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L. Nicastro

Publications and source records attributed to L. Nicastro.

At least 55 records · Page 3Linked to original sources
arXiv2007

The afterglow onset for GRB060418 and GRB060607A

Gamma-ray burst are thought to be produced by highly relativistic outflows. Although upper and lower limits for the outflow initial Lorentz factor $Γ_0$ are available, observational efforts to derive a direct determination of $Γ_0$ have so far failed or provided ambiguous results. As a matter of fact, the shape of the early-time afterglow light curve is strongly sensitive on $Γ_0$ which determines the time of the afterglow peak, i.e. when the outflow and the shocked circumburst material share a comparable amount of energy. We now comment early-time observations of the near-infrared afterglows of GRB 060418 and GRB 060607A performed by the REM robotic telescope. For both events, the afterglow peak was singled out and allowed us to determine the initial fireball Lorentz, $Γ_0\sim 400$.

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arXiv2007

GRB 070311: a direct link between the prompt emission and the afterglow

We present prompt gamma-ray, early NIR/optical, late optical and X-ray observations of the peculiar GRB 070311 discovered by INTEGRAL, in order to gain clues on the mechanisms responsible for the prompt gamma-ray pulse as well as for the early and late multi-band afterglow of GRB 070311. We fitted with empirical functions the gamma-ray and optical light curves and scaled the result to the late time X-rays. The H-band light curve taken by REM shows two pulses peaking 80 and 140 s after the peak of the gamma-ray burst and possibly accompanied by a faint gamma-ray tail. Remarkably, the late optical and X-ray afterglow underwent a major rebrightening between 3x10^4 and 2x10^5 s after the burst with an X-ray fluence comparable with that of the prompt emission extrapolated in the same band. Notably, the time profile of the late rebrightening can be described as the combination of a time-rescaled version of the prompt gamma-ray pulse and an underlying power law. This result supports a common origin for both prompt and late X-ray/optical afterglow rebrightening of GRB 070311 within the external shock scenario. The main fireball would be responsible for the prompt emission, while a second shell would produce the rebrightening when impacting the leading blastwave in a refreshed shock (abridged).

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arXiv2007

REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination

Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs. Currently, only lower limits were securely set to the initial fireball Lorentz factor Gamma_0. We aim to provide a direct measure of Gamma_0. The early-time afterglow light curve carries information about Gamma_0, which determines the time of the afterglow peak. We have obtained early observations of the near-infrared afterglows of GRB 060418 and GRB 060607A with the REM robotic telescope. For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of Gamma_0 ~ 400, fully confirming the highly relativistic nature of GRB fireballs. The deceleration radius was inferred to be R_dec ~ 10^17 cm. This is much larger than the internal shocks radius (believed to power the prompt emission), thus providing further evidence for a different origin of the prompt and afterglow stages of the GRB.

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arXiv2007

Simultaneous Swift and REM monitoring of the blazar PKS0537-441 in 2005

The blazar PKS0537-441 has been observed by Swift between the end of 2004 and November 2005. The BAT monitored it recurrently for a total of 2.7 Ms, and the XRT and UVOT pointed it on seven occasions for a total of 67 ks, making it one of the AGNs best monitored by Swift. The automatic optical and infrared telescope REM has monitored simultaneously the source at all times. In January-February 2005 PKS0537-441 has been detected at its brightest in optical and X-rays: more than a factor of 2 brighter in X-rays and about a factor 60 brighter in the optical than observed in December 2004. The July 2005 observation recorded a fainter X-ray state. The simultaneous optical state, monitored by both Swift UVOT and REM, is high, and in the VRI bands it is comparable to what was recorded in early January 2005, before the outburst. In November 2005, the source subsided both in X-rays and optical to a quiescent state, having decreased by factors of ~4 and ~60 with respect to the January-February 2005 outburst, respectively. Our monitoring shows an overall well correlated optical and X-ray decay. On the shorter time scales (days or hours), there is no obvious correlation between X-ray and optical variations, but the former tend to be more pronounced, opposite to what is observed on monthly time scales. The widely different amplitude of the long term variability in optical and X-rays is very unusual and makes this observation a unique case study for blazar activity. The spectral energy distributions are interpreted in terms of the synchrotron and inverse Compton mechanisms within a jet where the plasma radiates via internal shocks and the dissipation depends on the distance of the emitting region from the central engine (abridged).

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arXiv2007

Concepts for astronomical data accessibility and analysis via relational database

Relational databases (DBs) are ideal tools to manage bulky and structured data archives. In particular for Astronomy they can be used to fulfill all the requirements of a complex project, i.e. the management of: documents, software (s/w) packages and logs, observation schedules, object catalogues, quick-look, simulated, raw and processed data, etc. All the information gathered in a relational DB is easily and simultaneously accessible either from an interactive tool or a batch program. The user does not need to deal with traditional files I/O or editing, but has only to build the appropriate (SQL) query which will return the desired information/data, eventually producing the aforementioned files or even plots, tables, etc. in a variety of formats. What is then important for a generic user is to have the tools to easily and quickly develop, in any desired programming language, the custom s/w which can import/export the information into/from the DB. An example could be a Web interface which presents the available data and allows the user to select/retrieve (or even process) the data subset of interest. In the last years we have been implementing a package called MCS (see dedicated paper in this proceedings) which allows users to interact with MySQL based DBs through any programming language. MCS has a multi-thread (socket) architecture which means that several clients can submit queries to a server which in turn manages the communication with the MySQL server and other MCS servers. Here we'll focus on a the real-world case of the robotic IR-optical telescope REM (placed at La Silla, Chile) which performs real time images acquisition, processing and archiving by using some of the MCS capabilities.

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arXiv2007

MCS, a new approach to data treatment in astronomical projects

Today's astronomical projects need computational systems capable to store and analyze large amounts of scientific data, to effectively share data with other research Institutes and to easily implement information services to present data for different purposes (scientific, maintenance, outreach, etc.). Due to the wide scenario of astronomical projects there isn't yet a standardized approach to implement the software needed to support all the requirements of a project. The new approach we propose here is the use of a unified model where all data are stored into the same data base becoming available in different forms, to different users with different privileges.

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arXiv2006

An optical supernova associated with the X-ray flash XRF 060218

Long-duration gamma-ray bursts (GRBs) are associated with type Ic supernovae that are more luminous than average and that eject material at very high velocities. Less-luminous supernovae were not hitherto known to be associated with GRBs, and therefore GRB-supernovae were thought to be rare events. Whether X-ray flashes - analogues of GRBs, but with lower luminosities and fewer gamma-rays - can also be associated with supernovae, and whether they are intrinsically 'weak' events or typical GRBs viewed off the axis of the burst, is unclear. Here we report the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. Supernova 2006aj is intrinsically less luminous than the GRB-supernovae, but more luminous than many supernovae not accompanied by a GRB. The ejecta velocities derived from our spectra are intermediate between these two groups, which is consistent with the weakness of both the GRB output and the supernova radio flux. Our data, combined with radio and X-ray observations, suggest that XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB observed off-axis. This extends the GRB-supernova connection to X-ray flashes and fainter supernovae, implying a common origin. Events such as XRF 060218 are probably more numerous than GRB-supernovae.

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arXiv2006

Physics of the GRB 030328 afterglow and its environment

We report on the photometric, spectroscopic and polarimetric, monitoring of the optical afterglow of Gamma-Ray Burst (GRB) 030328 detected by HETE-2. Photometry, collected at 7 different telescopes, shows that a smoothly broken powerlaw decay, with indices alpha_1 = 0.76 +/- 0.03, alpha_2 = 1.50 +/- 0.07 and a break at t_b = 0.48 +/- 0.03 days after the GRB, provides the best fit of the optical afterglow decline. This shape is interpreted as due to collimated emission, for which we determine a jet opening angle theta_{jet} of about 3.2 degrees. An achromatic bump starting around 0.2 d after the GRB is possibly marginally detected in the optical light curves. Optical spectroscopy shows the presence of two rest-frame ultraviolet metal absorption systems at z = 1.5216 +/- 0.0006 and at z = 1.295 +/- 0.001, the former likely associated with the GRB host galaxy. Analysis of the absorption lines at z = 1.5216 suggests that the host of this GRB may be a Damped Lyman-alpha Absorber. The optical V-band afterglow appears polarized, with P= (2.4 +/- 0.6) % and theta = (170 +/- 7) degrees, suggesting an asymmetric blastwave expansion. An X-ray-to-optical spectral flux distribution of the GRB 030328 afterglow was obtained at 0.78 days after the GRB and fitted using a broken powerlaw, with an optical spectral slope beta_{opt} = 0.47 +/- 0.15, and an X-ray slope beta_{X} = 1.0 +/- 0.2. The discussion of these results in the context of the "fireball model" shows that the preferred scenario for this afterglow is collimated structured jet with fixed opening angle in a homogeneous medium.

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arXiv2005

A rapid and dramatic outburst in Blazar 3C 454.3 during May 2005 - Optical and infrared observations with REM and AIT

The flat-spectrum radio quasar 3C 454.3 is well known to be a highly active and variable source with outbursts occurring across the whole electromagnetic spectrum over the last decades. In spring 2005, 3C 454.3 has been reported to exhibit a strong optical outburst which subsequently triggered multi-frequency observations of the source covering the radio up to gamma-ray bands. Here, we present first results of our near-IR/optical (V, R, I, H band) photometry performed between May 11 and August 5, 2005 with the Rapid Eye Mount (REM) at La Silla in Chile and the Automatic Imaging Telescope (AIT) of the Perugia University Observatory. 3C 454.3 was observed during an exceptional and historical high state with a subsequent decrease in brightness over our 86 days observing period. The continuum spectral behaviour during the flaring and declining phase suggests a synchrotron peak below the near-IR band as well as a geometrical origin of the variations e.g. due to changes in the direction of forward beaming.

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arXiv2005

The BeppoSAX catalog of GRB X-ray afterglow observations

We present the X-ray afterglow catalog of BeppoSAX from the launch of the satellite to the end of the mission. Thirty-three X-ray afterglows were securely identified based on their fading behavior out of 39 observations. We have extracted the continuum parameters (decay index, spectral index, flux, absorption) for all available afterglows. We point out a possible correlation between the X-ray afterglow luminosity and the energy emitted during the prompt $γ$-ray event. We do not detect a significant jet signature within the afterglows, implying a lower limit on the beaming angle, neither a standard energy release when X-ray fluxes are corrected for beaming. Our data support the hypothesis that the burst should be surrounded by an interstellar medium rather than a wind environment, and that this environment should be dense. This may be explained by a termination shock located near the burst progenitor. We finally point out that some dark bursts may be explained by an intrinsic faintness of the event, while others may be strongly absorbed.

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arXiv2005

The puzzling case of GRB 990123: prompt emission and broad-band afterglow modeling

We report on BeppoSAX simultaneous X- and gamma-ray observations of the bright GRB 990123. We present the broad-band spectrum of the prompt emission, including optical, X- and gamma-rays, confirming the suggestion that the emission mechanisms at low and high frequencies must have different physical origins. In the framework of the standard fireball model, we discuss the X-ray afterglow observed by the NFIs and its hard X-ray emission up to 60 keV several hours after the burst, detected for about 20 ks by the PDS. Considering the 2-10 keV and optical light curves, the 0.1-60 keV spectrum during the 20 ks in which the PDS signal was present and the 8.46 GHz upper limits, we find that the multi-wavelength observations cannot be readily accommodated by basic afterglow models. While the temporal and spectral behavior of the optical afterglow is possibly explained by a synchrotron cooling frequency between the optical and the X-ray energy band during the NFIs observations, in X-rays this assumption only accounts for the slope of the 2-10 keV light curve, but not for the flatness of the 0.1-60 keV spectrum. Including the contribution of Inverse Compton (IC) scattering, we solve the problem of the flat X-ray spectrum and justify the hard X-ray emission; we suggest that the lack of a significant detection of 15-60 keV emission in the following 75 ks and last 70 ks spectra, should be related to poorer statistics rather than to an important suppression of IC contribution. However, considering also the radio band data, we find the 8.46 GHz upper limits violated. On the other hand, leaving unchanged the emission mechanism requires modifying the hydrodynamics by invoking an ambient medium whose density rises rapidly with radius and by having the shock losing energy. Thus we are left with an open puzzle which requires further inspection.

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arXiv2005

The GRB of 1999 January 23: prompt emission and broad-band afterglow modeling

We report on BeppoSAX simultaneous X- and gamma-ray observations of the bright gamma-ray burst (GRB) 990123. We present the broad-band spectrum of the prompt emission, including optical, X- and gamma-rays, confirming the suggestion that the emission mechanisms at low and high frequencies must have different physical origins. We discuss the X-ray afterglow observed by the Narrow Field Instruments (NFIs) on board BeppoSAX and its hard X-ray emission up to 60 keV several hours after the burst, in the framework of the standard fireball model. The effects of including an important contribution of Inverse Compton scattering or modifying the hydrodynamics are studied.

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arXiv2005

GRB990123: multiwavelength afterglow study

We report on the BeppoSAX and multiwavelength data analysis of the afterglow of Gamma-Ray Burst (GRB) 990123. Mainly due to its exceptional brightness, this is the only source for which the Wide Field Cameras have allowed an early detection of the X-ray afterglow between about 20 and 60 min after the GRB trigger. Besides, again for the first time, high-energy emission from the afterglow was detected up to 60 keV. The backwards extrapolation of the afterglow decay smoothly reconnects with the late GRB emission, thus indicating that both are consistent with being produced by the same phenomenon. For the X-ray afterglow we found a power-law decay with index alpha_X = 1.46 +/- 0.04; the spectrum has a power-law shape with photon index Gamma about 2. An extensive set of multiwavelength observations on the afterglow, collected from the literature and made during the BeppoSAX pointing, allowed constructing a Spectral Flux Distribution. We performed an analysis of the GRB afterglow emission in the context of the "fireball" model.

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arXiv2005

The puzzling case of GRB 990123: multiwavelength afterglow study

We report on the BeppoSAX data analysis of the afterglow of Gamma-Ray Burst (GRB) 990123, one of the brightest GRBs detected by BeppoSAX. Mainly due to its exceptional brightness, this is the only source for which the Wide Field Cameras have allowed an early detection of the X-ray afterglow between about 20 and 60 min after the GRB trigger. Besides, again for the first time, high-energy emission from the afterglow was detected up to 60 keV. For the X-ray afterglow we found a power-law decay with index alpha = 1.46 +/- 0.04; the spectrum has a power-law shape with photon index Gamma about 1.9. The backward extrapolation of the afterglow decay smoothly reconnects with the late GRB emission, thus suggesting that both emissions are produced by the same phenomenon. An extensive set of multiwavelength observations for the GRB 990123 afterglow made during the BeppoSAX pointing was collected from the literature. The hard X-ray to radio range coverage allowed to construct a spectral flux distribution and to perform an analysis of the GRB afterglow in the context of the "fireball" model. We also report the results of temporal and spectral analysis of an X-ray source serendipitously observed about 22 arcmin north of the GRB afterglow, along with the optical spectroscopy of its possible counterpart to establish the nature of this source.

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arXiv2005

Late-epoch optical and near-infrared observations of the GRB000911 afterglow and its host galaxy

We present the results of an optical and near-infrared (NIR) monitoring campaign of the counterpart of Gamma-Ray Burst (GRB) 000911, located at redshift z=1.06, from 5 days to more than 13 months after explosion. Our extensive dataset is a factor of 2 larger and spans a time interval about 4 times longer than the ones considered previously for this GRB afterglow; this allows a more thorough analysis of its light curve and of the GRB host galaxy properties. The afterglow light curves show a single power-law temporal decline, modified at late times by light from a host galaxy with moderate intrinsic extinction, and possibly by an emerging supernova (SN). The afterglow evolution is interpreted within the classical "fireball" scenario as a weakly collimated adiabatic shock propagating in the interstellar medium. The presence of a SN light curve superimposed on the non-thermal afterglow emission is investigated: while in the optical bands no significant contribution to the total light is found from a SN, the NIR J-band data show an excess which is consistent with a SN as bright as the known hypernova SN1998bw. If the SN interpretation is true, this would be the farthest GRB-associated SN, as well as the farthest core-collapse SN, discovered to date. However, other possible explanations of this NIR excess are also investigated. Finally, we studied the photometric properties of the host, and found that it is likely to be a slightly reddened, subluminous, extreme starburst compact galaxy, with luminosity about 0.1 L*, an age of about 0.5 Gyr and a specific Star Formation Rate (SFR) of approximately 30 Msol yr-1 (L/L*)-1. This is the highest specific SFR value for a GRB host inferred from optical/NIR data.

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arXiv2004

Probing the environment in Gamma-ray bursts: the case of an X-ray precursor, afterglow late onset and wind vs constant density profile in GRB011121 and GRB011211

In this paper we present BeppoSax and XMM observations of two long Gamma-ray bursts, the X-ray rich event of December 11, 2001 (GRB011211) and the hard and very bright event of November 21, 2001 (GRB011121). In both events we find evidence of a late X-ray burst, taking place several minutes after the prompt emission. The temporal and spectral behaviour of this phenomenon suggest that it represents the onset of the afterglow. Broad band spectral modelling of the afterglow indicate that the fireball evolution in the December burst takes place in a ISM environment. On the contrary in the November burst the wind case is revealed by an X-ray decay slower than that observed in the optical ($δ_{\rm X}=1.29\pm0.04$ vs $δ_{\rm O}=1.66\pm0.06$). The wind profile should change into a constant density profile at large radii, in order to reconcile late-time radio data with a jet. Two other results are obtained for this burst. An X-ray burst is preceding by about 30 s the much harder GRB. Contrary to the prediction of simple models of precursor activity for collapsars, its spectrum is not consistent with a black body. Finally, a substantial absorption column ($\NH=(7\pm2)\times10^{22}$ cm$^{-2}$) is detected during the early part of the prompt emission. This is much greater than that of the wind, and it is thus likely associated with the region surrounding the burst.

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arXiv2004

Prompt and afterglow X-ray emission from the X-Ray Flash of 2002 April 27

We report on the X-ray observations of the X-ray flash (XRF) which occurred on 2002 April 27, three days before BeppoSAX was switched off. The event was detected with the BeppoSAX Wide Field Cameras but not with the Gamma ray Burst Monitor. A follow-up observation with the BeppoSAX Narrow Field Instruments was soon performed and a candidate afterglow source was discovered. We present the results obtained. We also include the results obtained from the observations of the XRF field with the Chandra X-ray satellite. The spectral analysis of the prompt emission shows that the peak energy of the EF(E) spectrum is lower than 5.5 keV, with negligible spectral evolution. The X-ray afterglow spectrum is consistent with a power law model with photon index of about 2, while the 2-10 keV flux fades as a power law with a decay index -1.33. Both these indices are typical of GRBs. A very marginal excess around 4.5-5 keV is found in the afterglow spectrum measured by BeppoSAX . As for many GRBs, the extrapolation of the 2-10 keV fading law back to the time of the prompt emission is consistent with the X-ray flux measured during the second part of the event. We estimate a possible range of values of the redshift and discuss our results in the light of current models of XRFs.

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