arXiv subjects

G. Ghisellini

Publications and source records attributed to G. Ghisellini.

At least 271 records · Page 15Linked to original sources

arXiv2005

A flash in the dark: UVES/VLT high resolution spectroscopy of GRB afterglows

We present the first high resolution (R=20000--45000, corresponding to 14 km/s at 4200A to 6.6 km/s at 9000A) observations of the optical afterglow of Gamma Ray Bursts. GRB020813 and GRB021004 were observed by UVES@VLT 22.19 hours and 13.52 hours after the trigger, respectively. These spectra show that the inter--stellar matter of the GRB host galaxies is complex, with many components contributing to each main absorption system, and spanning a total velocity range of up to about 3000 km/s. Several narrow components are resolved down to a width of a few tens of km/s. In the case of GRB021004 we detected both low and high ionization lines. Combined with photoionization results obtained with CLOUDY, the ionization parameters of the various systems are consistent with a remarkably narrow range with no clear trend with system velocity. This can be interpreted as due to density fluctuations on top of a regular R^-2 wind density profile.

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arXiv2005

A new method optimized to use Gamma Ray Bursts as cosmic rulers

We present a new method aimed to handle long Gamma-Ray Burst (GRBs) as cosmic rulers. The recent discovery of a tight correlation between the collimation corrected GRB energy and the peak of the gamma-ray spectrum has opened the possibility to use GRBs as a new category of standard candles. Unfortunately, because of the lack of low-z GRBs, up to now this correlation is obtained from high-z GRBs with the consequence that it depends on the cosmological parameters we pretend to constrain. Hopefully this circularity problem will be solved when, in a few years, the low-z GRB sample will be increased enough. In the meanwhile we present here a new Bayesian method that eases the aforesaid circularity problem, and allows to introduce new constrains on the cosmological (Om,OL) diagram as well as to explore the universe kinematics up to z~3. The method we propose offers the further advantage to make handy the problem of the (Om,OL) loitering line singularity which inevitably appears when standard candles with z>2 are used. The combination of GRB with SN Ia data makes the popular LambdaCDM cosmology more consistent with the Hubble diagram at a 68% confidence level. For a flat cosmology we find Om=0.28\pm0.03 for the combined GRB+SN Ia data set. Correspondingly, the transition redshift between cosmic deceleration and acceleration is z_T=0.73\pm0.09, slightly larger than the value found by considering SNe Ia alone. We briefly discuss our results also in terms of non--LambdaCDM dark energy models.

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arXiv2004

Correlation between the TeV and X-ray emission in high--energy peaked BL Lac objects

We discuss the correlation between the evolution of the TeV emission and X-ray radiation observed in high-energy peaked BL Lac objects. We describe such a correlation by a simple power law. In the first part of this work we present correlations obtained for the activity of Mrk~501 observed in 1997 April and for the activity of Mrk~421 observed in 2000 February. We also discuss results of similar correlation obtained for this source by other authors. They report an almost quadratic correlations observed between the evolution of the TeV and X-ray emission. In the second part of this paper we present a phenomenological model which describes the evolution of the synchrotron and inverse Compton emission of a simple spherical homogeneous source. Neglecting the radiative cooling of the particles we derive analytical expressions that describe the evolution. Then we use a numerical code to investigate the impact of radiative cooling on the evolution. The quadratic correlation observed during the decay phase of the flare observed in Mrk~421 on 2001 March 19 appears problematic for this basic modeling. The quadratic correlation can be explained only for specific choices of the spectral bands used for the calculation. Therefore, looking for more robust solutions, we investigate the evolution of the emission generated by a cylindrical source. However this model does not provide robust solutions for the problem of a quadratic correlation. factor. Finally we briefly discuss the possible influence of the light travel time effect on our results.

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arXiv2004

Structured jets in TeV BL Lac objects and radiogalaxies

TeV BL Lacertae objects require extreme relativistic bulk motion in the gamma-ray emission region, but at the VLBI scale their radio knots hardly move. The same sources show evidence, in radio, of a structure made of a fast spine plus a slow layer. We propose that this structure exists even on the spatial scale of regions responsible for the gamma-ray emission. One component sees the (beamed) radiation produced by the other, and this enhances the inverse Compton emission of both components. In addition, this allows the magnetic field to be nearly in equipartition with the emitting particles. The inverse Compton emission of the spine is anisotropic in its frame, possibly producing the deceleration of the spine by the Compton rocket effect. In this scenario, also the slow layer is a relatively strong high energy emitter, and thus radiogalaxies become potentially detectable by GLAST.

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arXiv2004

The polarisation of afterglow emission reveals GRB jet structure

We numerically compute light and polarisation curves of gamma-ray burst afterglows for various configurations of the jet luminosity structure and for different dynamical evolutions. We especially consider the standard homogeneous ``top hat'' jet and the ``universal structured jet'' with power-law wings. We also investigate a possible more physical variation of the ``top hat'' model: the ``Gaussian jet''. The polarisation curves for the last two jet types are shown here for the first time together with the computation of X-ray and radio polarised fluxes. We show that the lightcurves of the total flux from these configurations are very similar to each other, and therefore only very high quality data could allow us to pin down the underlying jet structure. We demonstrate instead that polarisation curves are a powerful means to solve the jet structure, since the predicted behaviour of polarisation and its position angle at times around the jet break are very different if not opposite. We conclude that the afterglow polarisation measurements provide clear footprints of any outflow energy distribution (unlike the lightcurves of the total flux) and the joint analysis of the total and polarised flux should reveal GRBs jet structure.

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arXiv2004

Gamma Ray Bursts: new rulers to measure the Universe

The best measure of the Universe should be done using a standard "ruler" at any redshift. Type Ia Supernovae (SN Ia) probe the universe up to z$\sim$1.5, while the Cosmic Microwave Background (CMB) primary anisotropies concern basically $z\sim$1000. Apparently, Gamma--Ray Bursts (GRBs) are all but standard candles. However, their emission is collimated and the collimation--corrected energy correlates tightly with the frequency at which most of the radiation of the prompt is emitted, as found by Ghirlanda et al. (2004). Through this correlation we can infer the burst energy accurately enough to probe the intermediate redshift ($z<10$) Universe. Using the best known 15 GRBs we find very encouraging results that emphasize the cosmological GRB role. A combined fit with SN Ia yields $Ω_{\rm M}=0.37\pm0.10$ and $Ω_Λ=0.87\pm 0.23$. Assuming in addition a flat Universe, the parameters are constrained to be $Ω_{\rm M}=0.29\pm0.04$ and $Ω_Λ=0.71\pm 0.05$. GRBs accomplish the role of "missing link" between SN Ia and CMB primary anisotropies. They can provide a new insight on the cosmic effects of dark energy, complementary to the one supplied by CMB secondary anisotropies through the Integrated Sachs Wolfe effect. The unexpected Standard Candle cosmological role of GRBs motivates the most optimistic hopes for what can be obtained when the GRB-dedicated satellite, Swift, will be launched.

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arXiv2004

The collimation--corrected GRB energies correlate with the peak energy of their $νF_ν$ spectrum

We consider all bursts with known redshift and $νF_ν$ peak energy, $E^{obs}_{peak}$. For a good fraction of them an estimate of the jet opening angle is available from the achromatic break of their afterglow light curve. This allows the derivation of the collimation--corrected energy of the bursts, $E_γ$. The distribution of the values of $E_γ$ is more spread with respect to previous findings, covering about two orders of magnitude. We find a surprisingly tight correlation between $E_γ$ and the source frame $E_{peak}$: $E^{obs}_{peak}(1+z) \propto E_γ^{0.7}$. This correlation can shed light on the still uncertain radiation processes for the prompt GRB emission. More importantly, if the small scatter of this newly found correlation will be confirmed by forthcoming data, it will be possible to use it for cosmological purposes.

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arXiv2004

Probing a GRB progenitor at a redshift of z=2: a comprehensive observing campaign of the afterglow of GRB 030226

We report results from a comprehensive follow-up observing campaign of the afterglow of GRB 030226, including VLT spectroscopy, VLT polarimetry, and Chandra X-ray observations. In addition, we present BOOTES-1 wide-field observations at the time of the occurrence of the burst. First observations at ESO started 0.2 days after the event when the GRB afterglow was at a magnitude of R~19 and continued until the afterglow had faded below the detection threshold (R>26). No underlying host galaxy was found. The optical light curve shows a break around 0.8 days after the burst, which is achromatic within the observational errors, supporting the view that it was due to a jetted explosion. Close to the break time the degree of linear polarization of the afterglow light was less than 1.1%, which favors a uniform jet model rather than a structured one. VLT spectra show two absorption line systems at redshifts z=1.962+/-0.001 and at z=1.986+/-0.001, placing the lower limit for the redshift of the GRB close to 2. We emphasize that the kinematics and the composition of the absorbing clouds responsible for these line systems is very similar to those observed in the afterglow of GRB 021004. This corroborates the picture in which at least some GRBs are physically related to the explosion of a Wolf-Rayet star.

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arXiv2004

The spectra of short Gamma-Ray Bursts

We present the results of the spectral analysis of a sample of short bright $γ$--ray bursts (GRB) detected by BATSE and compare them with the average and time resolved spectral properties of long bright bursts. While the spectral parameters of short GRBs confirm, as expected from previous works based on the hardness ratio, that they are harder than long events, we find that this difference is mainly due to a harder low energy spectral component present in short bursts, rather than to a (marginally) different peak energy. Intriguingly our analysis also reveals that the emission properties of short GRBs are similar to the first 2 s of long events. This might suggest that the central engine of long and short GRBs is the same, just working for a longer time for long GRBs. We find that short bursts do not obey the correlation between peak frequency and isotropic emitted energy for any assumed redshift, while they can obey the similar correlation between the peak frequency and isotropic emitted luminosity. This is consistent with (although not a proof of) the idea that short GRBs emit a $γ$--ray luminosity similar to long GRBs. If they indeed obey the peak frequency -- isotropic luminosity relation, we can estimate the redshift distribution of short bursts, which turns out to be consistent with that of long bursts just with a slightly smaller average redshift.

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arXiv2004

Observing Mkn 421 with XMM--Newton: the EPIC--PN point of view

We present three observations (four exposures) of Mkn 421 performed by XMM--Newton in the Autumn of 2002, concentrating on the EPIC--PN camera data. The X-ray spectra were soft and steepening toward high energies. The source was highly variable and the hardness ratio plots displayed a clear harder--when--stronger correlation. During two complete flares the source showed strong spectral evolution: a hardness ratio and a time resolved spectral analysis revealed both a clockwise and a counterclockwise rotating loop patterns, suggesting the presence of temporal lags between different energy bands variations. We confirmed this result and estimated the delay amounts with a cross--correlation analysis performed on the single flares, discussing also variability patterns that could reproduce the asymmetry seen in the cross-correlation function. We verified our findings reproducing the two flares with analytical models. We obtained consistent results: during one flare, Mkn 421 displayed soft lags, while in the other case it showed hard lags. In both cases, the entity of the delays increases with the energy difference between the compared light curves. Finally, we discussed the presence and the frequency dependence of the temporal lags as an effect of particle acceleration, cooling and escape timescales, showing that our data are consistent with this picture.

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arXiv2004

On the jet structure and magnetic field configuration of GRB 020813

The polarization curve of GRB 020813 is discussed and compared to different models for the structure, evolution and magnetisation properties of the jet and the interstellar medium onto which the fireball impacts. GRB 020813 is best suited for this kind of analysis for the smoothness of its afterglow light curve, ensuring the applicability of current models. The polarization dataset allows us to rule out the standard GRB jet, in which the energy and Lorentz factor have a well defined value inside the jet opening angle and the magnetic field is generated at the shock front. We explore alternative models finding that a structured jet or a jet with a toroidal component of the magnetic field can fit equally well the polarization curve. Stronger conclusions cannot be drawn due to the incomplete sampling of the polarization curve. A more dense sampling, especially at early times, is required to pin down the structure of the jet and the geometry of its magnetic field.

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arXiv2004

Formation Rate, Evolving Luminosity Function, Jet Structure, and Progenitors for Long Gamma-Ray Bursts

We constrain the isotropic luminosity function (LF) and formation rate of long gamma-ray bursts (GRBs) by fitting models 'jointly' to both the observed differential peak flux and redshift distributions. We find evidence supporting an evolving LF, where the luminosity scales as (1+z)^delta with an optimal delta of 1.0\pm 0.2. For a single power-law LF, the best slope is ~ -1.57 with an upper luminosity of 10^53.3 erg/s, while the best slopes for a double power-law LF are approximately -1.6 and -2.6 with a break luminosity of 10^52.7 erg/s. Our finding implies a jet model intermediate between the universal structured epsilon(theta) proportional to theta^-2 model and the quasi-universal Gaussian structured model. For the uniform jet model our result is compatible with an angle distribution between 2 and 15 grades. Our best constrained GRB formation rate histories increase from z=0 to z=2 by a factor of ~30 and then continue increasing slightly. We connect these histories to that of the cosmic star formation history, and compare with observational inferences up to z~6. GRBs could be tracing the cosmic rates of both the normal and obscured star formation regimes. We estimate a current GRB event rate in the Milky Way of ~5 10^-5 yr^-1, and compare it with the birthrate of massive close WR+BH binaries with orbital periods of hours. The agreement is rather good suggesting that these systems could be the progenitors of the long GRBs.

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arXiv2004

Late evolution of the X-ray afterglow of GRB 030329

The X-ray afterglow of the Gamma-Ray Burst GRB 030329, associated to SN2003dh at z=0.1685, has been observed with XMM-Newton 258 days after the burst explosion. A source with flux of (6.2 +/- 2.3) 10^{-16} erg cm^{-2} s^{-1} (0.5-2 keV) has been detected at the GRB position. This measurement, together with a re-analysis of the previous X-ray observations, indicates a flattening of the X-ray light curve ~40 days after the burst. This is in remarkable agreement with the scenario invoking the presence of two jets with different opening angles. The wider jet should be responsible for the observed flattening due to its transition into the non-relativistic Sedov-Taylor phase.

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arXiv2004

GRB 020813: polarization in the case of a smooth optical decay

We present the results of a VLT polarimetric monitoring campaign of the GRB 020813 optical afterglow carried out in three nights, from 0.88 to 4.20 days after the gamma-ray event. The mean values of the degree of linear polarization (P) and its position angle (Theta) resulting from our campaign are =1.18+/-0.10% and Theta = 148.7+/-2.3 deg after correcting for Galactic interstellar polarization. Our VLT data set is most naturally described by a constant degree of linear polarization and position angle, nonetheless a slow Theta evolution cannot be entirely ruled out by our data. The VLT monitoring campaign did not reveal either a significant Theta rotation respect to the Keck spectropolarimetric observations performed ~0.25 days after the GRB (Barth et al. 2003). However, is a factor of two lower than the polarization degree measured from Keck. Additionally, the VLT polarization data allowed us to construct an accurate V-band light curve. The V-band photometric data revealed a smooth light curve with a break located between the last Keck and our first VLT polarimetric measurement, 0.33 < t_(break, V) < 0.88 days after the GRB. The typical magnitude fluctuations of the VLT V-band lightcurve are 0.003 mag, 0.010 mag and 0.016 mag for our three observing nights, respectively. We speculate that the stability of Theta might be related to the smoothness of the light curve.

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arXiv2004

The X-ray afterglow of GRB030329 at early and late times

Thanks to its extraordinary brightness, the X-ray afterglow of GRB030329 could be studied by XMM-Newton up to two months after the prompt Gamma-ray emission. We present the results of two XMM-Newton observations performed on May 5 and 29, as well as an analysis of the Rossi-XTE data of the early part of the afterglow, discussing in particular the stability of the X-ray spectrum and presenting upper limits on the presence of X-ray emission lines.

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arXiv2003

Short-Bright GRBs: spectral properties

We study the spectra of short-bright GRBs detected by BATSE and compare them with the average and time resolved spectral properties of long-bright bursts. We confirm that short events are harder than long bursts, as already found from the comparison of their (fluence) hardness ratio, but we find that this difference is mainly due to a harder low energy spectral component present in short bursts, rather than to a (marginally) different peak energy. Moreover, we find that short GRBs are similar to the first 1 sec emission of long bursts. The comparison of the energetic of short and long bursts also suggests that short GRBs do not obey the peak energy--equivalent isotropic energy correlation recently proposed for long events, implying that short GRBs emit lower energy than long ones. Nonetheless, short bursts seem to emit a luminosity similar to long GRBs and under such hypothesis their redshift distribution appears consistent with that observed for long events. These findings might suggest the presence of a common mechanism at the beginning of short and long bursts which operates on different timescales in the these two classes.

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arXiv2003

SN 2002lt and GRB 021211: a SN/GRB Connection at z = 1

We present spectroscopic and photometric observations of the afterglow of GRB 021211 and the discovery of its associated supernova, SN 2002lt. The spectrum shows a broad feature (FWHM = 150 A), around 3770 A (in the rest-frame of the GRB), which we interpret as Ca H+K blueshifted by 14400 km/s. Potential sources of contamination due to the host galaxy and/or residuals of telluric absorption have been analyzed and ruled out. Overall, the spectrum shows a suggestive resemblance with the one of the prototypical type-Ic SN 1994I. This might indicate that GRBs are produced also by standard type-Ic supernovae.

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arXiv2003

BeppoSAX Observations of 1-Jy BL Lacertae Objects - II

We present new BeppoSAX LECS and MECS observations, covering the energy range 0.1 - 10 keV (observer's frame), of four BL Lacertae objects selected from the 1 Jy sample. All sources display a flat (alpha_x ~ 0.7) X-ray spectrum, which we interpret as inverse Compton emission. One object shows evidence for a low-energy steepening (Delta alpha_x ~ 0.9) which is likely due to the synchrotron component merging into the inverse Compton one around ~ 2 keV. A variable synchrotron tail would explain why the ROSAT spectra of our sources are typically steeper than the BeppoSAX ones (Delta alpha_x} ~ 0.7). The broad-band spectral energy distributions fully confirm this picture and model fits using a synchrotron inverse Compton model allow us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. By combining the results of this paper with those previously obtained on other sources we present a detailed study of the BeppoSAX properties of a well-defined sub-sample of 14 X-ray bright (f_x (0.1 - 10 keV) > 3 x 10^{-12} erg/cm^2/s) 1-Jy BL Lacs. We find a very tight proportionality between nearly simultaneous radio and X-ray powers for the 1-Jy sources in which the X-ray band is dominated by inverse Compton emission, which points to a strong link between X-ray and radio emission components in these objects.

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