arXiv subjects

G. Ghirlanda

Publications and source records attributed to G. Ghirlanda.

At least 127 records · Page 7Linked to original sources

arXiv2011

Gamma Ray Bursts in the comoving frame

We estimate the bulk Lorentz factor Gamma_0 of 31 GRBs using the measured peak time of their afterglow light curves. We consider two possible scenarios for the estimate of Gamma_0: the case of a homogeneous circumburst medium or a wind density profile. The values of Gamma_0 are broadly distributed between few tens and several hundreds with average values ~138 and ~66 for the homogeneous and wind density profile, respectively. We find that the isotropic energy and luminosity correlate in a similar way with Gamma_0, i.e. Eiso Gamma_0^2 and Liso Gamma_0^2, while the peak energy Epeak Gamma_0. These correlations are less scattered in the wind density profile than in the homogeneous case. We then study the energetics, luminosities and spectral properties of our bursts in their comoving frame. The distribution of Liso' is very narrow with a dispersion of less than a decade in the wind case, clustering around Liso'=5x10^48 erg/s. Peak photon energies cluster around Epeak'=6 keV. The newly found correlations involving Gamma_0 offer a general interpretation scheme for the spectral-energy correlations of GRBs. The Epeak-Eiso and Epeak-Liso correlations are due to the different Gamma_0 factors and the collimation-corrected correlation, Epeak-Egamma (obtained by correcting the isotropic quantities for the jet opening angle theta_j), can be explained if theta_j^2*Gamma_0=constant. Assuming the Epeak-Egamma correlation as valid, we find a typical value of theta_j*Gamma_0 ~ 6-20, in agreement with the predictions of magnetically accelerated jet models.

astro-ph.HE↗

arXiv2011

GRB091127/SN2009nz and the VLT/X-shooter spectroscopy of its host galaxy: probing the faint end of the mass-metallicity relation

We perform a detailed study of the gamma-ray burst GRB091127/SN2009nz host galaxy at z=0.490 using the VLT/X-shooter spectrograph in slit and integral-field unit (IFU). From the analysis of the optical and X-ray afterglow data obtained from ground-based telescopes and Swift-XRT we confirm the presence of a bump associated with SN2009nz and find evidence of a possible jet break in the afterglow lightcurve. The X-shooter afterglow spectra reveal several emission lines from the underlying host, from which we derive its integrated properties. These are in agreement with those of previously studied GRB-SN hosts and, more generally, with those of the long GRB host population. We use the Hubble Space Telescope and ground based images of the host to determine its stellar mass (M_star). Our results extend to lower M_star values the M-Z plot derived for the sample of long GRB hosts at 0.3<z<1.0 adding new information to probe the faint end of the M-Z relation and the shift of the LGRB host M-Z relation from that found from emission line galaxy surveys. Thanks to the IFU spectroscopy we can build the 2D velocity, velocity dispersion and star formation rate (SFR) maps. They show that the host galaxy has a perturbed rotation kinematics with evidence of a SFR enhancement consistent with the afterglow position.

astro-ph.CO↗

arXiv2011

Short and Long GRBs: same emission mechanism?

We study the spectral evolution on second and sub--second timescales in 11 long and 12 short Gamma Ray Bursts (GRBs) with peak flux >8.5e-6 erg/cm2 s (8 keV-35 MeV) detected by the Fermi satellite. The peak flux correlates with the time-averaged peak energy in both classes of bursts. The peak energy evolution, as a function of time, tracks the evolution of the flux on short timescales in both short and long GRBs. We do not find evidence of an hard-to-soft spectral evolution. While short GRBs have observed peak energies larger than few MeV during most of their evolution, long GRBs can start with a softer peak energy (of few hundreds keV) and become as hard as short ones (i.e. with Ep,obs larger than few MeV) at the peak of their light curve. Six GRBs in our sample have a measured redshift. In these few cases we find that their correlations between the rest frame Ep and the luminosity Liso are less scattered than their correlations in the observer frame between the peak energy Ep,obs and the flux P. We find that the rest frame Ep of long bursts can be as high or even larger than that of short GRBs and that short and long GRBs follow the same Ep-Liso correlation, despite the fact that they likely have different progenitors.

astro-ph.HE↗

arXiv2011

ORIGIN: Metal Creation and Evolution from the Cosmic Dawn

ORIGIN is a proposal for the M3 mission call of ESA aimed at the study of metal creation from the epoch of cosmic dawn. Using high-spectral resolution in the soft X-ray band, ORIGIN will be able to identify the physical conditions of all abundant elements between C and Ni to red-shifts of z=10, and beyond. The mission will answer questions such as: When were the first metals created? How does the cosmic metal content evolve? Where do most of the metals reside in the Universe? What is the role of metals in structure formation and evolution? To reach out to the early Universe ORIGIN will use Gamma-Ray Bursts (GRBs) to study their local environments in their host galaxies. This requires the capability to slew the satellite in less than a minute to the GRB location. By studying the chemical composition and properties of clusters of galaxies we can extend the range of exploration to lower redshifts (z ~ 0.2). For this task we need a high-resolution spectral imaging instrument with a large field of view. Using the same instrument, we can also study the so far only partially detected baryons in the Warm-Hot Intergalactic Medium (WHIM). The less dense part of the WHIM will be studied using absorption lines at low redshift in the spectra for GRBs.

astro-ph.IM↗

arXiv2011

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 alpha_gamma in the [0.1-100 GeV] band greater than 1.2. In the alpha_gamma vs gamma-ray luminosity plane, these BL Lacs occupy the region populated by Flat Spectrum Radio Quasars (FSRQs). Studying the properties of their spectral energy distributions (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. equivalent 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 between 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 L_BLR/L_Edd ~ 5e-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 below a disk luminosity of the order of one per cent of the Eddington one.

astro-ph.CO↗

arXiv2011

Spectral properties of 438 GRBs detected by Fermi/GBM

We present the results of the spectral analysis of the public data of 438 Gamma Ray Bursts (GRBs) detected by the Fermi Gamma ray Burst Monitor (GBM) up to March 2010. For 432 bursts we could fit the time integrated spectrum. In 318 cases we can reliably constrain the peak energy Epeak of their νF_νspectrum by analyzing their time integrated spectrum between 8 keV and 35 MeV. 80% of these spectra are fitted by a power law with an exponential cutoff, and the remaining with the Band function. Among these 318 GRBs, 274 and 44 belong to the long and short GRB class, respectively. Long GRBs have a typical peak energy Epeak=160 keV and low energy spectral index alpha=-0.92. Short GRBs have harder peak energy (Epeak=490 keV) and harder low energy spectral index (alpha=-0.50) than long bursts. For each Fermi GRB we analyzed also the spectrum corresponding to the peak flux of the burst. On average, the peak spectrum has harder low energy spectral index but similar Epeak than the corresponding time-integrated spectrum for the same burst. The spectral parameters derived in our analysis of Fermi/GBM bursts are globally consistent with those reported in the GRB Cicular Network (GCN) archive after December 2008, while we found systematic differences, concerning the low energy power law index, for earlier bursts.

astro-ph.HE↗

arXiv2010

The radio/gamma-ray connection in Fermi-blazars

We study the correlation between the gamma-ray flux F_g, averaged over the first 11 months of Fermi survey and integrated above 100 MeV, and the radio flux density (F_r at 20 GHz) of Fermi sources associated with a radio counterpart in the AT20G survey. Considering the blazars detected in both bands, the correlation is highly significant and it is F_g~F_r^0.85+-0.04, similar for BL Lac and FSRQ sources. However, only a small fraction (~1/15) of the AT20G radio sources with flat radio spectrum, are detected by Fermi. To understand if this correlation is real, we examine the selection effects introduced by the flux limits of the radio and gamma-ray surveys, and the importance of variability of the gamma-ray flux. We find that the radio/gamma-ray flux correlation is real, but its slope is steeper than the observed one, i.e. F_g~F_r^delta with delta in the range 1.25-1.5. The observed F_g-F_r correlation and the fraction of radio sources detected by Fermi is reproduced assuming a long term gamma-ray flux variability following a log-normal probability distribution with standard deviation sigma>0.5 (corresponding to F_g varying by at least a factor 3). Such a variability is compatible with what observed when comparing, for the sources in common, the EGRET and the Fermi gamma-ray fluxes (even if the Fermi fluxes are averaged over ~1 year). Another indication of variability is the non detection of 12 out of 66 EGRET blazars by Fermi, despite its higher sensitivity.We also study the strong linear correlation between the gamma-ray and the radio luminosity of the 144 AT20G-Fermi associations with known redshift and show that it is statistically robust. Two possible implications of these correlations are discussed: the contribution of blazars to the extragalactic gamma-ray background and the prediction of blazars that might undergo extremely high states of gamma-ray emission in the next years.

astro-ph.HE↗

arXiv2010

Spectral evolution of Fermi/GBM short Gamma-Ray Bursts

We study the spectral evolution of 13 short duration Gamma Ray Bursts (GRBs) detected by the Gamma Burst Monitor (GBM) on board Fermi. We study spectra resolved in time at the level of 2-512 ms in the 8 keV-35 MeV energy range. We find a strong correlation between the observed peak energy Ep and the flux P within individual short GRBs. The slope of the Ep P^s correlation for individual bursts ranges between ~0.4 and ~1. There is no correlation between the low energy spectral index and the peak energy or the flux. Our results show that in our 13 short GRBs Ep evolves in time tracking the flux. This behavior is similar to what found in the population of long GRBs and it is in agreement with the evidence that long GRBs and (the still few) short GRBs with measured redshifts follow the same rest frame Ep-Liso correlation. Its origin is most likely to be found in the radiative mechanism that has to be the same in both classes of GRBs.

astro-ph.HE↗

arXiv2010

High redshift Fermi blazars

With the release of the first year Fermi catalogue, the number of blazars detected above 100 MeV lying at high redshift has been largely increased. There are 28 blazars at z>2 in the "clean" sample. All of them are Flat Spectrum Radio Quasars (FSRQs). We study and model their overall spectral energy distribution in order to find the physical parameters of the jet emitting region, and for all of them we estimate their black hole masses and accretion rates. We then compare the jet with the accretion disk properties, setting these sources in the broader context of all the other bright gamma-ray or hard X-ray blazars. We confirm that the jet power correlates with the accretion luminosity. We find that the high energy emission peak shifts to smaller frequencies as the observed luminosity increases, according to the blazar sequence, making the hard X-ray band the most suitable for searching the most luminous and distant blazars.

astro-ph.CO↗

arXiv2010

Relativistic jets in Narrow-Line Seyfert 1

Narrow-Line Seyfert 1 (NLS1) class of active galactic nuclei (AGNs) is generally radio-quiet, but a small percent of them are radio-loud. The recent discovery by Fermi/LAT of high-energy gamma-ray emission from 4 NLS1s proved the existence of relativistic jets in these systems. It is therefore important to study this new class of gamma-ray emitting AGNs. Here we report preliminary results about the observations of the July 2010 gamma-ray outburst of PMN J0948+0022, when the source flux exceeded for the first time 10^-6 ph cm^-2 s^-1 (E > 100 MeV).

astro-ph.HE↗

arXiv2010

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

In the first week of December 2009, the blazar 3C 454.3 became the brightest high energy source in the sky. Its photon flux reached and surpassed the level of 1e-5 ph/cm2/s above 100 MeV. The Swift satellite observed the source several times during the period of high gamma-ray flux, and we can construct really simultaneous spectral energy distributions (SED) before, during, and after the luminosity peak. Our main findings are: i) the optical, X-ray and gamma-ray fluxes correlate; ii) the gamma-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 gamma-ray vs optical flux correlation can be explained if the magnetic field is slightly fainter when the overall jet luminosity is stronger; v) the power that the jet spent to produce the peak gamma-ray luminosity is of the same order, or larger, than the accretion disk luminosity. During the flare, the total jet power surely surpassed the accretion power.

astro-ph.CO↗

arXiv2010

The intergalactic magnetic field constrained by Fermi/LAT observations of the TeV blazar 1ES 0229+200

TeV photons from blazars at relatively large distances, interacting with the optical-IR cosmic background, are efficiently converted into electron-positron pairs. The produced pairs are extremely relativistic (Lorentz factors of the order of 1e6 1e7 and promptly loose their energy through inverse Compton scatterings with the photons of the microwave cosmic background, producing emission in the GeV band. The spectrum and the flux level of this reprocessed emission is critically dependent on the intensity of the intergalactic magnetic field, B, that can deflect the pairs diluting the intrinsic emission over a large solid angle. We derive a simple relation for the reprocessed spectrum expected from a steady source. We apply this treatment to the blazar 1ES 0229+200, whose intrinsic very hard TeV spectrum is expected to be approximately steady. Comparing the predicted reprocessed emission with the upper limits measured by the Fermi/Large Area Telescope, we constrain the value of the intergalactic magnetic field to be larger than $B \simeq 5\times 10^{-15}$ Gauss, depending on the model of extragalactic background light.

astro-ph.CO↗

arXiv2010

Does the gamma-ray flux of the blazar 3C 454.3 vary on sub-hour timescales?

In the early days of April 2010, the blazar 3C 454.3 (z=0.859) underwent a strong gamma-ray outburst, reaching fluxes (E > 100 MeV) in excess of 10^-5 ph cm^-2 s^-1. The Fermi Gamma ray Space Telescope performed a 200 ks long pointed observation starting from 5 April 2010 19:38 UTC. This allowed us to try probing the variability of the gamma-ray emission on timescales of hours or less. We found the variability on a few hours timescale. On sub-hour timescale we found no evidence of significant variability, although the present statistics is not yet conclusive and further observations are needed.

astro-ph.HE↗

arXiv2010

Constraining the location of the emitting region in Fermi blazars through rapid gamma-ray variability

We consider the 1.5 years Fermi Large Area Telescope light curves (E > 100 MeV) of the flat spectrum radio quasars 3C 454.3 and PKS 1510-089, which show high activity in this period of time. We characterise the duty cycle of the source by comparing the time spent by the sources at different flux levels. We consider in detail the light curves covering periods of extreme flux. The large number of high-energy photons collected by LAT in these events allows us to find evidence of variability on timescales of few hours. We discuss the implications of significant variability on such short timescales, that challenge the scenario recently advanced in which the bulk of the gamma-ray luminosity is produced in regions of the jet at large distances (tens of parsec) from the black hole.

astro-ph.CO↗

arXiv2010

Spectral properties of long and short Gamma-Ray Bursts: comparison between BATSE and Fermi bursts

We compare the spectral properties of 227 Gamma Ray Bursts (GRBs) detected by the Fermi Gamma Ray Burst Monitor (GBM) up to February 2010 with those of bursts detected by the CGRO/BATSE instrument. Out of 227 Fermi GRBs, 166 have a measured peak energy E_peak_obs of their \nuF(ν) spectrum: of these 146 and 20 belong the long and short class, respectively. Fermi long bursts follow the correlations defined by BATSE bursts between their E_peak_obs vs fluence and peak flux: as already shown for the latter ones, these correlations and their slopes do not originate from instrumental selection effects. Fermi/GBM bursts extend such correlations toward lower fluence/peak energy values with respect to BATSE ones whereas no GBM long burst with E_peak_obs exceeding a few MeV is found, despite the possibility of detecting them. Again as for BATSE, $\sim$ 5% of long and almost all short GRBs detected by Fermi/GBM are outliers of the E_peak-isotropic equivalent energy ("Amati") correlation while no outlier (neither long nor short) of the E_peak-isotropic equivalent luminosity ("Yonetoku") correlation is found. Fermi long bursts have similar typical values of E_peak_obs but a harder low energy spectral index with respect to all BATSE events, exacerbating the inconsistency with the limiting slopes of the simplest synchrotron emission models. Although the short GRBs detected by Fermi are still only a few, we confirm that their E_peak_obs is greater and the low energy spectrum is harder than those of long ones. We discuss the robustness of these results with respect to observational biases induced by the differences between the GBM and BATSE instruments.

astro-ph.HE↗

arXiv2010

Correlation of Fermi-LAT sources with the AT20GHz radio survey

We cross correlate the Fermi 11 months survey catalogue (1FGL) with the 20 GHz Australia Telescope Compact Array radio survey catalogue (AT20G) composed by 5890 sources at declination <0 deg. Among the 738 Fermi sources distributed in the southern sky we find 230 highly probable candidate counterparts in the AT20G survey. Of these, 222 are already classified as blazars (176 of known type and 46 of unknown optical class) in the Fermi 1-year LAT AGN Catalogue (1LAC) and 8 are new associations. By studying the gamma-ray and radio properties of these associations we find a strong correlation between the gamma-ray flux (above 100 MeV) and the 20 GHz flux density. This correlation is more than 3 sigma statistically significant both for the population of BL Lacs and of FSRQ considered separately. We also find that the radio counterparts associated to the Fermi sources have on average flat radio spectra between 5 and 20 GHz and that Fermi gamma-ray sources are not preferentially associated with "ultra inverted spectrum" radio sources. For 2 of the 8 new associations we build the broad band spectral energy distribution combining Fermi, Swift and radio observations. One of these two sources is identified with the high redshift FSRQ Swift J1656.3-3302 (z=2.4) and we classify the other source as a candidate new FSRQ. We also study the brightest radio source of the 46 associations without an optical classification and classify it as a new BL Lac candidate "twin" of the prototypical BL Lac OJ 287 if its redshift is somewhat larger, z~0.4.

astro-ph.HE↗

arXiv2010

Fermi/LAT Gamma Ray Burst emission models and jet properties

The GeV emission of Gamma Ray Bursts, first detected by EGRET in an handful of bursts, is now an established property of roughly the 10% of all bursts, thanks to the Fermi/LAT observations. GRB 090510, a short burst, is particularly interesting because the good timing allows to derive a severe limit to theories of quantum gravity. With the dozen bursts detected in the 0.1-30 GeV band so far, we start to see some common properties: (i) the duration is often longer than the duration of the softer emission detected by the Gamma Burst Monitor (GBM) onboard Fermi; (ii) the spectrum is consistent with F(v)~v^{-1} with no strong spectral evolution; (iii) for the brightest bursts, the flux detected by the LAT decays as a power law with a typical slope: t^{-1.5}; iv) the peak energy of the GBM emission exceeds 500 keV (rest frame). These properties suggest a similar process for the origin of the GeV flux. We propose that it is afterglow synchrotron emission shortly following the start of the prompt phase. The steep decay slope suggests that the fireball emits in the radiative regime, i.e. all dissipated energy is radiated away. The large peak energy of the GBM flux suggests that electron-positron pairs might play a crucial role. The rapid onset, but with some delay, of the GeV flux with respect to the GBM one suggests that the bulk Lorentz factor Gamma of these bursts is of the order of 1000. Therefore the relatively small fraction of bursts detected at high energies might correspond to the fraction of bursts having the largest Gamma. If the emission occurs in the radiative regime we can start to understand why the observed X-ray and optical afterglow energetics are much smaller than the energetics emitted during the prompt phase.

astro-ph.HE↗

arXiv2010

Chasing the heaviest black holes of jetted Active Galactic Nuclei

We investigate the physical properties of the 10 blazars at redshift greater than 2 detected in the 3-years all sky survey performed by the Burst Alert Telescope (BAT) onboard the Swift satellite. We find that the jets of these blazars are among the most powerful known. Furthermore, the mass of their central black hole, inferred from the optical-UV bump, exceeds a few billions of solar masses, with accretion luminosities being a large fraction of the Eddington one. We compare their properties with those of the brightest blazars of the 3-months survey performed by the Large Area Telescope (LAT) onboard the Fermi satellite. We find that the BAT blazars have more powerful jets, more luminous accretion disks and larger black hole masses than LAT blazars. These findings can be simply understood on the basis of the blazar sequence, that suggests that the most powerful blazars have a spectral energy distribution with a high energy peak at MeV (or even sub-MeV) energies. This implies that the most extreme blazars can be found more efficiently in hard X-rays, rather than in the high energy gamma-ray band. We then discuss the implications of our findings for future missions, such as the New Hard X-ray Mission (NHXM) and especially the Energetic X-ray Imaging Survey Telescope (EXIST) mission which, during its planned 2 years all sky survey, is expected to detect thousands of blazars, with a few of them at z greater than 6.

astro-ph.HE↗