRBs reveals, however, that not all massive stars do produce a GRB. Only ~1% of core collapse SNe are able to produce a highly relativistic collimated outflow, and hence a GRB. The extra crucial parameter has long been suspected to be metallicity and/or rotation. We find observational evidence strongly supporting that both ingredients are necessary in order to make a GRB out of a core-collapsing star. A detailed study of the absorption pattern in the X-ray spectrum of GRB060218 reveals evidence of material highly enriched in low atomic number metals ejected before the SN/GRB explosion. We find that, within the current scenarios of stellar evolution, only a progenitor star characterized by a fast stellar rotation and sub-solar initial metallicity could produce such a metal enrichment in its close surrounding.

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arXiv2008

A Swift gaze into the 2006 March 29th burst forest of SGR 1900+14

We report on the intense burst ``forest'' recorded on 2006 March 29 which lasted for ~30s. More than 40 bursts were detected both by BAT and by XRT, seven of which are rare intermediate flares (IFs): several times 10^{42} ergs were released. The BAT data were used to carry out time-resolved spectroscopy in the 14-100keV range down to 8ms timescales. This unique dataset allowed us to test the magnetar model predictions such as the magnetically trapped fireball and the twisted magnetosphere over an unprecedented range of fluxes and with large statistics (in terms of both photons and IFs). We confirmed that a two blackbody component fits adequately the time-resolved and integrated spectra of IFs. However, Comptonization models give comparable good reduced chi^2. Moreover, we found: i) a change of behavior, around ~10^{41} erg/s, above which the softer blackbody shows a sort of saturation while the harder one still grows to a few times 10^{41} erg/s; ii) a rather sharp correlation between temperature and radii of the blackbodies (R^2 prop kT^{-3}), which holds for the most luminous parts of the flares (approximately for L_{tot} > 10^{41} erg/s). Within the magnetar model, the majority of these findings can be accounted for in terms of thermalised emission from the E-mode and O-mode photospheres. Interestingly, the maximum observed luminosity coming from a region of ~15km matches the magnetic Eddington luminosity at the same radius, for a surface dipole field of ~8 x 10^{14} G (virtually equal to the one deduced from the spindown of SGR 1900+14).

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arXiv2008

Testing the E_p,i - L_p,iso - T_0.45 correlation on a BeppoSAX and Swift sample of gamma-ray bursts

Using a sample of 14 BeppoSAX and 74 Swift GRBs with measured redshift we tested the correlation between the intrinsic peak energy of the time-integrated spectrum, E_p,i, the isotropic-equivalent peak luminosity, L_p,iso, and the duration of the most intense parts of the GRB computed as T_0.45 ("Firmani correlation"). For 41 out of 88 GRBs we could estimate all of the three required properties. Apart from 980425, which appears to be a definite outlier and notoriously peculiar in many respects, we used 40 GRBs to fit the correlation with the maximum likelihood method discussed by D'Agostini, suitable to account for the extrinsic scatter in addition to the intrinsic uncertainties affecting every single GRB. We confirm the correlation. However, unlike the results by Firmani et al., we found that the correlation does have a logarithmic scatter comparable with that of the E_p,i-E_iso ("Amati") correlation. We also find that the slope of the product L_p,iso T_0.45 is equal to ~0.5, which is consistent with the hypothesis that the E_p,i-L_p,iso-T_0.45 correlation is equivalent to the E_p,i-E_iso correlation (slope ~0.5). We conclude that, based on presently available data, there is no clear evidence that the E_p,i-L_p,iso-T_0.45 correlation is different (both in terms of slope and dispersion) from the E_p,i-E_iso correlation.

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arXiv2008

Infrared to X-ray observations of PKS 2155-304 in a low state

AIMS: Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy gamma-rays, through the investigation of the prototypical blazar PKS 2155-304, which shows complex behaviour. METHODS: We analyze simultaneous infrared-to-X-ray observations obtained with XMM-Newton and REM on November 7, 2006, when the source was in a low X-ray state. We perform a comparative analysis of these results with those obtained from previous observations in different brightness states. RESULTS: We found that the peak of the synchrotron emission moved from ultraviolet to optical wavelengths and the X-ray spectrum is best fit with a broken power law model with Gamma_2 ~ 2.4 harder than Gamma_1 ~ 2.6 and a break at about 3.5 keV. This suggests that the soft X-rays (E < 3.5 keV) are related to the high-energy tail of the synchrotron emission, while the hard X-rays (E > 3.5 keV) are from the energy region between the synchrotron and inverse-Compton humps. The different variability at energies below and above the break strengthens this hypothesis. Our results also stress the importance of monitoring this source at both low and high energies to better characterize its variability behaviour.

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arXiv2008

The complex light-curve of the afterglow of GRB071010A

We present and discuss the results of an extensive observational campaign devoted to GRB071010A, a long-duration gamma-ray burst detected by the Swift satellite. This event was followed for almost a month in the optical/near-infrared (NIR) with various telescopes starting from about 2min after the high-energy event. Swift-XRT observations started only later at about 0.4d. The light-curve evolution allows us to single out an initial rising phase with a maximum at about 7min, possibly the afterglow onset in the context of the standard fireball model, which is then followed by a smooth decay interrupted by a sharp rebrightening at about 0.6d. The rebrightening was visible in both the optical/NIR and X-rays and can be interpreted as an episode of discrete energy injection, although various alternatives are possible. A steepening of the afterglow light curve is recorded at about 1d. The entire evolution of the optical/NIR afterglow is consistent with being achromatic. This could be one of the few identified GRB afterglows with an achromatic break in the X-ray through the optical/NIR bands. Polarimetry was also obtained at about 1d, just after the rebrightening and almost coincident with the steepening. This provided a fairly tight upper limit of 0.9% for the polarized-flux fraction.

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arXiv2008

Observations of X-ray Flares from Gamma Ray Bursts

Swift-XRT observations of the X-ray emission from gamma ray bursts (GRBs) and during the GRB afterglow have led to many new results during the past two years. One of these exciting results is that approximately 1/3-1/2 of GRBs contain detectable X-ray flares. The mean fluence of the X-ray flares is ~10 times less than that of the initial prompt emission, but in some cases the flare is as energetic as the prompt emission itself. The flares display fast rises and decays, and they sometimes occur at very late times relative to the prompt emission (sometimes as late as 10^5 s after T_0) with very high peak fluxes relative to the underlying afterglow decay that has clearly begun prior to some flares. The temporal and spectral properties of the flares are found to favor models in which flares arise due to the same GRB internal engine processes that spawned the prompt GRB emission. Therefore, both long and short GRB internal engine models must be capable of producing high fluences in the X-ray band at very late times.

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arXiv2008

The Swift X-Ray Telescope: Status and Performance

We present science highlights and performance from the Swift X-ray Telescope (XRT), which was launched on November 20, 2004. The XRT covers the 0.2-10 keV band, and spends most of its time observing gamma-ray burst (GRB)afterglows, though it has also performed observations of many other objects. By mid-August 2007, the XRT had observed over 220 GRB afterglows, detecting about 96% of them. The XRT positions enable followup ground-based optical observations, with roughly 60% of the afterglows detected at optical or near IR wavelengths. Redshifts are measured for 33% of X-ray afterglows. Science highlights include the discovery of flaring behavior at quite late times, with implications for GRB central engines; localization of short GRBs, leading to observational support for compact merger progenitors for this class of bursts; a mysterious plateau phase to GRB afterglows; as well as many other interesting observations such as X-ray emission from comets, novae, galactic transients, and other objects.

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arXiv2008

Simultaneous multiwavelength observations of the blazar 1ES1959+650 at a low TeV flux

We present the results from a multiwavelength campaign on the TeV blazar 1ES 1959+650, performed in May, 2006. Data from the optical, UV, soft- and hard-X-ray and very high energy (VHE) gamma-ray (E > 100 GeV) bands were obtained with the SUZAKU and SWIFT satellites, with the MAGIC telescope and other ground based facilities. The source spectral energy distribution (SED), derived from SUZAKU and MAGIC observations at the end of May 2006, shows the usual double hump shape, with the synchrotron peak at a higher flux level than the Compton peak. With respect to historical values, during our campaign the source exhibited a relatively high state in X-rays and optical, while in the VHE band it was at one of the lowest level so far recorded. We also monitored the source for flux-spectral variability on a time window of 10 days in the optical-UV and X-ray bands and 7 days in the VHE band. The source varies more in the X-ray, than in the optical band, with the 2-10 keV X-ray flux varying by a factor of ~2. The synchrotron peak is located in the X-ray band and moves to higher energies as the source gets brighter, with the X-ray fluxes above it varying more rapidly than the X-ray fluxes at lower energies. The variability behaviour observed in the X-ray band cannot be produced by emitting regions varying independently, and suggests instead some sort of ``standing shock'' scenario. The overall SED is well represented by an homogeneous one-zone synchrotron inverse Compton emission model, from which we derive physical parameters that are typical of high energy peaked blazars.

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arXiv2008

Science with Simbol-X

Simbol-X is a French-Italian mission, with a participation of German laboratories, for X-ray astronomy in the wide 0.5-80 keV band. Taking advantage of emerging technology in mirror manufacturing and spacecraft formation flying, Simbol-X will push grazing incidence imaging up to ~80 keV, providing an improvement of roughly three orders of magnitude in sensitivity and angular resolution compared to all instruments that have operated so far above 10 keV. This will open a new window in X-ray astronomy, allowing breakthrough studies on black hole physics and census and particle acceleration mechanisms. We describe briefly the main scientific goals of the Simbol-X mission, giving a few examples aimed at highlighting key issues of the Simbol-X design.

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arXiv2007

Anomalous X-Ray emission in GRB060904B: a Nickel line?

The detection of an extra component in GRB060904B X-ray spectra in addition to the standard single power-law behaviour has recently been reported in the literature. This component can be fit with different models; in particular the addition of a spectral line provides the best representation.In this paper we investigate the physical properties that the surrounding medium must have in order to produce a spectral feature that can explain the detected emission. We analyse and discuss how and if the detected spectral excess fits in different theoretical models developed to explain the nature of line emission during the afterglow phase of Gamma-Ray Bursts (GRBs). Trasmission and reflection models have been considered. Given the high value (>>1) of the Thomson optical depth, the emission is likely to arise in a reflection scenario. Within reflection models, the external reflection geometry fails to predict the observed luminosity. On the contrary, the detected feature can be explained in a funnel scenario with typical opening angle theta of 5 degrees, Nickel mass of the order of 0.1 M_o and T=10^6 K. For theta=20 degrees, assuming the reprocessing material to be the SN shell, the detected emission implies a Nickel mass of 0.4 M_o at T=10^7 K and a metallicity 10 times the solar value. If the giant X-ray flare that dominates the early XRT light curve is identified with the ionizing source, the SN expansion began 3000 s before the GRB event.

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