arXiv subjects

G. Ghirlanda

Publications and source records attributed to G. Ghirlanda.

At least 181 records · Page 10Linked to original sources

arXiv2006

The unexpected clustering of the optical afterglow luminosities

We studied the behaviour of the optical afterglow lightcurves of a sample of 24 Gamma--Ray Bursts (GRBs) with known redshift and published estimates of the optical extinction in the source frame, detected before the SWIFT satellite launch. We found an unexpected clustering of the optical luminosities at 12 hours in the source frame. The distribution of the optical luminosities is narrower than the distribution of X-ray luminosities at the same time. Few (3) bursts stand apart from the main optical distribution, being fainter by a factor of about 15. We also analysed the optical luminosities of the SWIFT burst with known redshift finding that the luminosity distribution is similar to the pre SWIFT GRBs one, even if they have a different mean redshift. These results can suggest the existence of a family of intrinsically optically under--luminous dark GRBs.

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arXiv2006

Puzzled by GRB 060218

We study the optical-UV/X-ray spectral energy distribution of GRB 060218 during the prompt phase and during what seems to be the afterglow phase. The results are puzzling, since if the opt-UV and the X-ray emission belong to a single backbody (BB), then its luminosity is too large, and this BB cannot be interpreted as the signature of the shock breakout of the supernova. There are also serious problems in associating the emission expected by the supernova shock breakout with either the opt-UV or the X-ray emission. In the former case we derive too small ejecta velocities; in the latter case, on the contrary, the required velocity is too large, corresponding to the large radius of a BB required to peak close to the UV band. We then present what we think is the most conservative alternative explanation, namely a synchrotron spectrum, self-absorbed in the opt-UV and extending up to the X-ray band, where we observe the emission of the most energetic electrons, which are responsible for the exponential roll-over of the spectrum. The obtained fit can explain the entire spectrum except the BB observed in the X-rays, which must be a separate component. The puzzling feature of this interpretation is that the same model is required to explain the spectrum also at later times, up to 1e5 s, because the opt-UV emission remains constant in shape and also (approximately) in normalisation. In this case the observed X-ray flux is produced by self-Compton emission. Thus the prompt emission phase should last for ~1e5 s or more. Finally, we show that the BB observed in X-rays, up to 7000 seconds, can be photospheric emission from the cocoon or stellar material, energized by the GRB jet at radii comparable to the stellar radius (i.e. 1e10-1e11 cm), not very far from where this material becomes transparent (e.g. 1e12 cm).

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arXiv2006

Gamma Ray Bursts as standard candles to constrain the cosmological parameters

Gamma Ray Bursts (GRBs) are among the most powerful sources in the Universe: they emit up to 10^54 erg in the hard X-ray band in few tens of seconds. The cosmological origin of GRBs has been confirmed by several spectroscopic measurements of their redshifts, distributed in the range 0.1-6.3. These two properties make GRBs very appealing to investigate the far Universe. The energetics implied by the observed fluences and redshifts span at least four orders of magnitudes. Therefore, at first sight, GRBs are all but standard candles. But there are correlations among some observed quantities which allow us to know the total energy or the peak luminosity emitted by a specific burst with a great accuracy. Through these correlations, GRBs become "known" candles to constrain the cosmological parameters. One of these correlation is between the rest frame peak spectral energy E_peak and the total energy emitted in gamma--rays E_gamma, properly corrected for the collimation factor. Another correlation, discovered very recently, relates the total GRB luminosity L_iso, its peak spectral energy E_peak and a characteristic timescale T_0.45, related to the variability of the prompt emission. It is based only on prompt emission properties, it is completely phenomenological, model independent and assumption--free. The constraints found through these correlations on the Omega_M and Omega_Lambda parameters are consistent with the concordance model. The present limited sample of bursts and the lack of low redshift events, necessary to calibrate these correlations, makes the cosmological constraints obtained with GRBs still large compared to those obtained with other cosmological probes (e.g. SNIa or CMB). However, the newly born field of GRB--cosmology is very promising for the future.

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arXiv2006

The Hubble diagram extended to z>>1: the gamma-ray properties of GRBs confirm the Lambda-CDM model

Tight constraints on cosmological parameters can be obtained with standard candles spanning a range of redshifts as large as possible. We propose to treat SN Ia and long Gamma-Ray Bursts (GRBs) as a single class of candles. Taking advantage of the recent release of the Supernova Legacy Survey and {\it the recent finding of a tight correlation among the energetics and other prompt gamma-ray emission properties of GRBs}, we are able to standardize the luminosities/energetics of both classes of objects. In this way we can jointly use GRB and SNIa as cosmological probes to constrain Omega_m and Omega_L and the Dark Energy equation of state parameters through the same Bayesian method that we have, so far, applied to GRBs alone. Despite the large disparity in number (115 SNIa versus 19 GRBs) we show that the constraints on Omega_m and Omega_L are greatly improved by the inclusion of GRBs. More importantly, the result of the combined sample is in excellent agreement with the Lambda-CDM concordance cosmological model and does not require an evolving equation of state for the Dark Energy.

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arXiv2006

Are GRB 980425 and GRB 031203 real outliers or twins of GRB 060218?

GRB 980425 and GRB 031203 are apparently two outliers with respect to the correlation between the isotropic equivalent energy E_iso emitted in the prompt radiation phase and the peak frequency E_peak of the spectrum in a vF(v) representation (the so-called Amati relation). We discuss if these two bursts are really different from the others or if their location in the E_iso-E_peak plane is the result of other effects, such as viewing them off-axis, or through a scattering screen, or a misinterpretation of their spectral properties. The latter case seems particularly interesting after GRB 060218, that, unlike GRB 031203 and GRB 980425, had a prompt emission detected both in hard and soft X-rays which lasted ~2800 seconds. This allowed to determine its E_peak and total emitted energy. Although it shares with GRB 031203 the total energetics, it is not an outlier with respect to the Amati correlation. We then investigate if a hard-to-soft spectral evolution in GRB 031203 and GRB 980425, consistent with all the observed properties, can give rise to a time integrated spectrum with an E_peak consistent with the Amati relation.

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arXiv2006

Discovery of a tight correlation among the prompt emission properties of long Gamma Ray Bursts

We report the discovery of a correlation among three prompt emission properties of GRBs. These are the isotropic peak luminosity L_iso, the peak energy of the time-integrated prompt emission spectrum E_pk, and the ``high signal" timescale T_0.45, previously used to characterize the variability behavior of bursts. In the rest frame of the source the found correlation reads L_iso\propto E_pk^1.62 T_0.45^-0.49. We find other strong correlations, but at the cost of increasing the number of variables, involving the variability and the isotropic energy of the prompt emission. With respect to the previous tight correlations found in GRBs the newly found correlation does not require any information from the afterglow phase of the bursts, nor any model-dependent assumption. In the popular scenario in which we are receiving beamed radiation originating in a fireball pointing at us, the found correlation preserves its form in the comoving frame. This helps to explain the small scatter of the correlation, and underlines the role of the local brightness (i.e. the brightness of the visible fraction of the fireball surface). This correlation has been found for 19 objects, and it is hard to establish if any selection bias affects it. Its connection with the prompt local brightness is promising, but a solid physical understanding is still to be found. Despite all that, we find that some properties of the correlation, which we discuss, support its true existence, and this has important implications for the GRB physics. Furthermore, it is possible to use such correlation as an accurate redshift estimator, and its tightness will allow us to use it as a tool to constrain the cosmological parameters (abridged)

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arXiv2006

GRB 990413: Insight into the thermal phase evolution

GRB 990413 shows a very hard spectrum (with a low energy spectral component F(E) \propto E^{2.49}) which is well represented by a black body model with characteristic temperature ~70 keV. It thus belongs to the subset of GRBs which might be revealing a thermal phase. We find that the temperature/luminosity evolution is consistent with that found in the other ``thermal'' GRBs. The time resolved spectral analysis indicates the presence of a second non--thermal component contributing (for about 1 s) up to 30 per cent of the total flux. Differently from the other thermal GRBs, GRB 990413 shows significantly high level of variability and the evolution of the thermal/non--thermal spectral components is strongly correlated with the flux variations. This GRB thus offers the unique opportunity to test the standard fireball photospheric and internal shock phases and their reciprocal influence. GRB 990413 was not selected on the basis of its spectrum and thus hints to the possibility that this early behavior might be more common than currently known.

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arXiv2006

The present and the future of cosmology with Gamma Ray Bursts

Gamma Ray Bursts are among the most powerful astrophysical sources and they release up to 1.e54 erg, if isotropic, in less than few hundred seconds. Their detection in the hard X/gamma ray band (at energies >10 keV) and out to very high redshift (z~6.3) makes them a powerful new cosmological tool (a) to study the reionization epoch, (b) to unveil the properties of the IGM, (c) to study the present universe geometry and (d) to investigate the nature and cosmic evolution of the dark energy. While GRBs will surely help to understand the first two issues in the future, the present link between GRBs and cosmology has been made concrete by the recent discovery of a tight correlation between their rest frame prompt and afterglow emission properties which allowed their use as standard candles.

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arXiv2006

Long Gamma-Ray Bursts as standard candles

As soon as it was realized that long GRBs lie at cosmological distances, attempts have been made to use them as cosmological probes. Besides their use as lighthouses, a task that presents mainly the technological challenge of a rapid deep high resolution follow-up, researchers attempted to find the Holy Grail: a way to create a standard candle from GRB observables. We discuss here the attempts and the discovery of the Ghirlanda correlation, to date the best method to standardize the GRB candle. Together with discussing the promises of this method, we will underline the open issues, the required calibrations and how to understand them and keep them under control. Even though GRB cosmology is a field in its infancy, ongoing work and studies will clarify soon if and how GRBs will be able to keep up to the promises.

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arXiv2006

On the correlation of Short Gamma--Ray Bursts and Clusters of galaxies

We cross correlate Gamma--Ray Bursts and X--Ray selected clusters of galaxies at z\leq0.45. We find a positive 2σsignal for the angular cross--correlation function w_{bc}(θ) on scales θ\leq 3 deg between short GRBs and clusters. Conversely, no correlation is found between clusters and the population of long GRBs. The comparison with the cluster autocorrelation function shows that short GRBs do not trace the cluster distribution as not all short GRBs are found in clusters. A higher signal in w_{bc}(θ) is found if we only consider the cluster population up to z=0.1. By comparing the short burst autocorrelation function with model predictions we then constrain short bursts to mostly originate within \sim 270 Mpc (i.e. z\leq 0.06). Our analysis also reveals that short GRBs are better correlated with ``normal'' galaxies. The double compact object merger model for short GRBs would associate them preferentially to early--type galaxies but the present statistics do not allow us to exclude that at least a fraction of these events might also take place in late--type galaxies, in agreement with recent evidences.

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arXiv2006

The clustering of the luminosities of optical afterglows of long Gamma Ray Bursts

We studied the optical afterglows of the 24 pre-Swift Gamma-Ray Bursts with known spectroscopic redshift and published estimates of the optical extinction in the source frame. We find an unexpected clustering of the optical afterglow luminosities measured 12 hours (source frame time) after the trigger. For 21 out of 24 bursts, the distribution of the optical luminosities is narrower than the distribution of the X-ray luminosities, and even narrower than the distribution of the ratio between the monochromatic optical luminosities and the total isotropic emitted prompt energy. Three bursts stand apart from the distribution of the other sources, being underluminous by a factor ~15. We compare this result with the somewhat analogous result concerning the luminosity of the X-ray afterglows studied by Gendre & Boer. For all our GRBs we construct the optical to X-ray spectral energy distribution. For all but a minority of them, the optical and the X-ray emissions are consistent with being produced by the same radiation process. We discuss our results in the framework of the "standard" external shock synchrotron model. Finally, we consider the behavior of the first GRBs of known redshifts detected by Swift. We find that these Swift GRBs entirely confirm our findings.

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arXiv2005

Gamma Ray Bursts as cosmological tools

The use of Gamma Ray Bursts as ``standard candles'' has been made possible by the recent discovery of a very tight correlation between their rest frame intrinsic properties. This correlation relates the GRB prompt emission peak spectral energy E_peak to the energy E_gamma corrected for the collimation angle theta_jet of these sources. The possibility to use GRBs to constrain the cosmological parameters and to study the nature of Dark Energy are very promising.

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arXiv2005

Cosmological constraints with GRBs: homogeneous medium vs wind density profile

We present the constraints on the cosmological parameters obtained with the $E_{\rm peak}$--$E_γ$ correlation found with the most recent sample of 19 GRBs with spectroscopically measured redshift and well determined prompt emission spectral and afterglow parameters. We compare our results obtained in the two possible uniform jet scenarios, i.e. assuming a homogeneous density profile (HM) or a wind density profile (WM) for the circumburst medium. Better constraints on $Ω_{M}$ and $Ω_Λ$ are obtained with the (tighter) $E_{\rm peak}$--$E_γ$ correlation derived in the wind density scenario. We explore the improvements to the constraints of the cosmological parameters that could be reached with a large sample, $\sim$ 150 GRBs, in the future. We study the possibility to calibrate the slope of these correlations. Our optimization analysis suggests that $\sim 12$ GRBs with redshift $z\in(0.9,1.1)$ can be used to calibrate the $E_{\rm peak}$--$E_γ$ with a precision better than 1%. The same precision is expected for the same number of bursts with $z\in(0.45,0.75)$. This result suggests that we do not necessarily need a large sample of low z GRBs for calibrating the slope of these correlations.

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arXiv2005

On the interpretation of the spectral--energy correlations in long Gamma--Ray Bursts

Recently, Liang & Zhang (2005) found a tight correlation involving only observable quantities, namely the isotropic emitted energy $E_{γ,iso}$, the energy of the peak of the prompt spectrum $E^\prime_{p}$, and the jet break time $t^\prime_{j}$. This phenomenological correlation can have a first explanation in the framework of jetted fireballs, whose semiaperture angle $θ_{j}$ is measured by the jet break time $t^\prime_{j}$. By correcting $E_{γ, iso}$ for the angle $θ_{j}$ one obtains the so called Ghirlanda correlation linking the collimation corrected energy $E_γ$ and $E^\prime_{p}$. There are two ways to derive $θ_{j}$ from $t^\prime_{j}$ in the standard scenario, corresponding to an homogeneous or to a wind-like circumburst medium. We show that the Ghirlanda correlation with a wind-like medium is as tight as (if not tighter) than the Ghirlanda correlation found in the case of an homogeneous medium. There are hence two Ghirlanda correlations, both entirely consistent with the phenomenological Liang & Zhang relation. We consider the difference between the observed correlations and the ones one would see in the comoving frame (i.e. moving with the same bulk Lorentz factor of the fireball). Since both $E_{p}$ and $E_γ$ transform in the same way, the wind-like Ghirlanda relation, which is linear, remains linear also in the comoving frame, no matter the distribution of bulk Lorentz factors. Instead, in the homogeneous density case, one is forced to assume the existence of a strict relation between the bulk Lorentz factor and the total energy, which in turn put constraints on the radiation mechanisms of the prompt emission. The wind-like Ghirlanda correlation, being linear, corresponds to different bursts having the same number of photons.

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arXiv2005

Gamma--Ray Bursts associated with Supernovae: A systematic analysis of BATSE GRB candidates

We examined the properties of a sample of BATSE Gamma--Ray Bursts (GRBs) comprising events which have indications of association with a supernova (SN), some on the basis of indications of re--brightening in the optical afterglow light curve, but in most cases based only on the `loose' temporal and directional coincidence inferred from the cross correlation of catalogs. Despite of the large uncertainties in the latter selection method, the temporal and spectral analysis reveal three interesting statistical results when the sample is compared with that of all the BATSE GRBs: the GRBs tentatively associated with SNe are found to predominantly (in $\sim$ 80% of the cases) have single-peaked light curves, a softer spectrum (i.e. low energy power law index $α\sim$ --1.5) and tend not to follow the Lag-Luminosity and Isotropic Energy--Peak Energy correlations. These three independent statistical properties point toward the existence of a significant number of under-luminous,GRB 980425-like events constituting -- at least from an observational point of view -- a tail or a separate class with respect to the whole of the BATSE GRB events. The unusually high percentage of SN Ibc among those identified by the catalog cross--correlation (factor $\sim 4$ higher than expected from SN catalog statistics) reinforces the non-randomness of (some of) the selected events.

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arXiv2005

A Gamma-Ray Burst Mission to Investigate the Properties of Dark Energy

[Abridged] Very recently, relations between the peak energy of Gamma-Ray Burst burst spectra, the isotropic-equivalent energy of the burst, and the radiated energy of the burst have been found. In a way that is exactly analogous to the way in which the relation between the peak luminosity and the rate of decline of the light curve of Type Ia supernovae can be used to make Type Ia supernovae excellent standard candles for cosmology, so too, the relations between Epk, Eiso, and Egamma point toward a methodology for using GRBs as excellent standard candles for cosmology. In addition, GRBs occur over the broad redshift range from z=0.1 to at least z=4.5, and both they and their afterglows are easily detectable out to z > 8. Thus GRBs show great promise as cosmological ``yardsticks'' to measure the rate of expansion of the universe over time, and therefore the properties of dark energy (i.e., Omega_M, Omega_Lambda, w_0, and w_a). We describe a concept for a possible MIDEX-class mission dedicated to using GRBs to constrain the properties of dark energy that would obtain these quantities for > 800 bursts in the redshift range 0.1 >~ z <~ 10 during a 2-year mission. This burst sample would enable both Omega_M and w_0 to be determined to +/- 0.07 and +/- 0.06 (68% CL), respectively, and w_a to be significantly constrained.

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arXiv2005

Probing the existence of the E_peak-E_iso correlation in long Gamma Ray Bursts

We probe the existence of the E_peak-E_iso correlation in long GRBs using a sample of 442 BATSE bursts with known E_peak and with redshift estimated through the lag-luminosity correlation. This sample confirms that the rest frame peak energy is correlated with the isotropic equivalent energy. The distribution of the scatter of the points around the best fitting line is similar to that obtained with the 27 bursts with spectroscopic redshifts. We interpret the scatter in the E_peak-E_iso plane as due to the opening angle distribution of GRB jets. By assuming that the collimation corrected energy correlates with E_peak we can derive the observed distribution of the jet opening angles, which turns out to be log-normal with a peak value of ~6.5 degrees.

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arXiv2005

Cosmology with Gamma Ray Bursts

Apparently, Gamma-Ray Bursts (GRBs) are all but standard candles. Their emission is collimated into a cone and the received flux depends on the cone aperture angle. Fortunately we can derive the aperture angle through an achromatic steepening of the lightcurve of the afterglow, and thus we can measure the "true" energetics of the prompt emission. Ghirlanda et al. (2004) found that this collimation-corrected energy correlates tightly with thefrequency at which most of the radiation of the prompt is emitted. Through this correlation we can infer the burst energy accurately enough for a cosmological use. Using the best known 15 GRBs we find very encouraging results that emphasize the cosmological GRB role. Probing the universe with high accuracy up to high redshifts, GRBs establish a new insight on the cosmic expanding acceleration history and accomplish the role of "missing link" between the Cosmic Microwave Background and type Ia supernovae, motivating the most optimistic hopes for what can be obtained from the bursts detected by SWIFT.

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