Showing papers by "M. Perri published in 2011"

Relativistic jet activity from the tidal disruption of a star by a massive black hole

Abstract: Two groups report observations of the X-ray source Swift J164449.3+573451, which was discovered when it triggered the Swift Burst Alert Telescope on 28 March 2011. Burrows et al. report that the source has increased in brightness in the X-ray band more than 10,000-fold since 1990, and by more than 100-fold since early 2010. They conclude that we are observing the onset of relativistic jet activity from a supermassive black hole. Zauderer et al. arrive at a similar conclusion based on their observation of a radio transient associated with the source, and extensive monitoring at centimetre to millimetre wavelengths during the first month of its evolution. They estimate the mass of the black hole at around 106 solar masses. Supermassive black holes have powerful gravitational fields with strong gradients that can destroy stars that get too close1,2, producing a bright flare in ultraviolet and X-ray spectral regions from stellar debris that forms an accretion disk around the black hole3,4,5,6,7. The aftermath of this process may have been seen several times over the past two decades in the form of sparsely sampled, slowly fading emission from distant galaxies8,9,10,11,12,13,14, but the onset of the stellar disruption event has not hitherto been observed. Here we report observations of a bright X-ray flare from the extragalactic transient Swift J164449.3+573451. This source increased in brightness in the X-ray band by a factor of at least 10,000 since 1990 and by a factor of at least 100 since early 2010. We conclude that we have captured the onset of relativistic jet activity from a supermassive black hole. A companion paper15 comes to similar conclusions on the basis of radio observations. This event is probably due to the tidal disruption of a star falling into a supermassive black hole, but the detailed behaviour differs from current theoretical models of such events.

Discovery of the Onset of Rapid Accretion by a Dormant Massive Black Hole

Abstract: Massive black holes are believed to reside at the centres of most galaxies. They can be- come detectable by accretion of matter, either continuously from a large gas reservoir or impulsively from the tidal disruption of a passing star, and conversion of the gravitational energy of the infalling matter to light. Continuous accretion drives Active Galactic Nuclei (AGN), which are known to be variable but have never been observed to turn on or off. Tidal disruption of stars by dormant massive black holes has been inferred indirectly but the on- set of a tidal disruption event has never been observed. Here we report the first discovery of the onset of a relativistic accretion-powered jet in the new extragalactic transient, Swift J164449.3+573451. The behaviour of this new source differs from both theoretical models of tidal disruption events and observations of the jet-dominated AGN known as blazars. These differences may stem from transient effects associated with the onset of a powerful jet. Such an event in the massive black hole at the centre of our Milky Way galaxy could strongly ionize the upper atmosphere of the Earth, if beamed towards us.

Fermi Large Area Telescope Observations of Markarian 421: The Missing Piece of its Spectral Energy Distribution

Abstract: We report on the gamma-ray activity of the high-synchrotron-peaked BL Lacertae object Markarian 421 (Mrk 421) during the first 1.5 years of Fermi operation, from 2008 August 5 to 2010 March 12. We find that the Large Area Telescope (LAT) gamma-ray spectrum above 0.3 GeV can be well described by a power-law function with photon index Gamma = 1.78 +/- 0.02 and average photon flux F(>0.3 GeV) = (7.23 +/- 0.16) x 10(-8) ph cm(-2) s(-1). Over this time period, the Fermi-LAT spectrum above 0.3 GeV was evaluated on seven-day-long time intervals, showing significant variations in the photon flux (up to a factor similar to 3 from the minimum to the maximum flux) but mild spectral variations. The variability amplitude at X-ray frequencies measured by RXTE/ASM and Swift/BAT is substantially larger than that in gamma-rays measured by Fermi-LAT, and these two energy ranges are not significantly correlated. We also present the first results from the 4.5 month long multifrequency campaign on Mrk 421, which included the VLBA, Swift, RXTE, MAGIC, the F-GAMMA, GASP-WEBT, and other collaborations and instruments that provided excellent temporal and energy coverage of the source throughout the entire campaign (2009 January 19 to 2009 June 1). During this campaign, Mrk 421 showed a low activity at all wavebands. The extensive multi-instrument (radio to TeV) data set provides an unprecedented, complete look at the quiescent spectral energy distribution (SED) for this source. The broadband SED was reproduced with a leptonic (one-zone synchrotron self-Compton) and a hadronic model (synchrotron proton blazar). Both frameworks are able to describe the average SED reasonably well, implying comparable jet powers but very different characteristics for the blazar emission site.

Insights into the high-energy γ-ray emission of Markarian 501 from extensive multifrequency observations in the Fermi era

Abstract: We report on the gamma-ray activity of the blazar Mrk 501 during the first 480 days of Fermi operation. We find that the average LAT gamma-ray spectrum of Mrk 501 can be well described by a single power-law function with a photon index of 1.78 +/- 0.03. While we observe relatively mild flux variations with the Fermi-LAT (within less than a factor of 2), we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 +/- 0.14, and the softest one is 2.51 +/- 0.20. These unexpected spectral changes do not correlate with the measured flux variations above 0.3GeV. In this paper, we also present the first results from the 4.5-month-long multifrequency campaign (2009 March 15 - August 1) on Mrk 501, which included the VLBA, Swift, RXTE, MAGIC and VERITAS, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign. The average spectral energy distribution of Mrk 501 is well described by the standard one-zone synchrotron self-Compton model. In the framework of this model, we find that the dominant emission region is characterized by a size <~ 0.1 pc (comparable within a factor of few to the size of the partially-resolved VLBA core at 15-43 GHz), and that the total jet power (~10^{44} erg s^{-1}) constitutes only a small fraction (~10^{-3}) of the Eddington luminosity. The energy distribution of the freshly-accelerated radiating electrons required to fit the time-averaged data has a broken power-law form in the energy range 0.3GeV-10TeV, with spectral indices 2.2 and 2.7 below and above the break energy of 20GeV. We argue that such a form is consistent with a scenario in which the bulk of the energy dissipation within the dominant emission zone of Mrk 501 is due to relativistic, proton-mediated shocks.

Simultaneous Planck, Swift, and Fermi observations of X-ray and gamma-ray selected blazars

Abstract: We present simultaneous Planck, Swift, Fermi, and ground-based data for 105 blazars belonging to three samples with flux limits in the soft X-ray, hard X-ray, and gamma-ray bands. Our unique data set has allowed us to demonstrate that the selection method strongly influences the results, producing biases that cannot be ignored. Almost all the BL Lac objects have been detected by Fermi-LAT, whereas ~40% of the flat-spectrum radio quasars (FSRQs) in the radio, soft X-ray, and hard X-ray selected samples are still below the gamma-ray detection limit even after integrating 27 months of Fermi-LAT data. The radio to sub-mm spectral slope of blazars is quite flat up to ~70GHz, above which it steepens to ~-0.65. BL Lacs have significantly flatter spectra than FSRQs at higher frequencies. The distribution of the rest-frame synchrotron peak frequency ( upS) in the SED of FSRQs is the same in all the blazar samples with =10^13.1 Hz, while the mean inverse-Compton peak frequency, , ranges from 10^21 to 10^22 Hz. The distributions of upS and of upIC of BL Lacs are much broader and are shifted to higher energies than those of FSRQs and strongly depend on the selection method. The Compton dominance of blazars ranges from ~0.2 to ~100, with only FSRQs reaching values >3. Its distribution is broad and depends strongly on the selection method, with gamma-ray selected blazars peaking at ~7 or more, and radio-selected blazars at values ~1, thus implying that the assumption that the blazar power is dominated by high-energy emission is a selection effect. Simple SSC models cannot explain the SEDs of most of the gamma-ray detected blazars in all samples. The SED of the blazars that were not detected by Fermi-LAT may instead be consistent with SSC emission. Our data challenge the correlation between bolometric luminosity and upS predicted by the blazar sequence.

Planck early results. XV. Spectral energy distributions and radio continuum spectra of northern extragalactic radio sources

Abstract: Spectral energy distributions (SEDs) and radio continuum spectra are presented for a northern sample of 104 extragalactic radio sources. based on the Planck Early Release Compact Source Catalogue (ERCSC) and simultaneous multi frequency data. The nine Planck frequencies, from 30 to 857 GHz, are complemented by a set of simultaneous observations ranging from radio to gamma-rays. This is the first extensive frequency coverage in the radio and millimetre domains for an essentially complete sample of extragalactic radio sources, and it shows how the individual shocks, each in their own phase of development, shape the radio spectra as they move in the relativistic jet. The SEDs presented in this paper were fitted with second and third degree polynomials to estimate the frequencies of the synchrotron and inverse Compton (IC) peaks, and the spectral indices of low and high frequency radio data, including the Planck ERCSC data, were calculated. SED modelling methods are discussed, with an emphasis on proper. physical modelling of the synchrotron bump using multiple components. Planck ERCSC data also suggest that the original accelerated electron energy spectrum could be much harder than commonly thought, with power-law index around 1.5 instead of the canonical 2.5. The implications of this are discussed for the acceleration mechanisms effective in blazar shock. Furthermore in many cases the Planck data indicate that gamma-ray emission must originate in the same shocks that produce the radio emission.

Planck early results. XV. Spectral energy distributions and radio continuum spectra of northern extragalactic radio sources

Abstract: Spectral energy distributions (SEDs) and radio continuum spectra are presented for a northern sample of 104 extragalactic radio sources, based on the Planck Early Release Compact Source Catalogue (ERCSC) and simultaneous multifrequency data. The nine Planck frequencies, from 30 to 857 GHz, are complemented by a set of simultaneous observations ranging from radio to gamma-rays. This is the first extensive frequency coverage in the radio and millimetre domains for an essentially complete sample of extragalactic radio sources, and it shows how the individual shocks, each in their own phase of development, shape the radio spectra as they move in the relativistic jet. The SEDs presented in this paper were fitted with second and third degree polynomials to estimate the frequencies of the synchrotron and inverse Compton (IC) peaks, and the spectral indices of low and high frequency radio data, including the Planck ERCSC data, were calculated. SED modelling methods are discussed, with an emphasis on proper, physical modelling of the synchrotron bump using multiplecomponents. Planck ERCSC data also suggest that the original accelerated electron energy spectrum could be much harder than commonly thought, with power-law index around 1.5 instead of the canonical 2.5. The implications of this are discussed for the acceleration mechanisms effective in blazar shock. Furthermore in many cases the Planck data indicate that gamma-ray emission must originate in the same shocks that produce the radio emission.

The Swift Serendipitous Survey in deep XRT GRB fields (SwiftFT) ? I. The X-ray catalog and number counts

Abstract: Aims. An accurate census of the active galactic nuclei (AGN) is a key step in investigating the nature of the correlation between the growth and evolution of super massive black holes and galaxy evolution. X-ray surveys provide one of the most efficient ways of selecting AGN. Methods. We searched for X-ray serendipitous sources in over 370 Swift-XRT fields centered on gamma ray bursts detected between 2004 and 2008 and observed with total exposures ranging from 10 ks to over 1 Ms. This defines the Swift Serendipitous Survey in deep XRT GRB fields, which is quite broad compared to existing surveys (∼33 square degrees) and medium depth, with a faintest flux limit of 7.2 × 10 −16 erg cm −2 s −1 in the 0.5 to 2 keV energy range (4.8 × 10 −15 erg cm −2 s −1 at 50% completeness). The survey has a high degree of uniformity thanks to the stable point spread function and small vignetting correction factors of the XRT, moreover is completely random on the sky as GRBs explode in totally unrelated parts of the sky. Results. In this paper we present the sample and the X-ray number counts of the high Galactic-latitude sample, estimated with high statistics over a wide flux range (i.e., 7.2×10 −16 ÷∼5×10 −13 erg cm −2 s −1 in the 0.5–2 keV band and 3.4×10 −15 ÷∼6×10 −13 erg cm −2 s −1 in the 2–10 keV band). We detect 9387 point-like sources with a detection Poisson probability threshold of ≤2× 10 −5 , in at least one of the three energy bands considered (i.e. 0.3–3 keV, 2–10 keV, and 0.3–10 keV), for the total sample, while 7071 point-like sources are found at high Galactic-latitudes (i.e. | b |≥ 20 deg). The large number of detected sources resulting from the combination of large area and deep flux limits make this survey a new important tool for investigating the evolution of AGN. In particular, the large area permits finding rare high-luminosity objects like QSO2, which are poorly sampled by other surveys, adding precious information for the luminosity function bright end. The high Galactic-latitude log N–log S relation is well determined over all the flux coverage, and it is nicely consistent with previous results at 1σ confidence level. By the hard X-ray color analysis, we find that the Swift Serendipitous Survey in deep XRT GRB fields samples relatively unobscured and mildly obscured AGN, with a fraction of obscured sources of ∼37% (∼15%) in the 2–10 (0.3–3 keV) band.

Radioactive synovectomy with Yttrium⁹⁰ citrate in haemophilic synovitis: Brazilian experience.

Abstract: Recurrent haemarthroses leading to chronic synovitis and arthropathy remain a major cause of morbidity in patients with haemophilia. Radioactive synovectomy (RS) is considered the first choice of treatment for chronic haemophilic synovitis. The aim of this study was to evaluate the effect of RS with Yttrium(90) citrate (C-Y(90)) in the joints of patients with chronic haemophilic synovitis. From 2003 to 2007, 245 joints (118 knees, 76 elbows, 49 ankles and two shoulders) of 190 patients with haemophilia or von Willebrand disease were submitted to RS with C-Y(90) at Hemocentro de Mato Grosso, Brazil. Forty joints had radiographic Pettersson scores above 8. There were 36 joints of 22 patients with inhibitors to factor VIII. The procedure was safe with low occurrence of adverse events. The main effect was the overall reduction in joint bleeding frequency, from 19.8 to 2.6 per year post-RS. Similar results were obtained in cases with high radiographic scores and in inhibitor patients. Pain reduction was observed in most cases. Average range of motion was maintained or increased 1 year post-RS in most joints. Extension was stable or increased in 88.2% of the knees and 86.5% of the elbows. Ankle plantarflexion was stable or increased in 90.9%, whereas dorsiflexion was maintained or increased in 87.9%. Worsening of the range of motion, when present, ranged from 14 to 17 degrees. We concluded that RS with C-Y(90) represents an important resource for the treatment of chronic haemophilic synovitis, markedly reducing joint bleeding frequency and pain, irrespective of the radiographic stage and inhibitor status.

Recovering Swift-XRT energy resolution through CCD charge trap mapping

Abstract: The X-ray telescope on board the Swift satellite for gamma-ray burst astronomy has been exposed to the radiation of the space environment since launch in November 2004. Radiation causes damage to the detector, with the generation of dark current and charge trapping sites that result in the degradation of the spectral resolution and an increase of the instrumental background. The Swift team has a dedicated calibration program with the goal of recovering a significant proportion of the lost spectroscopic performance. Calibration observations of supernova remnants with strong emission lines are analysed to map the detector charge traps and to derive position-dependent corrections to the measured photon energies. We have achieved a substantial recovery in the XRT resolution by implementing these corrections in an updated version of the Swift XRT gain file and in corresponding improvements to the Swift XRT HEAsoft software. We provide illustrations of the impact of the enhanced energy resolution, and show that we have recovered most of the spectral resolution lost since launch

X-ray spectral evolution of TeV BL Lacertae objects: eleven years of observations withBeppoSAX,XMM-NewtonandSwiftsatellites (Corrigendum)

Abstract: Due to an error in reporting the spectral parameters of Mrk 421 from literature, the upper panels of Figs. 1, 2, 4, 6, 8 have a mistake. In these figures, the height of the SED at the peak energy for Mrk 421 was incorrectly reported from the literature (see Tramacere, A., Massaro, F., & Cavaliere, A., 2007, A&A, 466, 521) as being a factor of 10 lower than it’s true value. However, the conclusions and the results of the paper are not affected by the above mistake. Here in the following there are the corrected version of the above figures.

Recovering Swift-XRT energy resolution through CCD charge trap mapping

Abstract: The X-ray telescope on board the Swift satellite for gamma-ray burst astronomy has been exposed to the radiation of the space environment since launch in November 2004. Radiation causes damage to the detector, with the generation of dark current and charge trapping sites that result in the degradation of the spectral resolution and an increase of the instrumental background. The Swift team has a dedicated calibration program with the goal of recovering a significant proportion of the lost spectroscopic performance. Calibration observations of supernova remnants with strong emission lines are analysed to map the detector charge traps and to derive position-dependent corrections to the measured photon energies. We have achieved a substantial recovery in the XRT resolution by implementing these corrections in an updated version of the Swift XRT gain file and in corresponding improvements to the Swift XRT HEAsoft software. We provide illustrations of the impact of the enhanced energy resolution, and show that we have recovered most of the spectral resolution lost since launch.

The Swift Serendipitous Survey in deep XRT GRB fields (SwiftFT) I. The X-ray catalog and number counts

Abstract: We searched for X-ray serendipitous sources in over 370 Swift-XRT fields centered on gamma ray bursts detected between 2004 and 2008 and observed with total exposures ranging from 10 ks to over 1 Ms. This defines the Swift Serendipitous Survey in deep XRT GRB fields, which is quite broad compared to existing surveys (~33 square degrees) and medium depth, with a faintest flux limit of 7.2e-16 erg cm^-2 s^-1 in the 0.5 to 2 keV energy range. The survey has a high degree of uniformity thanks to the stable point spread function and small vignetting correction factors of the XRT, moreover is completely random on the sky as GRBs explode in totally unrelated parts of the sky. In this paper we present the sample and the X-ray number counts of the high Galactic-latitude sample, estimated with high statistics over a wide flux range (i.e., 7.2e-16 to ~5e-13 erg cm^-2 s^-1 in the 0.5-2 keV band and 3.4e-15 to ~6e-13 erg cm^-2 s^-1 in the 2-10 keV band). We detect 9387 point-like sources, while 7071 point-like sources are found at high Galactic-latitudes (i.e. >=20 deg). The large number of detected sources resulting from the combination of large area and deep flux limits make this survey a new important tool for investigating the evolution of AGN. In particular, the large area permits finding rare high-luminosity objects like QSO2, which are poorly sampled by other surveys, adding precious information for the luminosity function bright end. The high Galactic-latitude logN-logS relation is well determined over all the flux coverage, and it is nicely consistent with previous results at 1 sigma confidence level. By the hard X-ray color analysis, we find that the Swift Serendipitous Survey in deep XRT GRB fields samples relatively unobscured and mildly obscured AGN, with a fraction of obscured sources of ~37% (~15%) in the 2-10 (0.3-3 keV) band.

Broad band spectral energy distribution studies of Fermi bright blazars

Abstract: The Fermi Gamma-ray Space Telescope was successfully launched on June 11, 2008 and has already opened a new era for gamma-ray astronomy. The Large Area Telescope (LAT), the main instrument on board Fermi, presents a significant improvement in sensitivity over its predecessor EGRET, due to its large field of view and effective area, combined with its excellent timing capabilities. The preliminary results of the Spectral Energy Distribution Analysis performed on a sample of bright blazars are presented. For this study, the data from the first three months of data collection of Fermi have been used. The analysis is extended down to radio, mm, near-IR, optical, UV and X-ray bands and up to TeV energies based on unprecedented sample of simultaneous multi-wavelength observations by GASP-WEBT.