Imprint of galaxy clustering in the cosmic gamma-ray background

Star-forming galaxies are predicted to contribute considerably to the cosmic gamma-ray background (CGB) as they are confirmed γ-ray emitters and are the most numerous population of γ-ray sources, although individually faint. Even though the Fermi Gamma-ray Space Telescope will be able to resolve few star-forming galaxies individually, their fractional contribution to the CGB should become far more significant than it was for past measurements of the CGB as many of the brighter, formerly unresolved sources will be resolved out. Thus, the clustering feature of galaxies imprinted on the CGB might be detectable by Fermi. In anticipation of such measurements, we calculate the predicted angular auto- and cross-power spectra of the CGB from normal galaxies. We find that the amplitude of the auto-power spectrum is smaller than that for other sources, such as blazars and dark matter annihilation; the shape is also characteristic. We also show that the cross-power spectrum with galaxy surveys features larger amplitude. Fermi should be able to detect the correlation signature in both the auto- and cross-power spectra at angular scales of ~1°–10° after 5 years of operation. Such a detection would be valuable in confirming the level of the star-forming galaxy contribution to the CGB and, more importantly, in serving as a tool in the effort to discriminate between possible origins of the CGB

The Spectral Index Distribution of EGRET Blazars: Prospects for GLAST

The intrinsic distribution of spectral indices in GeV energies of gamma-ray--loud blazars is a critical input in determining the spectral shape of the unresolved blazar contribution to the diffuse extragalactic gamma-ray background, as well as an important test of blazar emission theories. We present a maximum-likelihood method of determining the intrinsic spectral index distribution (ISID) of a population of gamma-ray emitters which accounts for error in measurement of individual spectral indices, and we apply it to EGRET blazars. We find that the most likely Gaussian ISID for EGRET blazars has a mean of 2.27 and a standard deviation of 0.20. We additionally find some indication that FSRQs and BL Lacs may have different ISIDs (with BL Lacs being harder). We also test for spectral index hardening associated with blazar variability for which we find no evidence. Finally, we produce simulated GLAST spectral index datasets and perform the same analyses. With improved statistics due to the much larger number of resolvable blazars, GLAST data will help us determine the ISIDs with much improved accuracy. Should any difference exist between the ISIDs of BL Lacs and FSRQs or between the ISIDs of blazars in the quiescent and flaring states, GLAST data will be adequate to separate these ISIDs at a significance better than 3 sigma.Comment: 11 pages, 9 figures, emulateapj; accepted for publication in Ap

Probing the Intergalactic Magnetic Field with the Anisotropy of the Extragalactic Gamma-ray Background

The intergalactic magnetic field (IGMF) may leave an imprint on the angular anisotropy of the extragalactic gamma-ray background through its effect on electromagnetic cascades triggered by interactions between very high energy photons and the extragalactic background light. A strong IGMF will deflect secondary particles produced in these cascades and will thus tend to isotropize lower energy cascade photons, thereby inducing a modulation in the anisotropy energy spectrum of the gamma-ray background. Here we present a simple, proof-of-concept calculation of the magnitude of this effect and demonstrate that current Fermi data already seem to prefer non-negligible IGMF values. The anisotropy energy spectrum of the Fermi gamma-ray background could thus be used as a probe of the IGMF strength.Comment: 11 pages, 4 figures, MN LaTeX style; accepted for publication in MNRA

High Energy Polarization of Blazars : Detection Prospects

Emission from blazar jets in the ultraviolet, optical, and infrared is polarized. If these low-energy photons were inverse-Compton scattered, the upscattered high-energy photons retain a fraction of the polarization. Current and future X-ray and gamma-ray polarimeters such as INTEGRAL-SPI, PoGOLITE, X-Calibur, Gamma-Ray Burst Polarimeter, GEMS-like missions, ASTRO-H, and POLARIX have the potential to discover polarized X-rays and gamma-rays from blazar jets for the first time. Detection of such polarization will open a qualitatively new window into high-energy blazar emission; actual measurements of polarization degree and angle will quantitatively test theories of jet emission mechanisms. We examine the detection prospects of blazars by these polarimetry missions using examples of 3C 279, PKS 1510-089, and 3C 454.3, bright sources with relatively high degrees of low-energy polarization. We conclude that while balloon polarimeters will be challenged to detect blazars within reasonable observational times (with X-Calibur offering the most promising prospects), space-based missions should detect the brightest blazars for polarization fractions down to a few percent. Typical flaring activity of blazars could boost the overall number of polarimetric detections by nearly a factor of five to six purely accounting for flux increase of the brightest of the comprehensive, all-sky, Fermi-LAT blazar distribution. The instantaneous increase in the number of detections is approximately a factor of two, assuming a duty cycle of 20% for every source. The detectability of particular blazars may be reduced if variations in the flux and polarization fraction are anticorrelated. Simultaneous use of variability and polarization trends could guide the selection of blazars for high-energy polarimetric observations.Comment: Matches published version in Ap

Magnetic field tomography in two clouds towards Ursa Major using HI fibers

The atomic interstellar medium (ISM) is observed to be full of linear structures that are referred to as "fibers". Fibers exhibit similar properties to linear structures found in molecular clouds known as striations. Suggestive of a similar formation mechanism, both striations and fibers appear to be ordered, quasi-periodic, and well-aligned with the magnetic field. The prevailing formation mechanism for striations involves the excitation of fast magnetosonic waves. Based on this theoretical model, and through a combination of velocity centroids and column density maps, Tritsis et al. (2018) developed a method for estimating the plane-of-sky (POS) magnetic field from molecular cloud striations. We apply this method in two H\textsc{I} clouds with fibers along the same line-of-sight (LOS) towards the ultra-high-energy cosmic-ray (UHECR) hotspot, at the boundaries of Ursa Major. For the cloud located closer to Earth, where Zeeman observations from the literature were also available, we find general agreement in the distributions of the LOS and POS components of the magnetic field. We find relatively large values for the total magnetic field (ranging from $\sim$$\rm{10}$ to $\sim$$\rm{20} ~\rm{\mu G}$) and an average projection angle with respect to the LOS of $\sim$ 50$^\circ$. For the cloud located further away, we also find a large value for the POS component of the magnetic field of $15^{+8}_{-3}~\rm{\mu G}$. We discuss the potential of our new magnetic-field tomography method for large-scale application. We consider the implications of our findings for the accuracy of current reconstructions of the Galactic magnetic field and on the propagation of UHECR through the ISM.Comment: 11 pages, 8 figures, published in Ap