G. Ghirlanda

The Collimation-corrected Gamma-Ray Burst Energies Correlate with the Peak Energy of Their νFν Spectrum

We consider all bursts with known redshift and νFν peak energy, E. 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, Eγ. The distribution of the values of Eγ is more spread out than in previous findings, covering about 2 orders of magnitude. We find a surprisingly tight correlation between Eγ and the source frame Epeak: E(1 + z) ∝ E. 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 could be confirmed by forthcoming data, it would be possible to use it for cosmological purposes.

General physical properties of bright Fermi blazars

We studied all blazars of known redshift detected by the Fermi satellite during its first 3-month survey. For the majority of them, pointed Swift observations ensure a good multiwavelength coverage, enabling us to reliably construct their spectral energy distributions (SEDs). We model the SEDs using a one-zone leptonic model and study the distributions of the derived interesting physical parameters as a function of the observed γ-ray luminosity. We confirm previous findings concerning the relation of the physical parameters with source luminosity which are at the origin of the blazar sequence. The SEDs allow to estimate the luminosity of the accretion disc for the majority of broad emitting line blazars, while for the lineless BL Lac objects in the sample upper limits can be derived. We find a positive correlation between the jet power and the luminosity of the accretion disc in broad-line blazars. In these objects, we argue that the jet must be proton dominated, and that the total jet power is of the same order of (or slightly larger than) the disc luminosity. We discuss two alternative scenarios to explain this result.

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 superposed onto the host galaxy in our first infrared JHK observations, carried out ~9 hr 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 MV = -19.75 ± 0.15.

GeV emission from gamma-ray bursts: a radiative fireball?

We study the emission observed at energies > 100 MeV of 11 gamma-ray bursts (GRBs) detected by the Fermi-Large Area Telescope (LAT) until 2009 October. The GeV emission has three main properties: (i) its duration is often longer than the duration of the softer emission detected by the Gamma Burst Monitor onboard Fermi (this confirms earlier results from the Energetic Gamma-Ray Experiment Telescope); (ii) its spectrum is consistent with F ν ∝ ν ―1 and does not show strong spectral evolution; and (iii) for the brightest bursts the flux detected by the LAT decays as a power law with a typical slope t ―1.5 . We argue that the observed >0.1 GeV flux can be interpreted as afterglow emission shortly following the start of the prompt phase emission as seen at smaller frequencies. The decay slope is what is expected if the fireball emission is produced in the radiative regime, i.e. all dissipated energy is radiated away. We also argue that the detectability in the GeV energy range depends on the bulk Lorentz factor Γ of the bursts, being strongly favoured in the case of large Γ. This implies that the fraction of bursts detected at high energies corresponds to the fraction of bursts having the largest r. The radiative interpretation can help to explain why the observed X-ray and optical afterglow energetics are much smaller than the energetics emitted during the prompt phase, despite the fact that the collision with the external medium should be more efficient than internal shocks in producing the radiation that we see.

The transition between BL Lac objects and flat spectrum radio quasars

We study the BL Lac objects detected in the one year all sky survey of the Fermi satellite, with a energy spectral slope α in the [0.1‐100 GeV] band greater than 1.2. In the α vs γ‐ray luminosity plane, these BL Lacs occupy the region populated by Flat Spectrum Radio Quasars (FSRQs). Studying the properties of their spectral energy d istributions (SED) and of their emitting lines, we find that several of these BL Lacs have a SED similar to FSRQs and that they do have broad lines of large equivalent width, and should be reclassified as FSRQs even adopting the current phenomenological definition (i.e. equ ivalent width EW of the emitting line greater than 5 ˚ A). In other cases, even if the EW width is small, the emitting lines can be as luminous as in quasars, and again their SED is similar to the SED of FSRQs. Sources classified as BL Lacs with a SED appearing as intermediate bet ween BL Lacs and FSRQs also have relatively weak broad emission lines and small EW, and can be considered as transition sources. These properties are confirmed also by model fitting , that allows to derive the relevant intrinsic jet parameters and the jet power. This study leads us to propose a physical distinction between the two classes of blazars, based on the luminosity of the broad line region measured in Eddington units. The dividing line is of the order of LBLR/LEdd � 5 × 10 4 , in good agreement with the idea that the presence of strong emitting lines is related to a transition in the accretion regime, becoming radiatively inefficient bel ow a disk luminosity of the order of one per cent of the Eddington one.

TeV BL Lac objects at the dawn of the Fermi era

We reconsider the emission properties of BL Lac objects emitting in the high-energy y-ray band, by exploiting the information in the MeV-GeV band obtained by the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope in its first three months of operation. To this aim we construct the spectral energy distribution of all the BL Lacs revealed by LAT and of the known TeV BL Lacs not detected by LAT, also including data from the Swift satellite, and model them with a simple one-zone leptonic model. The analysis shows that the BL Lacs detected by LAT (whether or not already detected in the TeV band) share similar physical parameters. While some of the TeV BL Lacs not revealed by LAT have spectral energy distributions and physical parameters very similar to the LAT BL Lacs, one group of objects displays peculiar properties (larger electron energies and smaller magnetic fields), suggesting different physical conditions in the emission region. Finally, we discuss possible criteria to select in an effective manner good new candidates for the Cherenkov telescopes among the LAT sources, presenting a list of predicted fluxes in the very high-energy band calculated including the effects of absorption by extragalactic background light.

Spectral-luminosity relation within individual Fermi gamma rays bursts

We study the spectra of all long gamma ray bursts (GRBs) of known redshift detected by the Fermi satellite untill the end of July 2009. Their fluxes and fluences are large enough to allow a time dependent study of their spectral characteristics in the 8 keV–1 MeV energy range. We find that the peak energy Epeak of their EL(E) spectrum correlates with the luminosity in a remarkably tight way within individual bursts. This time-resolved Epeak − Liso correlation is very similar for all the considered bursts and has a slope and normalisation similar to the analogous Epeak − Liso correlation defined by the time-integrated spectra of different bursts detected by several different satellites. For a few of the considered GRBs, we could also study the behaviour of the Epeak − Liso correlation during the rising and decaying phases of individual pulses within each burst, finding no differences. Our results indicate the presence of a similar physical mechanism, operating for the duration of different GRBs, tightly linking the burst luminosity with the peak energy of the spectrum emitted at different times. Such a physical mechanism is the same during the rise and decay phase of individual pulses composing a GRB. While calling for a robust physical interpretation, these results strongly indicate that the Epeak −Liso spectral energy correlation found considering the time-integrated spectra of different bursts is real and not the result of instrumental selection effects.

A unifying view of gamma-ray burst afterglows

We selected a sample of 33 Gamma‐Ray Bursts (GRBs) detected by Swift, with known redshift and optical extinction at the host frame. For these, we constructed the de‐absorbed and K‐corrected X‐ray and optical rest frame light curves. These are modelled as the sum of two components: emission from the forward shock due to the interaction of a fireball with the circum‐burst medium and an additional component, treated in a completely phenomenological way. The latter can be identified, among other possibili ties, as “late prompt” emission produced by a long lived central engine with mechanisms similar to those responsible for the production of the “standard” early prompt radiation. Apart from flares or re‐brightenings, that we do not model, we find a good agreement with the data, despite of their complexity and diversity. Although based in part on a phenomenological model with a relatively large number of free parameters, we believe that our findings are a first ste p towards the construction of a more physical scenario. Our approach allows us to interpret the behaviour of the optical and X‐ray afterglows in a coherent way, by a relatively simple scenario. Within this context it is possible to explain why sometimes no jet break is observed; why, even if a jet break is observed, it is often chromatic; why the steepening after th e jet break time is often shallower than predicted. Finally, the decay slope of the late prompt emission after the shallow phase is found to be remarkably similar to the time profile expected by the accretion rate of fall‐back material (i.e. / t 5/3 ), suggesting that this can be the reason why the central engine can be active for a long time.

The γ-ray brightest days of the blazar 3C 454.3

In the first week of 2009 December, the blazar 3C 454.3 became the brightest high-energy source in the sky. Its photon flux reached and surpassed the level of 10 −5 ph cm −2 s −1 above 100 MeV. The Swift satellite observed the source several times during the period of high γ -ray flux, and we can construct really simultaneous spectral energy distributions (SEDs) before, during and after the luminosity peak. Our main findings are (i) the optical, X-ray and γ -ray fluxes correlate; (ii) the γ -ray flux varies quadratically (or even more) with the optical flux; (iii) a simple one-zone synchrotron inverse Compton model can account for all the considered SED; (iv) in this framework the γ -ray versus optical flux correlation can be explained if the magnetic field is slightly fainter when the overall jet luminosity is stronger and (v) the power that the jet spent to produce the peak γ -ray luminosity is of the same order, or larger, than the accretion disc luminosity.

Short versus long gamma-ray bursts: spectra, energetics, and luminosities

We compare the spectral properties of 79 short and 79 long Gamma-Ray Bursts (GRBs) detected by BATSE and selected with the same limiting peak flux. Short GRBs have a low-energy spectral component harder and a peak energy slightly higher than long GRBs, but no difference is found when comparing short GRB spectra with those of the first 1-2 s emission of long GRBs. These results confirm earlier findings for brighter GRBs. The bolometric peak flux of short GRBs correlates with their peak energy in a similar way to long bursts. Short and long GRBs populate different regions of the bolometric fluence-peak energy plane, short bursts being less energetic by a factor similar to the ratio of their durations. If short and long GRBs had similar redshift distributions, they would have similar luminosities yet different energies, which correlate with the peak energy Epeak for the population of long GRBs. We also test whether short GRBs are consistent with the Epeak−Eiso and Epeak−Liso correlations for the available sample of short (6 events) and long (92 events) GRBs with measured redshifts and E obs : while short GRBs are inconsistent with the Epeak−Eiso correlation of long GRBs, they could follow the Epeak−Liso correlation of long bursts. All the above indications point to short GRBs being similar to the first phases of long bursts. This suggests that a similar central engine (except for its duration) operates in GRBs of different durations.