Extreme TeV blazars and the intergalactic magnetic field

We study the four BL Lac objects (RGB J0152+017, 1ES 0229+200, 1ES 0347-121 and PKS 0548-322) detected in the TeV band but not present in the 1FGL catalogue of the Fermi/Large Area Telescope. We analize the 24 months of LAT data deriving gamma-ray fluxes or upper limits that we use to assemble their spectral energy distributions (SED). We model the SEDs with a standard one-zone leptonic model, also including the contribution of the reprocessed radiation in the multi GeV band, emitted by the pairs produced through the conversion of the primary TeV emission by interaction with the cosmic optical-IR background. For simplicity, in the calculation of this component we adopt an analytical approach including some simplifying assumptions, in particular i) the blazar high energy emission is considered on average stable over times of the order of 10^7 years and ii) the observer is exactly on-axis. We compare the physical parameters derived by the emission model with those of other high-energy emitting BL Lacs, confirming that TeV BL Lacs with a rather small GeV flux are characterized by extremely low values of the magnetic field and large values of the electron energies. The comparison between the flux in the GeV band and that expected from the reprocessed TeV emission allows us to confirm and strengthen the lower limit of B >10^{-15} G for the intergalactic magnetic field using a theoretically motivated spectrum for the primary high-energy photons.Comment: 11 pages, 7 figures, accepted for publication in M.N.R.A.

The broad-band X-ray spectrum of the blazar PKS B1830-211 by Chandra and INTEGRAL

In this paper we present a broad-band study of the X-ray emission of the blazar PKS1830-211 based on Chandra and Integral observations. Notwithstanding the high redshift (z=2.507), it is a bright X-ray source (F(2-10 keV)~10^{-11} erg cm^{-2} s^{-1}), due to gravitational lensing by an intervening galaxy at z=0.89. Previous X-ray observations attribute the observed absorption at E<2 keV to the lensing galaxy. Our analysis, although not in contrast with this hypothesis, suggests also the possibility of an intrinsic (ionized) absorption, taking place at the front side of the jet. This scenario is also supported by some evidence, in the same data, of a feature observed at 2.15 keV which can be interpreted as a blueshifted iron line (v/c ~ 0.18). The SED of PKS1830-211 can be well modelled by combining a Synchrotron Self-Compton component and an external source of photons to be scattered up to \gamma-ray energies by relativistic electrons moving outward in the jet. The main source of low energy photons is a dust torus at the temperature of 10^3 K as expected in MeV blazars.Comment: Accepted for publication in A&

Exploring the nature of the unidentified VHE gamma-ray source HESS J1507-622

The nature of the first unidentified VHE gamma-ray source with significant angular offset from the Galactic plane of 3.5 degrees, HESS J1507-622, is explored. Fermi-LAT data in the high-energy (HE, 100 MeV < E < 100 GeV) gamma-ray range collected over 34 month are used to describe the spectral energy distribution (SED) of the source. HESS J1507-622 is detected in the Fermi energy range and its spectrum is best described by a power law in energy with Gamma=1.7 +/- 0.1 stat +/- 0.2_sys and integral flux between (0.3-300) GeV of F = (2.0 +/-0.5_stat +/- 1.0_sys) x 10^-9 cm^-2 s^-1. With the available data it is not possible to discriminate between a hadronic and a leptonic scenario for HESS J1507-622. The location and compactness of the source indicate a considerable physical offset from the Galactic plane for this object. In case of a multiple-kpc distance, this challenges a pulsar wind nebula (PWN) origin for HESS J1507-622 since the time of travel for a pulsar born in the Galactic disk to reach such a location would exceed the inverse Compton (IC) cooling time of electrons that are energetic enough to produce VHE gamma-rays. However, an origin of this gamma-ray source connected to a pulsar that was born off the Galactic plane in the explosion of a hypervelocity star cannot be excluded. The nature of HESS J1507-622 is still unknown to date, and a PWN scenario cannot be ruled out in general. On the contrary HESS J1507-622 could be the first discovered representative of a population of spatially extended VHE gamma-ray emitters with HE gamma-ray counterpart that are located at considerable offsets from the Galactic plane. Future surveys in the VHE gamma-ray range are necessary to probe the presence or absence of such a source population.(abridged)Comment: accepted for publication in A&A, 7 pages, 3 figure

Parsec Scale Properties of Markarian 501

We present the results of a high angular resolution study of the BL Lac object Markarian 501 in the radio band. We consider data taken at 14 different epochs, ranging between 1.6 GHz and 22 GHz in frequency, and including new Space VLBI observations obtained on 2001 March 5 and 6 at 1.6 and 5 GHz. We study the kinematics of the parsec-scale jet and estimate its bulk velocity and orientation with respect to the line of sight. Limb brightened structure in the jet is clearly visible in our data and we discuss its possible origin in terms of velocity gradients in the jet. Quasi-simultaneous multi-wavelength observations allow us to map the spectral index distribution and to compare it to the jet morphology. Finally, we estimate the physical parameters of the parsec-scale jet.Comment: accepted for publication in ApJ; 24 pages with 17 figures (fig. 1 and fig. 2 available only as .jpg files

Discovering and Locating High-Energy Extra-Galactic Sources by Bayesian Mixture Modelling

Discovering and locating gamma-ray sources in the whole sky map is a declared target of the Fermi Gamma-ray Space Telescope collaboration. In this paper, we carry out an unsupervised analysis of the collection of high-energy photons accumulated by the Large Area Telescope, the principal instrument on board the Fermi spacecraft, over a period of around 7.5 years using a Bayesian mixture model. A fixed, though unknown, number of parametric components identify the extra-galactic emitting sources we are searching for, while a further component represents parametrically the dffuse gamma ray background due to both, extragalactic and galactic high energy photon emission. We determine the number of sources, their coordinates on the map and their intensities. The model parameters are estimated using a reversible jump MCMC algorithm which implements four different types of moves. These allow us to explore the dimension of the parameter space. The possible transitions remove from or add a source to the model, while leaving the background component unchanged. We furthermore present an heuristic procedure, based on the posterior distribution of the mixture weights, to qualify the nature of each detected source

Particle Dark Matter Constraints from the Draco Dwarf Galaxy

It is widely thought that neutralinos, the lightest supersymmetric particles, could comprise most of the dark matter. If so, then dark halos will emit radio and gamma ray signals initiated by neutralino annihilation. A particularly promising place to look for these indicators is at the center of the local group dwarf spheroidal galaxy Draco, and recent measurements of the motion of its stars have revealed it to be an even better target for dark matter detection than previously thought. We compute limits on WIMP properties for various models of Draco's dark matter halo. We find that if the halo is nearly isothermal, as the new measurements indicate, then current gamma ray flux limits prohibit much of the neutralino parameter space. If Draco has a moderate magnetic field, then current radio limits can rule out more of it. These results are appreciably stronger than other current constraints, and so acquiring more detailed data on Draco's density profile becomes one of the most promising avenues for identifying dark matter.Comment: 13 pages, 6 figure

Modeling the flaring activity of the high z, hard X-ray selected blazar IGR J22517+2217

We present new Suzaku and Fermi data, and re-analyzed archival hard X-ray data from INTEGRAL and Swift-BAT survey, to investigate the physical properties of the luminous, high-redshift, hard X-ray selected blazar IGR J22517+2217, through the modelization of its broad band spectral energy distribution (SED) in two different activity states. Through the analysis of the new Suzaku data and the flux selected data from archival hard X-ray observations, we build the source SED in two different states, one for the newly discovered flare occurred in 2005 and one for the following quiescent period. Both SEDs are strongly dominated by the high energy hump peaked at 10^20 -10^22 Hz, that is at least two orders of magnitude higher than the low energy (synchrotron) one at 10^11 -10^14 Hz, and varies by a factor of 10 between the two states. In both states the high energy hump is modeled as inverse Compton emission between relativistic electrons and seed photons produced externally to the jet, while the synchrotron self-Compton component is found to be negligible. In our model the observed variability can be accounted for by a variation of the total number of emitting electrons, and by a dissipation region radius changing from within to outside the broad line region as the luminosity increases. In its flaring activity, IGR J22517+2217 shows one of the most powerful jet among the population of extreme, hard X-ray selected, high redshift blazar observed so far.Comment: Accepted for publication in MNRA

Multiwavelength Observations of the Gamma-Ray Blazar PKS 0528+134 in Quiescence

We present multiwavelength observations of the ultraluminous blazar-type radio loud quasar PKS 0528+134 in quiescence during the period July to December 2009. Significant flux variability on a time scale of several hours was found in the optical regime, accompanied by a weak trend of spectral softening with increasing flux. We suggest that this might be the signature of a contribution from the accretion disk at the blue end of the optical spectrum. The optical flux is weakly polarized with rapid variations of the degree and direction of polarization, while the polarization of the 43 GHz radio core remains steady. Optical spectropolarimetry suggests a trend of increasing degree of polarization with increasing wavelength, providing additional evidence for an accretion disc contribution towards the blue end of the optical spectrum. We constructed four SEDs indicating that even in the quiescent state, the bolometric luminosity of PKS 0528+134 is dominated by its gamma-ray emission. A leptonic single-zone jet model produced acceptable fits to the SEDs with contributions to the high-energy emission from synchrotron self-Compton radiation and Comptonization of direct accretion disk emission. Fit parameters close to equipartition were obtained. The moderate variability on long time scales implies the existence of on-going particle acceleration, while the observed optical polarization variability seems to point towards a turbulent acceleration process. Turbulent particle acceleration at stationary features along the jet therefore appears to be a viable possibility for the quiescent state of PKS 0528+134.Comment: Accepted for Publication in The Astrophysical Journal. - Acknowledgement adde

AGILE detection of GeV gamma-ray emission from the SNR W28

Supernova remnants (SNRs) are believed to be the main sources of Galactic cosmic rays. Molecular clouds associated with SNRs can produce gamma-ray emission through the interaction of accelerated particles with the concentrated gas. The middle aged SNR W28, for its associated system of dense molecular clouds, provides an excellent opportunity to test this hypothesis. We present the AGILE/GRID observations of SNR W28, and compare them with observations at other wavelengths (TeV and 12CO J=1-->0 molecular line emission). The gamma-ray flux detected by AGILE from the dominant source associated with W28 is (14 +- 5) 10^-8 ph cm^-2 s^-1 for E > 400 MeV. This source is positionally well correlated with the TeV emission observed by the HESS telescope. The local variations of the GeV to TeV flux ratio suggest a difference between the CR spectra of the north-west and south molecular cloud complexes. A model based on a hadronic-induced interaction and diffusion with two molecular clouds at different distances from the W28 shell can explain both the morphological and spectral features observed by AGILE in the MeV-GeV energy range and by the HESS telescope in the TeV energy range. The combined set of AGILE and H.E.S.S. data strongly support a hadronic model for the gamma-ray production in W28.Comment: Accepted for publication in Astronomy & Astrophysics Letter

The bright unidentified gamma-ray source 1FGL J1227.9-4852: Can it be associated with an LMXB?

We present an analysis of high energy (HE; 0.1-300 GeV) gamma-ray observations of 1FGL J1227.9-4852 with the Fermi Gamma-ray Space Telescope, follow-up radio observations with the Australia Telescope Compact Array, Giant Metrewave Radio Telescope and Parkes radio telescopes of the same field and follow-up optical observations with the ESO VLT. We also examine archival XMM-Newton and INTEGRAL X-ray observations of the region around this source. The gamma-ray spectrum of 1FGL J1227.9-4852 is best fit with an exponentially cutoff power-law, reminiscent of the population of pulsars observed by Fermi. A previously unknown, compact radio source within the 99.7% error circle of 1FGL J1227.9-4852 is discovered and has a morphology consistent either with an AGN core/jet structure or with two roughly symmetric lobes of a distant radio galaxy. A single bright X-ray source XSS J12270-4859, a low-mass X-ray binary, also lies within the 1FGL J1227.9-4852 error circle and we report the first detection of radio emission from this source. The potential association of 1FGL J1227.9-4852 with each of these counterparts is discussed. Based upon the available data we find the association of the gamma-ray source to the compact double radio source unlikely and suggest that XSS J12270-4859 is a more likely counterpart to the new HE source. We propose that XSS J12270-4859 may be a millisecond binary pulsar and draw comparisons with PSR J1023+0038.Comment: Accepted for publication in MNRAS; 9 pages, 8 figures, 2 table