arXiv subjects

G. Ghirlanda

Publications and source records attributed to G. Ghirlanda.

197 records · Page 11Linked to original sources

arXiv2005

The updated E_peak - E_gamma correlation in GRBs

The recently discovered correlation between the rest frame GRB peak spectral energy $E_{\rm peak}$ and the collimation corrected energy $E_γ$ in long GRBs is potentially very important, yet awaits confirmation from an independent sample. It may help to shed light on the radiation mechanism of the prompt GRB phase and on the way -- and in which form -- the energy is released from the central engine. We here present some additional evidence for the correlation (two new bursts) and re-derive the best-fit parameters. The tightness of the correlation is confirmed (sigma=0.1 dex). We show that this correlation allows us, for the first time, to use GRBs as cosmological probes to constrain the expansion history of the universe.

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arXiv2005

GRB jets inside and outside the star: precursors and cosmological implications

After many years of speculation, recent observations have confirmed the association of gamma-ray bursts with core-collapse supernova explosions from massive stars. This association carries with it important consequences. The burst relativistic jet has to propagate through the cold dense stellar material before it reaches the transparency radius and the burst photons are produced. This propagation is likely to affect the initial properties of the jet, shaping it and changing its energy composition. The variability injected at the base of the jet is also likely to be erased by the jet-star interaction. Despite this, GRBs seem to have remarkably predictable properties once the radiative phase sets in, as emphasized by the recent discovery of several tight correlation between spectral, geometric and energetic properties of the jet. In this contribution we discuss the jet interaction with the star, emphasizing its time-dependent properties and the resulting energy distribution. We finally emphasize the surprising predictability of jet and radiation properties outside the star and underline its implication for standardizing the GRB candle.

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arXiv2005

The peak luminosity - peak energy correlation in GRBs

We derive the peak luminosity - peak energy (L_iso - E_peak) correlation using 22 long Gamma-Ray Bursts (GRBs) with firm redshift measurements. We find that its slope is similar to the correlation between the time integrated isotropic emitted energy E_iso and E_peak (Amati et al. 2002). For the 15 GRBs in our sample with estimated jet opening angle we compute the collimation corrected peak luminosity L_gamma, and find that it correlates with E_peak. This has, however, a scatter larger than the correlation between E_peak and E_gamma (the time integrated emitted energy, corrected for collimation; Ghirlanda et al. 2004), which we ascribe to the fact that the opening angle is estimated through the global energetics. We have then selected a large sample of 442 GRBs with pseudo--redshifts, derived through the lag-luminosity relation, to test the existence of the L_iso-E_peak correlation. With this sample we also explore the possibility of a correlation between time resolved quantities, namely L_iso,p and the peak energy at the peak of emission E_peak,p.

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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

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

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

SN 2003lw and GRB 031203: A Bright Supernova for a Faint Gamma-ray Burst

Optical and near-infrared observations of the gamma-ray burst GRB 031203, at z = 0.1055, are reported. A very faint afterglow is detected superimposed to the host galaxy in our first infrared JHK observations, carried out ~9 hours after the burst. Subsequently, a rebrightening is detected in all bands, peaking in the R band about 18 rest-frame days after the burst. The rebrightening closely resembles the light curve of a supernova like SN 1998bw, assuming that the GRB and the SN went off almost simultaneously, but with a somewhat slower evolution. Spectra taken close to the maximum of the rebrightening show extremely broad features as in SN 1998bw. The determination of the absolute magnitude of this SN (SN 2003lw) is difficult owing to the large and uncertain extinction, but likely this event was brighter than SN 1998bw by 0.5 mag in the VRI bands, reaching an absolute magnitude M_V = -19.75+-0.15.

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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

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

The X-ray afterglow of GRB030329

We report on XMM-Newton and Rossi-XTE observations of the bright (fluence $\sim$ 10$^{-4}$ erg cm$^{-2}$) and nearby (z=0.1685) Gamma-Ray Burst GRB030329 associated to SN2003dh. The first Rossi-XTE observation, 5 hours after the burst, shows a flux decreasing with time as a power law with index 0.9$\pm$0.3. Such a decay law is only marginally consistent with a further Rossi-XTE measurement (at t-t$_{GRB}\sim$30 hr). Late time observations of this bright afterglow at X-ray wavelengths have the advantage, compared to optical observations, of not being affected by contributions from the supernova and host galaxy. A first XMM-Newton observation, at t-t$_{GRB}\sim$37 days, shows a flux of 4$\times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ (0.2-10 keV). The spectrum is a power law with photon index $Γ$=1.9 and absorption $<2.5\times10^{20}$ cm$^{-2}$, consistent with the Galactic value. A further XMM-Newton pointing at t-t$_{GRB}\sim$61 days shows a flux fainter by a factor $\sim$2. The combined Rossi-XTE and XMM-Newton measurements require a break at t$\sim$0.5 days in the afterglow decay, with a power law index increasing from 0.9 to 1.9, similar to what is observed in the early part of the optical afterglow. The extrapolation of the XMM-Newton spectra to optical frequencies lies a factor of $\sim10$ below simultaneous measurements. This is likely due to the presence of SN2003dh.

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arXiv2003

Evidence for anisotropy in the distribution of short-lived gamma-ray bursts

Measurements of the two-point angular correlation function w(θ) for 407 short gamma-ray bursts collected in the Current BATSE Catalogue reveal a ~2 σdeviation from isotropy on angular scales θ~ 2-4 degrees. Such an anisotropy is not observed in the distribution of long gamma-ray bursts and hints to the presence of repeated bursts for up to ~13% of the sources under exam. However, the available data cannot exclude the signal as due to the presence of large-scale structure. Under this assumption, the amplitude of the observed w(θ) is compatible with those derived for different populations of galaxies up to redshifts ~0.5, result that suggests short gamma-ray bursts to be relatively local sources.

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arXiv2003

Hard GRB spectra: thermal vs non-thermal emission

We consider the evidence for very hard low energy spectra during the prompt phase of Gamma Ray Bursts (GRB). In particular we examine the spectral evolution of GRB 980306 together with the detailed analysis of some other bursts already presented in the literature (GRB 911118, GRB 910807, GRB 910927 and GRB 970111), and check for the significance of their hardness by applying different tests. The hard spectra of these bursts and their evolution constrain several non--thermal emission models, which appear inadequate to account for these cases. The extremely hard spectra at the beginning of their prompt emission are also compared with a black body spectral model: the resulting fits are remarkably good. These findings on the possible thermal character of the evolving spectrum and their implications on the GRB physical scenario can be considered in the frameworks of photospheric models for a fireball which is becoming optically thin, and of the Compton drag model. Both models appear to be qualitatively and quantitatively consistent with the found spectral characteristics.

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arXiv2002

Time Resolved Spectral Analysis of Bright Gamma Ray Bursts

We present the time integrated and time resolved spectral analysis of a sample of bright bursts selected with F_{peak}>20 phot cm^{-2} sec^{-1} from the BATSE archive. We fitted four different spectral models to the pulse time integrated and time resolved spectra. We compare the low energy slope of the fitted spectra with the prediction of the synchrotron theory [predicting photon spectra softer than E^{-2/3}], and test, through direct spectral fitting, the synchrotron shock model. We point out that differences in the parameters distribution can be ascribed to the different spectral shape of the models employed and that in most cases the spectrum can be described by a smoothly curved function. The synchrotron shock model does not give satisfactory fits to the time averaged and time resolved spectra. Finally, we derive that the synchrotron low energy limit is violated in a considerable number of spectra both during the rise and decay phase around the peak.

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arXiv2001

Spectral Analysis of Bright Gamma Ray Bursts

We present the time integrated and time resolved spectral analysis of a sample of bright bursts selected from the BATSE archive. We fitted four different spectral models to the time integrated and time resolved spectra of the flux pulses. We point out that the found (marginal) differences in the parameter distributions can be ascribed to the different spectral shape of the employed models and that a smoothly curved model best fits the observed spectra. We characterize the spectral shape of bright bursts and compare the low energy slope of the fitted spectra with the prediction N(E) = E^{-2/3} of the synchrotron theory, finding that this limit is violated in a considerable number of time resolved spectra around the peaks, both during the rise and decay phase.

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arXiv2000

Time Resolved GRB Spectroscopy

We present the main results of a study of time-resolved spectra of 43 intense GRBs detected by BATSE. We considered the 4-parameter Band model and the Optically Thin Synchrotron Shock model (OTSSM). We find that the large majority of time-resolved spectra of GRBs are in remarkable agreement with the OTSSM. However, about 15 % of initial GRB pulses show an apparent low-energy photon suppression. This phenomenon indicates that complex radiative conditions modifying optically thin emission may occur during the initial phases of some GRBs.

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