arXiv subjects

M. Ajello

Publications and source records attributed to M. Ajello.

At least 163 records · Page 9Linked to original sources

arXiv2010

Three years Swift-BAT Survey of AGN: reconciling theory and observations?

It is well accepted that unabsorbed as well as absorbed AGN are needed to explain the nature and the shape of the Cosmic X-ray background, even if the fraction of highly absorbed objects (dubbed Compton-thick sources) substantially still escapes detection. We derive and analyze the absorption distribution using a complete sample of AGN detected by Swift-BAT in the first three years of the survey. The fraction of Compton-thick AGN represents only 4.6% of the total AGN population detected by Swift-BAT. However, we show that once corrected for the bias against the detection of very absorbed sources the real intrinsic fraction of Compton-thick AGN is 20$^{+9}_{-6}$%. We proved for the first time (also in the BAT band) that the anti-correlation of the fraction of absorbed AGN and luminosity it tightly connected to the different behavior of the luminosity functions (XLFs) of absorbed and unabsorbed AGN. This points towards a difference between the two subsamples of objects with absorbed AGN being, on average, intrinsically less luminous than unobscured ones. Moreover the XLFs show that the fraction of obscured AGN might also decrease at very low luminosity. This can be successfully interpreted in the framework of a disk cloud outflow scenario as the disappearance of the obscuring region below a critical luminosity. Our results are discussed in the framework of population synthesis models and the origin of the Cosmic X-ray Background.

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arXiv2010

Probing the Transition Between the Synchrotron and Inverse-compton Spectral Components of 1ES 1959+650

1ES 1959+650 is one of the most remarkable high-peaked BL Lacertae objects (HBL). In 2002, it exhibited a TeV gamma-ray flare without a similar brightening of the synchrotron component at lower energies. We present the results of a multifrequency campaign, triggered by the INTEGRAL IBIS detection of 1ES 1959+650. Our data range from the optical to hard X-ray energies, thus covering the synchrotron and inverse-Compton components simultaneously. We observed the source with INTEGRAL, the Swift X-Ray Telescope, and the UV-Optical Telescope, and nearly simultaneously with a ground-based optical telescope. The steep spectral component at X-ray energies is most likely due to synchrotron emission, while at soft gamma-ray energies the hard spectral index may be interpreted as the onset of the high-energy component of the blazar spectral energy distribution (SED). This is the first clear measurement of a concave X-ray-soft gamma-ray spectrum for an HBL. The SED can be well modeled with a leptonic synchrotron self-Compton model. When the SED is fitted this model requires a very hard electron spectral index of q ~ 1.85, possibly indicating the relevance of second-order Fermi acceleration.

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arXiv2010

Galaxy Clusters in the Swift/BAT era II: 10 more Clusters detected above 15 keV

We report on the discovery of 10 additional galaxy clusters detected in the ongoing Swift/BAT all-sky survey. Among the newly BAT-discovered clusters there are: Bullet, Abell 85, Norma, and PKS 0745-19. Norma is the only cluster, among those presented here, which is resolved by BAT. For all the clusters we perform a detailed spectral analysis using XMM-Newton and Swift/BAT data to investigate the presence of a hard (non-thermal) X-ray excess. We find that in most cases the clusters' emission in the 0.3-200keV band can be explained by a multi-temperature thermal model confirming our previous results. For two clusters (Bullet and Abell 3667) we find evidence for the presence of a hard X-ray excess. In the case of the Bullet cluster, our analysis confirms the presence of a non-thermal, power-law like, component with a 20-100 keV flux of 3.4 \times 10-12 erg cm-2 s-1 as detected in previous studies. For Abell 3667 the excess emission can be successfully modeled as a hot component (kT=~13keV). We thus conclude that the hard X-ray emission from galaxy clusters (except the Bullet) has most likely thermal origin.

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arXiv2010

Swift-BAT Survey of Galactic Sources: Catalog and Properties of the populations

We study the populations of X-ray sources in the Milky Way in the 15-55 keV band using a deep survey with the BAT instrument aboard the Swift observatory. We present the logN-logS distributions of the various source types and we analyze their variability and spectra. For the low-mass X-ray binaries (LMXBs) and the high-mass X-ray binaries (HMXBs) we derive the luminosity functions to a limiting luminosity of L_X~7 times10^{34} erg s/s. Our results confirm the previously found flattening of the LMXB luminosity function below a luminosity of L_X~10^{37} erg s/s. The luminosity function of the HMXBs is found to be significantly flatter in the 15-55 keV band than in the 2-10 keV band. From the luminosity functions we estimate the ratios of the hard X-ray luminosity from HMXBs to the star-formation rate, and the LMXB luminosity to the stellar mass. We use these to estimate the X-ray emissivity in the local universe from X-ray binaries and show that it constitutes only a small fraction of the hard X-ray background.

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arXiv2010

AGN clustering in the local Universe: an unbiased picture from Swift-BAT

We present the clustering measurement of hard X-ray selected AGN in the local Universe. We used a sample of 199 sources spectroscopically confirmed detected by Swift-BAT in its 15-55 keV all-sky survey. We measured the real space projected auto-correlation function and detected a signal significant on projected scales lower than 200 Mpc/h. We measured a correlation length of r0=5.56+0.49-0.43 Mpc/h and a slope γ=1.64-0.08 -0.07. We also measured the auto-correlation function of Type I and Type II AGN and found higher correlation length for Type I AGN. We have a marginal evidence of luminosity dependent clustering of AGN, as we detected a larger correlation length of luminous AGN than that of low luminosity sources. The corresponding typical host DM halo masses of Swift-BAT are log(MDMH) 12-14 h^-1 M/M_sun, depending on the subsample. For the whole sample we measured log(MDMH)\sim 13.15 h-1 M/M_sun which is the typical mass of a galaxy group. We estimated that the local AGN population has a typical lifetime tau_AGN \sim 0.7 Gyr, it is powered by SMBH with mass MBH \sim 1-10x10^8 M_\odot and accreting with very low efficiency, log(epsilon)-2.0. We also conclude that local AGN host galaxies are typically red-massive galaxies with stellar mass of the order 2-80x10^10 h^-1 M_sun. We compared our results with clustering predictions of merger-driven AGN triggering models and found a good agreement.

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arXiv2010

The Discovery of gamma-Ray Emission From The Blazar RGB J0710+591

The high-frequency-peaked BL Lacertae object RGB J0710+591 was observed in the very high-energy (VHE; E > 100 GeV) wave band by the VERITAS array of atmospheric Cherenkov telescopes. The observations, taken between 2008 December and 2009 March and totaling 22.1 hr, yield the discovery of VHE gamma rays from the source. RGB J0710+591 is detected at a statistical significance of 5.5 standard deviations (5.5σ) above the background, corresponding to an integral flux of (3.9 +/- 0.8) x 10-12 cm-2 s-1 (3% of the Crab Nebula's flux) above 300 GeV. The observed spectrum can be fit by a power law from 0.31 to 4.6 TeV with a photon spectral index of 2.69 +/- 0.26stat +/- 0.20sys. These data are complemented by contemporaneous multiwavelength data from the Fermi Large Area Telescope, the Swift X-ray Telescope, the Swift Ultra-Violet and Optical Telescope, and the Michigan-Dartmouth-MIT observatory. Modeling the broadband spectral energy distribution (SED) with an equilibrium synchrotron self-Compton model yields a good statistical fit to the data. The addition of an external-Compton component to the model does not improve the fit nor brings the system closer to equipartition. The combined Fermi and VERITAS data constrain the properties of the high-energy emission component of the source over 4 orders of magnitude and give measurements of the rising and falling sections of the SED.

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arXiv2010

Discovery of a GeV blazar shining through the Galactic plane

The \emph{Fermi} Large Area Telescope (LAT) discovered a new gamma-ray source near the Galactic plane, \object{Fermi J0109+6134}, when it flared brightly in 2010 February. The low Galactic latitude (b =-1.2\degr) indicated that the source could be located within the Galaxy, which motivated rapid multi-wavelength follow-up including radio, optical, and X-ray observations. We report the results of analyzing all 19 months of LAT data for the source, and of X-ray observations with both \emph{Swift} and the \emph{Chandra X-ray Observatory}. We determined the source redshift, z =0.783, using a Keck LRIS observation. Finally, we compiled a broadband spectral energy distribution (SED) from both historical and new observations contemporaneous with the 2010 February flare. The redshift, SED, optical line width, X-ray absorption, and multi-band variability indicate that this new GeV source is a blazar seen through the Galactic plane. Because several of the optical emission lines have equivalent width >5Å, this blazar belongs in the flat-spectrum radio quasar category.

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arXiv2010

Constraints on Cosmological Dark Matter Annihilation from the Fermi-LAT Isotropic Diffuse Gamma-Ray Measurement

The first published Fermi large area telescope (Fermi-LAT) measurement of the isotropic diffuse gamma-ray emission is in good agreement with a single power law, and is not showing any signature of a dominant contribution from dark matter sources in the energy range from 20 to 100 GeV. We use the absolute size and spectral shape of this measured flux to derive cross section limits on three types of generic dark matter candidates: annihilating into quarks, charged leptons and monochromatic photons. Predicted gamma-ray fluxes from annihilating dark matter are strongly affected by the underlying distribution of dark matter, and by using different available results of matter structure formation we assess these uncertainties. We also quantify how the dark matter constraints depend on the assumed conventional backgrounds and on the Universe's transparency to high-energy gamma-rays. In reasonable background and dark matter structure scenarios (but not in all scenarios we consider) it is possible to exclude models proposed to explain the excess of electrons and positrons measured by the Fermi-LAT and PAMELA experiments. Derived limits also start to probe cross sections expected from thermally produced relics (e.g. in minimal supersymmetry models) annihilating predominantly into quarks. For the monochromatic gamma-ray signature, the current measurement constrains only dark matter scenarios with very strong signals.

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arXiv2010

Fermi LAT Search for Photon Lines from 30 to 200 GeV and Dark Matter Implications

Dark matter (DM) particle annihilation or decay can produce monochromatic $γ$-rays readily distinguishable from astrophysical sources. $γ$-ray line limits from 30 GeV to 200 GeV obtained from 11 months of Fermi Large Area Space Telescope data from 20-300 GeV are presented using a selection based on requirements for a $γ$-ray line analysis, and integrated over most of the sky. We obtain $γ$-ray line flux upper limits in the range $0.6-4.5\times 10^{-9}\mathrm{cm}^{-2}\mathrm{s}^{-1}$, and give corresponding DM annihilation cross-section and decay lifetime limits. Theoretical implications are briefly discussed.

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arXiv2010

Observations of Milky Way Dwarf Spheroidal galaxies with the Fermi-LAT detector and constraints on Dark Matter models

We report on the observations of 14 dwarf spheroidal galaxies with the Fermi Gamma-Ray Space Telescope taken during the first 11 months of survey mode operations. The Fermi telescope provides a new opportunity to test particle dark matter models through the expected gamma-ray emission produced by pair annihilation of weakly interacting massive particles (WIMPs). Local Group dwarf spheroidal galaxies, the largest galactic substructures predicted by the cold dark matter scenario, are attractive targets for such indirect searches for dark matter because they are nearby and among the most extreme dark matter dominated environments. No significant gamma-ray emission was detected above 100 MeV from the candidate dwarf galaxies. We determine upper limits to the gamma-ray flux assuming both power-law spectra and representative spectra from WIMP annihilation. The resulting integral flux above 100 MeV is constrained to be at a level below around 10^-9 photons cm^-2 s^-1. Using recent stellar kinematic data, the gamma-ray flux limits are combined with improved determinations of the dark matter density profile in 8 of the 14 candidate dwarfs to place limits on the pair annihilation cross-section of WIMPs in several widely studied extensions of the standard model. With the present data, we are able to rule out large parts of the parameter space where the thermal relic density is below the observed cosmological dark matter density and WIMPs (neutralinos here) are dominantly produced non-thermally, e.g. in models where supersymmetry breaking occurs via anomaly mediation. The gamma-ray limits presented here also constrain some WIMP models proposed to explain the Fermi and PAMELA e^+e^- data, including low-mass wino-like neutralinos and models with TeV masses pair-annihilating into muon-antimuon pairs. (Abridged)

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arXiv2009

Swift BAT, Fermi LAT, and the Blazar Sequence

Using public \fermi LAT and \swift BAT observations, we constructed the first sample of blazars selected at both hard X-rays and gamma-rays. Studying its spectral properties, we find a luminosity dependence of the spectral slopes at both energies. Specifically, luminous blazars, generally classified as FSRQs, have {\it hard} continua in the medium-hard X-ray range but {\it soft} continua in the LAT gamma-ray range (photon indices $Γ_X$ \ltsima 2 and $Γ_G$ \gtsima 2), while lower luminosity blazars, classified as BL Lacs, have opposite behavior, i.e., {\it soft} X-ray and {\it hard} gamma-ray continua ($Γ_X$ \gtsima 2.4 and $Γ_G < 2$). The trends are confirmed by detailed Monte Carlo simulations explicitly taking into account the observational biases of both instruments. Our results support the so-called ``blazar sequence'' which was originally based on radio samples of blazars and radio luminosities. We also argue that the X-ray-to-gamma-ray continua of blazars may provide independent insights into the physical conditions around the jet, complementing/superseding the ambiguities of the traditional classification based on optical properties.

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arXiv2009

The Spectral Energy Distribution of Fermi bright blazars

(Abridged) We have conducted a detailed investigation of the broad-band spectral properties of the γ-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray spectra with Swift, radio, infra-red, optical and other hard X-ray/gamma-ray data, collected within three months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous Spectral Energy Distributions (SED) for 48 LBAS blazars.The SED of these gamma-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual Log $ν$ - Log $ν$ F$_ν$ representation, the typical broad-band spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SEDs to characterize the peak intensity of both the low and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broad-band colors (i.e. the radio to optical and optical to X-ray spectral slopes) and from the gamma-ray spectral index. Our data show that the synchrotron peak frequency $ν_p^S$ is positioned between 10$^{12.5}$ and 10$^{14.5}$ Hz in broad-lined FSRQs and between $10^{13}$ and $10^{17}$ Hz in featureless BL Lacertae objects.We find that the gamma-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron - inverse Compton scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton (SSC) models cannot explain most of our SEDs, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. (...)

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arXiv2009

PKS 0537-286, carrying the information of the environment of SMBHs in the early Universe

We present the results of a multifrequency campaign on the high-redshift (z = 3.1) blazar PKS 0537-286. The source was observed at different epochs from 2006 to 2008 with INTEGRAL and Swift, and nearly simultaneously with ground-based near-IR/optical telescopes. The SEDs are compatible with a model based on synchrotron radiation and external inverse Compton scattering. The campaign gives an insight into the physical environment of the blazar.

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arXiv2009

The 22-Month Swift-BAT All-Sky Hard X-ray Survey

We present the catalog of sources detected in the first 22 months of data from the hard X-ray survey (14--195 keV) conducted with the BAT coded mask imager on the \swift satellite. The catalog contains 461 sources detected above the 4.8 sigma level with BAT. High angular resolution X-ray data for every source from Swift XRT or archival data have allowed associations to be made with known counterparts in other wavelength bands for over 97% of the detections, including the discovery of ~30 galaxies previously unknown as AGN and several new Galactic sources. A total of 266 of the sources are associated with Seyfert galaxies (median redshift z ~ 0.03) or blazars, with the majority of the remaining sources associated with X-ray binaries in our Galaxy. This ongoing survey is the first uniform all sky hard X-ray survey since HEAO-1 in 1977. Since the publication of the 9-month BAT survey we have increased the number of energy channels from 4 to 8 and have substantially increased the number of sources with accurate average spectra. The BAT 22-month catalog is the product of the most sensitive all-sky survey in the hard X-ray band, with a detection sensitivity (4.8 sigma) of 2.2e-11 erg/cm2/s (1 mCrab) over most of the sky in the 14--195 keV band.

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arXiv2009

NTT, Spitzer and Chandra spectroscopy of SDSSJ095209.56+214313.3: the most luminous coronal-line supernova ever observed, or a stellar tidal disruption event ?

The galaxy SDSSJ0952+2143 showed remarkable emission-line properties first reported in 2008 (paper I), which are the consequence of a powerful high-energy flare. Here we report follow-up observations of SDSSJ0952+2143, and discuss outburst scenarios in terms of stellar tidal disruption by a SMBH, peculiar variability of an AGN, and a supernova explosion. The optical spectrum of SDSSJ0952+2143 exhibits several peculiarities: an exceptional ratio of [FeVII] transitions over [OIII], a dramatic decrease by a factor of 10 of the highest-ionization lines, a very unusual and variable Balmer line profile including a triple-peaked narrow component with two unresolved horns, and a large Balmer decrement. The MIR emission measured with the Spitzer IRS in the narrow 10-20mu band is extraordinarily luminous (3.5 x 10^{43} erg\s). The IRS spectrum shows a bump around ~11mu and an increase towards longer wavelengths, reminiscent of silicate emission. The strong MIR excess over the NIR implies the dominance of relatively cold dust. The X-ray luminosity of 10^{41} erg\s measured with Chandra is below that typically observed in AGN. Similarities of SDSSJ0952+2143 with some extreme supernovae suggest the explosion of a supernova of Type IIn. However, an extreme accretion event in a low-luminosity AGN or inactive galaxy, especially stellar tidal disruption, remain possibilities, which could potentially produce a very similar emission-line response. If indeed a supernova, SDSSJ0952+2143 is one of the most distant X-ray and MIR detected SNe known so far, the most MIR luminous, and one of the most X-ray luminous. It is also by far the most luminous (>10^{40} erg\s) in high-ionization coronal lines, exceeding previous SNe by at least a factor of 100 [abridged].

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arXiv2009

The Evolution of Swift/BAT blazars and the origin of the MeV background

We use 3 years of data from the Swift/BAT survey to select a complete sample of X-ray blazars above 15 keV. This sample comprises 26 Flat-Spectrum Radio Quasars (FSRQs) and 12 BL Lac objects detected over a redshift range of 0.03<z<4.0. We use this sample to determine, for the first time in the 15--55 keV band, the evolution of blazars. We find that, contrary to the Seyfert-like AGNs detected by BAT, the population of blazars shows strong positive evolution. This evolution is comparable to the evolution of luminous optical QSOs and luminous X-ray selected AGNs. We also find evidence for an epoch-dependence of the evolution as determined previously for radio-quiet AGNs. We interpret both these findings as a strong link between accretion and jet activity. In our sample, the FSRQs evolve strongly, while our best-fit shows that BL Lacs might not evolve at all. The blazar population accounts for 10-20 % (depending on the evolution of the BL Lacs) of the Cosmic X-ray background (CXB) in the 15--55 keV band. We find that FSRQs can explain the entire CXB emission for energies above 500 keV solving the mystery of the generation of the MeV background. The evolution of luminous FSRQs shows a peak in redshift ($z_c$=4.3$\pm0.5$) which is larger than the one observed in QSOs and X-ray selected AGNs. We argue that FSRQs can be used as tracers of massive elliptical galaxies in the early Universe.

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arXiv2009

The redshift and afterglow of the extremely energetic gamma-ray burst GRB 080916C

The detection of GeV photons from gamma-ray bursts (GRBs) has important consequences for the interpretation and modelling of these most-energetic cosmological explosions. The full exploitation of the high-energy measurements relies, however, on the accurate knowledge of the distance to the events. Here we report on the discovery of the afterglow and subsequent redshift determination of GRB 080916C, the first GRB detected by the Fermi Gamma-Ray Space Telescope with high significance detection of photons at >0.1 GeV. Observations were done with 7-channel imager GROND at the 2.2m MPI/ESO telescope, the SIRIUS instrument at the Nagoya-SAAO 1.4m telescope in South Africa, and the GMOS instrument at Gemini-S. The afterglow photometric redshift of z=4.35+-0.15, based on simultaneous 7-filter observations with the Gamma-Ray Optical and Near-infrared Detector (GROND), places GRB 080916C among the top 5% most distant GRBs, and makes it the most energetic GRB known to date. The detection of GeV photons from such a distant event is rather surprising. The observed gamma-ray variability in the prompt emission together with the redshift suggests a lower limit for the Lorentz factor of the ultra-relativistic ejecta of Gamma > 1090. This value rivals any previous measurements of Gamma in GRBs and strengthens the extreme nature of GRB 080916C.

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arXiv2009

A candidate tidal disruption event in the Galaxy cluster Abell 3571

Tidal disruption events are possible sources of temporary nuclear activity in galactic nuclei and can be considered as good indicators of the existence of super massive black holes in the centers of galaxies. A new X-ray source has been serendipitously detected with ROSAT in a PSPC pointed observation of the galaxy cluster A3571. Given the strong flux decay of the object in subsequent detections, the tidal disruption scenario is investigated as possible explanationof the event. We followed up the evolution of the X-ray transient with ROSAT, XMM-Newton and Chandra for a total period of ~13 years. We also obtained 7-band optical/NIR photometry with GROND at the ESO/MPI 2.2m telescope. We report a very large decay of the X-ray flux of ROSAT source identified with the galaxy LEDA 095953 a member of the cluster Abell 3571. We measured a maximum 0.3-2.4 keV luminosity Log(L_X)=42.8 erg s^-1. The high state of the source lasted at least 150 ks, afterwards L_X declined as ~t^-2. The spectrum of the brightest epoch is consistent with a black body with temperature kT ~0.12 keV. The total energy released by this event in 10 yr was estimated to be Delta_E>2x10^50 erg. We interpret this event as a tidal disruption of a solar type star by the central supermassive black hole (i.e. ~10^7 M_sun) of the galaxy.

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