arXiv subjects

G. Tagliaferri

Publications and source records attributed to G. Tagliaferri.

At least 253 records · Page 14Linked to original sources

arXiv2006

Discovery of the optical afterglow of XRF 040812: VLT and Chandra observations

We present Chandra and VLT observations of the X-Ray Flash XRF 040812. The X-ray analysis reveals with high precision the position of a hard, fading source. A careful analysis of our I-band VLT images taken starting 17 hours after the burst led to the discovery of the optical afterglow superimposed to a bright (I=21.5) host galaxy. The optical afterglow is seen decaying with an index of 1.1. We do not detect any jet break and supernova rebrightening in the optical light curve. The bright apparent luminosity of the host galaxy allows us to get a rough estimate of the redshift, comparing with a set of GRB/XRF host galaxies with known luminosity and redshift. Such comparison suggests a redshift of XRF 040812 in the range 0.3<z<0.7. This is also consistent with the lack of emission features in our spectrum. The low inferred redshift is in agreement with the idea that XRFs are low-luminosity, closer events.

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arXiv2006

A metal rich molecular cloud surrounds GRB 050904 at redshift 6.3

GRB050904 is the gamma-ray burst with the highest measured redshift. We performed time resolved X-ray spectroscopy of the late GRB and early afterglow emission. We find robust evidence for a decrease with time of the soft X-ray absorbing column. We model the evolution of the column density due to the flash ionization of the GRB and early afterglow photons. This allows us to constrain the metallicity and geometry of the absorbing cloud. We conclude that the progenitor of GRB050904 was a massive star embedded in a dense metal enriched molecular cloud with Z~0.03 Z_solar. This is the first local measurement of metallicity in the close environment of a GRB and one of the highest redshift metallicity measurements. We also find that the dust associated with the cloud cannot be similar to that of our Galaxy but must be either sizably depleted or dominated by silicate grains. We discuss the implications of these results for GRB progenitors and high redshift star formation.

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arXiv2006

Swift observations of GRB050904: the most distant cosmic explosion ever observed

Swift discovered the high redshift (z=6.29) GRB050904 with the Burst Alert Telescope (BAT) and began observing with its narrow field instruments 161 s after the burst onset. This gamma-ray burst is the most distant cosmic explosion ever observed. Because of its high redshift, the X-ray Telescope (XRT) and BAT simultaneous observations provide 4 orders of magnitude of spectral coverage (0.2-150 keV; 1.4-1090 keV in the source rest frame) at a very early source-frame time (22 s). GRB050904 was a long, multi-peaked, bright GRB with strong variability during its entire evolution. The light curve observed by the XRT is characterized by the presence of a long flaring activity lasting up to 1-2 hours 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, 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. The dips of the late X-ray flares may be consistent with an underlying X-ray emission arising from the forward shock and with the properties of the optical afterglow reported by Tagliaferri et al. (2005b). We interpret the BAT and XRT data as a single continuous observation of the prompt emission from a very long GRB. The peculiarities observed in GRB050904 could be due to its origin within one of the first star-forming regions in the Universe; very low metallicities of the progenitor at these epochs may provide an explanation.

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arXiv2006

SWIFT observations of TeV BL Lac objects

Context: We present the results of a set of observations of nine TeV detected BL Lac objects performed by the XRT and UVOT detectors on board the Swift satellite between March and December 2005. Aims: We are mainly interested in measuring the spectral parameters, and particularly the intrinsic curvature in the X-ray band. Methods: We perform X-ray spectral analysis of observed BL Lac TeV objects using either a log-parabolic or a simple power-law model . Results: We found that many of the objects in our sample do show significant spectral curvature, whereas those having the peak of the spectral energies distribution at energies lower than ~0.1 keV show power law spectra. In these cases, however, the statistics are generally low thus preventing a good estimate of the curvature. Simultaneous UVOT observations are important to verify how X-ray spectra can be extrapolated at lower frequencies and to search for multiple emission components. Conclusions: The results of our analysis are useful for the study of possible signatures of statistical acceleration processes predicting intrinsically curved spectra and for modelling the SED of BL Lacertae objects up to TeV energies where a corresponding curvature is likely to be present.

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arXiv2006

Long-term monitoring of the X-ray afterglow of GRB 050408 with Swift/XRT

We present observations of the X-ray afterglow of GRB 050408, a gamma-ray burst discovered by HETE-II. Swift began observing the field 42 min after the burst, performing follow-up over a period of 38 d (thus spanning three decades in time).The X-ray light curve showed a steepening with time, similar to many other afterglows. However, the steepening was unusually smooth, over the duration of the XRT observation, with no clear break time. The early decay was too flat to be described in terms of standard models. We therefore explore alternative explanations, such as the presence of a structured afterglow or of long-lasting energy injection into the fireball from the central GRB engine. The lack of a sharp break puts constraints on these two models. In the former case, it may indicate that the angular energy profile of the jet was not a simple power law, while in the second model it implies that injection did not stop abruptly. The late decay may be due either to a standard afterglow (that is, with no energy injection), or to a jetted outflow still being refreshed. A significant amount of absorption was present in the X-ray spectrum, corresponding to a rest-frame Hydrogen column density NH = 1.2 (-0.3,+0.4)*10^22 cm^-2, indicative of a dense environment.

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arXiv2006

Near real-time selection of high redshift GRBs with Swift

A good fraction of GRBs detected by Swift are at a large redshift (up to z=6.3, so far). Their study allows us to investigate, among other things, the cosmic star formation in the early Universe (possibly up to the re-ionization era) and the chemical enrichment of the high-redshift gas. Here we present and discuss a method of selection which identifies high-redshift candidates based only upon promptly-available information provided by Swift. This method relies upon, Galactic extinction, GRB duration time and absence of an optical counterpart in the UVOT telescope onboard Swift. This tool may provide an extremely effective way to locate high-redshift astrophysical objects and to follow them in the optical/NIR band in near real time.

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arXiv2006

Swift XRT and UVOT deep observations of the high energy peaked BL Lac object PKS 0548-322 close to its brightest state

We present the results of a spectral analysis of 5 Swift XRT and UVOT observations of the BL Lac object PKS 0548-322 carried out over the period April-June 2005. The X-ray flux of this high energy peaked BL Lac (HBL) source was found to be approximately constant at a level of F(2-10 keV) ~ 4x10^-11 erg cm^-2 s^-1, a factor of 2 brighter than when observed by BeppoSAX in 1999 and close to the maximum intensity reported in the Einstein Slew Survey. The very good statistics obtained in the 0.3-10 keV Swift X-ray spectrum allowed us to detect highly significant deviations from a simple power law spectral distribution. A log-parabolic model describes well the X-ray data and gives a best fit curvature parameter of 0.18 and peak energy in the Spectral Energy Distribution of about 2 keV. The UV spectral data from Swift UVOT join well with a power law extrapolation of the soft X-ray data points suggesting that the same component is responsible for the observed emission in the two bands. The combination of synchrotron peak in the X-ray band and high intensity state confirms PKS 0548-322 as a prime target for TeV observations. X-ray monitoring and coordinated TeV campaigns are highly advisable.

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arXiv2006

GRB 050223: A dark GRB in a dusty starburst galaxy

Aims: We aim at detecting and determining the properties of the host galaxy of the dark GRB 050223. Methods: We use VLT optical/NIR images coupled to Swift X-ray positioning, and optical spectra of the host galaxy to measure its properties. Results: We find a single galaxy within the Swift error box of GRB 050223. It is located at z = 0.584 and its luminosity is L ~ 0.4 L*. Emission lines in the galaxy spectrum imply an intrinsic SFR > 7 Msun/yr, and a large extinction A_V > 2 mag within it. We also detect absorption lines, which reveal an underlying stellar population with an age between 40 Myr and 1.5 Gyr. Conclusions: The identification of a host galaxy with atypical properties using only the X-ray transient suggests that a bias may be present in the former sample of host galaxies. Dust obscuration together with intrinsic faintness are the most probable causes for the darkness of this burst.

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arXiv2006

Very Early Optical Afterglows of Gamma-Ray Bursts: Evidence for Relative Paucity of Detection

Very early observations with the Swift satellite of gamma-ray burst (GRB) afterglows reveal that the optical component is not detected in a large number of cases. This is in contrast to the bright optical flashes previously discovered in some GRBs (e.g. GRB 990123 and GRB 021211). Comparisons of the X-ray afterglow flux to the optical afterglow flux and prompt gamma-ray fluence is used to quantify the seemingly deficient optical, and in some cases X-ray, light at these early epochs. This comparison reveals that some of these bursts appear to have higher than normal gamma-ray efficiencies. We discuss possible mechanisms and their feasibility for explaining the apparent lack of early optical emission. The mechanisms considered include: foreground extinction, circumburst absorption, Ly-alpha blanketing and absorption due to high redshift, low density environments, rapid temporal decay, and intrinsic weakness of the reverse shock. Of these, foreground extinction, circumburst absorption, and high redshift provide the best explanations for most of the non-detections in our sample. There is tentative evidence of suppression of the strong reverse shock emission. This could be because of a Poynting-flux-dominated flow or a pure non-relativistic hydrodynamical reverse shock.

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arXiv2006

The variable X-ray light curve of GRB 050713A: the case of refreshed shocks

We present a detailed study of the spectral and temporal properties of the X-ray and optical emission of GRB050713a up to 0.5 day after the main GRB event. The X-ray light curve exhibits large amplitude variations with several rebrightenings superposed on the underlying three-segment broken powerlaw that is often seen in Swift GRBs. Our time-resolved spectral analysis supports the interpretation of a long-lived central engine, with rebrightenings consistent with energy injection in refreshed shocks as slower shells generated in the central engine prompt phase catch up with the afterglow shock at later times. Our sparsely-sampled light curve of the optical afterglow can be fitted with a single power law without large flares. The optical decay index appears flatter than the X-ray one, especially at later times.

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arXiv2006

The multiwavelength afterglow of GRB 050721: a puzzling rebrightening seen in the optical but not in the X-ray

Context. GRB050721 was detected by Swift and promptly followed-up, in the X-ray by Swift itself and, in the optical band, by the VLT operated, for the first time, in rapid response mode. A multiwavelength monitoring campaign was performed in order to study its afterglow behavior. Aims. We present the analysis of the early and late afterglow emission in both the X-ray and optical bands, as observed by Swift, a robotic telescope, and the VLT. We compare early observations with late afterglow observations obtained with Swift and the VLT in different bands in order to constrain the density of the medium in which the fireball is expanding. Methods. We have analyzed both the X-ray and the optical light curves and compared the spectral energy distribution of the afterglow at two different epochs. Results. We observed an intense rebrightening in the optical band at about one day after the burst which was not seen in the X-ray band. This is the first observation of a GRB afterglow in which a rebrightening is observed in the optical but not in the X-ray band. The lack of detection in X-ray of such a strong rebrightening at lower energies can be described with a variable external density profile. In such a scenario, the combined X-ray and optical observations allow us to derive the matter density at 10^15 cm from the burst. This is about a factor of 10 higher than in the inner region.

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arXiv2006

The early X-ray emission from GRBs

We present observations of the early X-ray emission for a sample of 40 gamma-ray bursts (GRBs) obtained using the Swift satellite for which the narrow-field instruments were pointed at the burst within 10 minutes of the trigger. Using data from the Burst Alert and X-Ray Telescopes, we show that the X-ray light curve can be well described by an exponential that relaxes into a power law, often with flares superimposed. The transition time between the exponential and the power law provides a physically defined timescale for the burst duration. In most bursts the power law breaks to a shallower decay within the first hour, and a late emission "hump" is observed which can last for many hours. In other GRBs the hump is weak or absent. The observed variety in the shape of the early X-ray light curve can be explained as a combination of three components: prompt emission from the central engine; afterglow; and the late hump. In this scenario, afterglow emission begins during or soon after the burst and the observed shape of the X-ray light curve depends on the relative strengths of the emission due to the central engine and that of the afterglow. There is a strong correlation such that those GRBs with stronger afterglow components have brighter early optical emission. The late emission hump can have a total fluence equivalent to that of the prompt phase. GRBs with the strongest late humps have weak or no X-ray flares.

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arXiv2006

Simultaneous X-ray and optical observations of S5 0716+714 after the outburst of March 2004

At the end of March 2004, the blazar S5 0716+714 underwent an optical outburst that prompted for quasi-simultaneous target-of-opportunity observations with the INTEGRAL and XMM-Newton satellites. In this paper, we report the results of the XMM-Newton and INTEGRAL OMC data analysis. The X-ray spectrum is well-represented by a concave broken power-law model, with the break at about 2 keV. In the framework of the synchrotron self-Compton model, the softer part of the spectrum, which is described by a power law of index $α\simeq 1.8$ ($f_ν\propto ν^{-α}$), is probably due to synchrotron emission, while the harder part of the spectrum, which has $α\simeq 1$, is due to inverse Compton emission. The blazar shows the long and short-term variability typical of low-frequency peaked BL Lac (LBL): the former is manifested by a gradual decrease in the optical flux from the peak as observed by ground telescopes at the end of March 2004, while the latter is characterized by soft X-ray and optical flares on time scales from a few thousand seconds to few hours. We can follow spectral variations on sub-hour time scales and study their correlation with the flux variability. We find evidence that the peak energy of the time-resolved spectra is increasing with flux. The modeling of the spectral energy distribution compared with archival observations suggests that the long-term variability (from outburst to quiescence or viceversa) could be due to a change in the injected power, while the short-term variability (flares) could be explained with changes in the slope of the distribution of the electrons.

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arXiv2006

Swift and infra-red observations of the blazar 3C~454.3 during the giant X-ray flare of May 2005

We present the results of a series of Swift and quasi simultaneous ground-based infra-red observations of the blazar 3C 454.3 carried out in April-May 2005 when the source was 10 to 30 times brighter than previously observed. We found 3C 454.3 to be very bright and variable at all frequencies covered by our instrumentation. The broad-band Spectral Energy Distribution (SED) shows the usual two-bump shape (in nu-nu f(nu) space) with the Infra-red, optical and UV data sampling the declining part of the synchrotron emission that, even during this extremely large outburst, had its maximum in the far-infrared. The X-ray spectral data from the XRT and BAT instruments are flat and due to inverse Compton emission. The remarkable SED observed implies that at the time of the Swift pointings 3C 454.3 was one of the brightest objects in the extragalactic sky with a gamma-ray emission similar or brighter than that of 3C 279 when observed in a high state by EGRET. Time variability in the optical-UV flux is very different from that in the X-ray data: while the first component varied by about a factor two within a single exposure, but remained approximately constant between different observations, the inverse Compton component did not vary on short time-scales but changed by more than a factor of 3 between observations separated by a few days. This different dynamical behaviour illustrates the need to collect simultaneous multi-frequency data over a wide range of time-scales to fully constrain physical parameters in blazars.

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arXiv2006

Panchromatic study of GRB 060124: from precursor to afterglow

We present observations of GRB 060124, the first event for which both the prompt and the afterglow emission could be observed simultaneously and in their entirety by the three Swift instruments. Indeed, Swift-BAT triggered on a precursor ~570s before the main burst peak, and this allowed Swift to repoint the narrow field instruments to the burst position ~350s before the main burst occurred. GRB 060124 also triggered Konus-Wind, which observed the prompt emission in a harder gamma-ray band (up to 2MeV). Thanks to these exceptional circumstances, the temporal and spectral properties of the prompt emission can be studied in the optical, X-ray and gamma-ray ranges. While the X-ray emission (0.2-10keV) clearly tracks the gamma-ray burst, the optical component follows a different pattern, likely indicating a different origin, possibly the onset of external shocks. The prompt GRB spectrum shows significant spectral evolution, with both the peak energy and the spectral index varying. As observed in several long GRBs, significant lags are measured between the hard- and low-energy components, showing that this behaviour extends over 3 decades in energy. The GRB peaks are also much broader at soft energies. This is related to the temporal evolution of the spectrum, and can be accounted for by assuming that the electron spectral index softened with time. The burst energy (E_iso~5x10^{53} erg) and average peak energy (E_p~300keV) make GRB 060124 consistent with the Amati relation. The X-ray afterglow is characterized by a decay which presents a break at t_b~10^5s.

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arXiv2006

The shock break-out of GRB 060218/SN 2006aj

Although the link between long Gamma Ray Bursts (GRBs) and supernovae (SNe) has been established, hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know however how a GRB jet emerges from the star surface nor how a GRB progenitor explodes. Here we report on observations of the close GRB060218 and its connection to SN2006aj. In addition to the classical non-thermal emission, GRB060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/UV band as time passes. We interpret these features as arising from the break out of a shock driven by a mildly relativistic shell into the dense wind surrounding the progenitor. Our observations allow us for the first time to catch a SN in the act of exploding, to directly observe the shock break-out and to provide strong evidence that the GRB progenitor was a Wolf-Rayet star.

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arXiv2006

The first Swift X-ray Flash: The faint afterglow of XRF 050215B

We present the discovery of XRF 050215B and its afterglow. The burst was detected by the Swift BAT during the check-out phase and observations with the X-ray telescope began approximately 30 minutes after the burst. These observations found a faint, slowly fading X-ray afterglow near the centre of the error box as reported by the BAT. Infrared data, obtained at UKIRT after 10 hours also revealed a very faint K-band afterglow. The afterglow appear unusual since it is very faint, especially in the infrared with K>20 only 9 hours post burst. The X-ray and infrared lightcurves exhibit a slow, monotonic decay with alpha=0.8 and no evidence for steepening associated with the jet break to 10 days post burst. We discuss possible explanations for the faintness and slow decay in the context of present models for the production of X-ray Flashes.

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arXiv2006

Hypernova Signatures in the Late Rebrightening of GRB 050525A

We report observations of GRB 050525A, for which a Gemini North spectrum shows its redshift to be z = 0.606. This is the third closest long GRB discovered by Swift. We observed its afterglow using the VLT, Gemini and TNG telescopes to search for an associated SN. We find that the early-time light curve is described by a broken power law with a break at t ~ 0.3 d after the burst. About 5 d after the burst, a flattening is apparent, followed by a further dimming. Both the magnitude and the shape of the light curve suggest that a supernova was emerging during the late decay of the afterglow. This supernova, dubbed SN 2005nc, had a rise time faster than SN 1998bw and a long-lasting maximum. A spectrum obtained about 20 d (rest-frame) after the GRB resembles the spectrum of SN 1998bw obtained close to maximum light.

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