Constraining sources of ultra high energy cosmic rays using high energy observations with the Fermi satellite

Abstract

We analyze the conditions that enable acceleration of particles to ultra-high energies, ∼ 10 20 eV (UHECRs). We show that broad band photon data recently provided by WMAP, ISOCAM, Swift and Fermi satellites, yield constraints on the ability of active galactic nuclei (AGN) to produce UHECRs. The high energy (MeV -GeV) photons are produced by Compton scattering of the emitted low energy photons and the cosmic microwave background or extra-galactic background light. The ratio of the luminosities at high and low photon energies can therefore be used as a probe of the physical conditions in the acceleration site. We find that existing data excludes core regions of nearby radio-loud AGN as possible acceleration sites of UHECR protons. However, we show that giant radio lobes are not excluded. We apply our method to Cen A, and show that acceleration of protons to ∼ 10 20 eV can only occur at distances 100 kpc from the core. The origin of ultra-high energy cosmic rays (UHECRs), with energies 10 18.5 eV, is still under debate. Astrophysical sources of UHECRs are limited by two requirements: a strong magnetic field is needed to confine the accelerated cosmic rays, while the magnetic field cannot be too strong in order to avoid excessive synchrotron radiation and photo-meson energy losses [1]. In addition, energy losses by photo-meson production through scattering off the cosmic microwave background (CMB) limit the distance of UHECR sources to 100 Mpc (the socalled GZK cutoff) [2,3]. Several possible sources of UHECRs that fulfill these constraints are discussed in the literature. The leading candidates are gamma-ray bursts (GRBs) [4][5][6], low luminosity gamma-ray bursts and hypernovae [7][8][9][10], and active galactic nuclei (AGNs) [11][12][13][14][15][16][17].The possibility that AGNs are the main source of UHECRs is challenged by the need for an unusually high photon luminosity. The required strong magnetic field, combined with the assumption that the energy density in the photon field is at least comparable to the energy density in the magnetic field, constrain the power output from the source to be L > 10 45 Γ 2 β −1 erg s −1 [13,18,19]. Here, Γ and βc are the Lorentz factor and characteristic velocity within the source. Only very few AGNs within the GZK horizon fulfill this requirement. A detailed analysis of those AGNs whose position was found to be correlated with the arrival direction of UHECRs [20], resulted in a similar conclusion [21].The underlying assumption of the above mentioned constraint is that the energy density in the photon field is comparable or higher than the energy density in the magnetic field. It is indeed difficult to obtain direct constraints on the value of the magnetic fields in AGNs and test this assumption.In this paper, we re-analyze the constraints on AGNs as UHECR sources by combining low energy (radio-optical) and high energy (∼ MeV band) data, enabled by recent Swift and Fermi observatories. These data provide constraints on the magnetic field strength, and avoid the need to adopt t...

Mass entrainment and turbulence-driven acceleration of ultra-high energy cosmic rays in Centaurus A

“…Cavallo 1978;Biermann & Strittmatter 1987;Stecker et al 1991;Mannheim 1995;Benford & Protheroe 2008;Hardcastle et al 2009;Kachelrieß et al 2009;Hardcastle 2010;Pe'er & Loeb 2012). The synchrotron and inverse-Compton emission from these structures is seen in all wavebands from low-frequency radio to TeV gamma-ray.…”

“…Due to its brightness and proximity, Cen A is an excellent laboratory for detailed studies of particle acceleration, production of A&A 558, A19 (2013) UHECRs, and the evolution of low-power radio galaxies in general. Several authors (Moskalenko et al 2009;Hardcastle et al 2009;O'Sullivan et al 2009;Gopal-Krishna et al 2010;Pe'er & Loeb 2012) have alluded to the possibility that the production of UHECRs occurs at Cen A's intermediate to large scales. Hardcastle et al (2009), O'Sullivan et al (2009) and more recently O'Sullivan (2011) have considered stochastic acceleration by magnetohydrodynamic (MHD) turbulence as the driver for this in the large-scale lobes.…”

“…Relativistic jets and giant lobes of radio galaxies are potential sources of ultra-high energy cosmic rays (UHECRs) and very-high energy (VHE) neutrinos (e.g. Cavallo 1978;Biermann & Strittmatter 1987;Stecker et al 1991;Mannheim 1995;Benford & Protheroe 2008;Hardcastle et al 2009;Kachelrieß et al 2009;Hardcastle 2010;Pe'er & Loeb 2012). The synchrotron and inverse-Compton emission from these structures is seen in all wavebands from low-frequency radio to TeV gamma-ray.…”

Fermi-LAT andSuzakuobservations of the radio galaxy Centaurus B

“…The motivation of the study came from the robust identification of several other radio galaxies with Fermi-LAT sources (Abdo et al 2009a(Abdo et al ,b, 2010cKataoka et al 2011), including the case of Centaurus A for which the 600 kpc-scale lobes have in fact been successfully resolved at GeV photon energies (Abdo et al 2010b,a). These studies have provided important clues for understanding energy dissipation processes within the extended lobes of radio-loud AGN in general and particle acceleration mechanisms related to the production of UHECRs in particular (see in this context Hardcastle et al 2009a;O'Sullivan et al 2009;Pe'er & Loeb 2012).…”

The Centaurus a Ultrahigh-Energy Cosmic-Ray Excess and the Local Extragalactic Magnetic Field

“…There are a number of possibilities for accelerating UHECR in a radio galaxy like Cen A (Colgate 2004;Hardcastle et al 2009;Fraschetti & Melia 2008;O'Sullivan et al 2009;Dermer et al 2009;Rieger & Aharonian 2009;Pe'er & Loeb 2012;Sahu et al 2012). Acceleration in the inner jets or near the black hole both would occur on a scale much smaller than the radio lobes.…”