Gamma-ray emission from AGNs

Blazars, radio-loud active galactic nuclei with the relativistic jet closely aligned with the line of sight, dominate the extragalactic sky observed at gamma-ray energies, above 100 MeV. We discuss some of the emission properties of these sources, focusing in particular on the "blazar sequence" and the interpretative models of the high-energy emission of BL Lac objects.Comment: 8 pages, 4 figures, to appear in the proceedings of the HEPRO II conference, Buenos Aires, October 26-30 200

Conservative upper limits on WIMP annihilation cross section from Fermi-LAT γ\gamma-rays

The spectrum of an isotropic extragalactic $\gamma$-ray background (EGB) has been measured by the Fermi-LAT telescope at high latitudes. Two new models for the EGB are derived from the subtraction of unresolved point sources and extragalactic diffuse processes, which could explain from 30% to 70% of the Fermi-LAT EGB. Within the hypothesis that the two residual EGBs are entirely due to the annihilation of dark matter (DM) particles in the Galactic halo, we obtain $conservative$ upper limits on their annihilation cross section \sigmav. Severe bounds on a possible Sommerfeld enhancement of the annihilation cross section are set as well. Finally, would {\sigmav} be inversely proportional to the WIMP velocity, very severe limits are derived for the velocity-independent part of the annihilation cross section.Comment: Proceedings of XII Taup Conference, Munich, September 201

Cosmic rays in the surroundings of SNR G35.6-0.4

HESS J1858+020 is a TeV gamma-ray source that was reported not to have any clear cataloged counterpart at any wavelength. However, it has been recently proposed that this source is indirectly associated with the radio source, re-identified as a supernova remnant (SNR), G35.6-0.4. The latter is found to be middle-aged ($\sim 30$ kyr) and to have nearby molecular clouds (MCs). HESS J1858+020 was proposed to be the result of the interaction of protons accelerated in the SNR shell with target ions residing in the clouds. The Fermi Large Area Telescope (LAT) First Source Catalog does not list any source coincident with the position of HESS J1858+020, but some lie close. Here, we analyse more than 2 years of data obtained with the Fermi-LAT for the region of interest, and consider whether it is indeed possible that the closest LAT source, 1FGL J1857.1+0212c, is related to HESS J1858+020. We conclude it is not, and we impose upper limits on the GeV emission originating from HESS J1858+020. Using a simplified 3D model for the cosmic-ray propagation out from the shell of the SNR, we consider whether the interaction between SNR G35.6-0.4 and the MCs nearby could give rise to the TeV emission of HESS J1858+020 without producing a GeV counterpart. If so, the pair of SNR/TeV source with no GeV detection would be reminiscent of other similarly-aged SNRs, such as some of the TeV hotspots near W28, for which cosmic-ray diffusion may be used to explain their multi-frequency phenomenology. However, for HESS J1858+020, we found that although the phase space in principle allows for such GeV--TeV non-correlation to appear, usual and/or observationally constrained values of the parameters (e.g., diffusion coefficients and cloud-SNR likely distances) would disfavor it.Comment: In press in MNRA

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

Impact of the orbital uncertainties on the timing of pulsars in binary systems

The detection of pulsations from an X-ray binary is an unambiguous signature of the presence of a neutron star in the system. When the pulsations are missed in the radio band, their detection at other wavelengths, like X-ray or gamma-rays, requires orbital demodulation, since the length of the observations are often comparable to, or longer than the system orbital period. The detailed knowledge of the orbital parameters of binary systems plays a crucial role in the detection of the spin period of pulsars, since any uncertainty in their determination translates into a loss in the coherence of the signal during the demodulation process. In this paper, we present an analytical study aimed at unveiling how the uncertainties in the orbital parameters might impact on periodicity searches. We find a correlation between the power of the signal in the demodulated arrival time series and the uncertainty in each of the orbital parameters. This correlation is also a function of the pulsar frequency. We test our analytical results with numerical simulations, finding good agreement between them. Finally, we apply our study to the cases of LS 5039 and LS I +61 303 and consider the current level of uncertainties in the orbital parameters of these systems and their impact on a possible detection of a hosted pulsar. We also discuss the possible appearance of a sideband ambiguity in real data. The latter can occur when, due to the use of uncertain orbital parameters, the power of a putative pulsar is distributed in frequencies lying nearby the pulsar period. Even if the appearance of a sideband is already a signature of a pulsar component, it may introduce an ambiguity in the determination of its period. We present here a method to solve the sideband issue.Comment: Accepted 2012 September 08 by MNRAS. The paper contains 18 figures and 5 table

Fermi Large Area Telescope Observations of the Cygnus Loop Supernova Remnant

We present an analysis of the gamma-ray measurements by the Large Area Telescope(LAT) onboard the \textit{Fermi Gamma-ray Space Telescope} in the region of the supernova remnant(SNR) Cygnus Loop(G74.0$-$8.5). We detect significant gamma-ray emission associated with the SNR in the energy band 0.2--100 GeV. The gamma-ray spectrum shows a break in the range 2--3 GeV. The gamma-ray luminosity is $\sim$ $1 \times 10^{33}$erg s$^{-1}$ between 1--100 GeV, much lower than those of other GeV-emitting SNRs. The morphology is best represented by a ring shape, with inner/outer radii 0$^\circ$.7 $\pm$ 0$^\circ$.1 and 1$^\circ$.6 $\pm$ 0$^\circ$.1. Given the association among X-ray rims, \halpha filaments and gamma-ray emission, we argue that gamma rays originate in interactions between particles accelerated in the SNR and interstellar gas or radiation fields adjacent to the shock regions. The decay of neutral pions produced in nucleon-nucleon interactions between accelerated hadrons and interstellar gas provides a reasonable explanation for the gamma-ray spectrum.Comment: accepted by ApJ, 34 pages, 6 figure

PSR J2030+3641: radio discovery and gamma-ray study of a middle-aged pulsar in the now identified Fermi-LAT source 1FGL J2030.0+3641

In a radio search with the Green Bank Telescope of three unidentified low Galactic latitude Fermi-LAT sources, we have discovered the middle-aged pulsar J2030+3641, associated with 1FGL J2030.0+3641 (2FGL J2030.0+3640). Following the detection of gamma-ray pulsations using a radio ephemeris, we have obtained a phase-coherent timing solution based on gamma-ray and radio pulse arrival times that spans the entire Fermi mission. With a rotation period of 0.2 s, spin-down luminosity of 3e34 erg/s, and characteristic age of 0.5 Myr, PSR J2030+3641 is a middle-aged neutron star with spin parameters similar to those of the exceedingly gamma-ray-bright and radio-undetected Geminga. Its gamma-ray flux is 1% that of Geminga, primarily because of its much larger distance, as suggested by the large integrated column density of free electrons, DM=246 pc/cc. We fit the gamma-ray light curve, along with limited radio polarimetric constraints, to four geometrical models of magnetospheric emission, and while none of the fits have high significance some are encouraging and suggest that further refinements of these models may be worthwhile. We argue that not many more non-millisecond radio pulsars may be detected along the Galactic plane that are responsible for LAT sources, but that modified methods to search for gamma-ray pulsations should be productive -- PSR J2030+3641 would have been found blindly in gamma rays if only >0.8 GeV photons had been considered, owing to its relatively flat spectrum and location in a region of high soft background.Comment: Accepted for publication in ApJ, 9 pages, 6 figure

Blazar properties: an update from recent results

After a brief critical overview of the main properties of blazars and their classification, some significant results from recent multiwavelength observations are summarized, in the context of the jet physics.Comment: 5 pages, 2 figures. Invited talk at the 2nd Heidelberg workshop: "High-Energy Gamma-rays and Neutrinos from Extra-Galactic Sources", January 13-16, 2009, to be published in Int. J. Mod. Phys. D. Updated reference

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.Comment: 4 pages, 2 figures. Accepted for publication on MNRAS. Section 1 completely rewritten and enlarge

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.Comment: 12 pages, 10 figures, 3 tables. Submitted to MNRAS on the 22nd March 2010