Diffuse Hard X-Ray Emission in Starburst Galaxies as Synchrotron From Very High Energy Electrons

Abstract

The origin of the diffuse hard X-ray (2-10 keV) emission from starburst galaxies is a long-standing problem. We suggest that synchrotron emission of 10-100 TeV electrons and positrons (e ± ) can contribute to this emission, because starbursts have strong magnetic fields. We consider three sources of e ± at these energies: (1) primary electrons directly accelerated by supernova remnants, (2) pionic secondary e ± created by inelastic collisions between cosmic ray (CR) protons and gas nuclei in the dense interstellar medium of starbursts, and (3) pair e ± produced between the interactions between 10 and 100 TeV γ -rays and the intense far-infrared (FIR) radiation fields of starbursts. We create one-zone steady-state models of the CR population in the Galactic center (R 112 pc), NGC 253, M82, and Arp 220's nuclei, assuming a power-law injection spectrum for electrons and protons. We consider different injection spectral slopes, magnetic field strengths, CR acceleration efficiencies, and diffusive escape times, and include advective escape, radiative cooling processes, and secondary and pair e ± . We compare these models to extant radio and GeV and TeV γ -ray data for these starbursts, and calculate the diffuse synchrotron X-ray and inverse Compton (IC) luminosities of these starbursts in the models which satisfy multiwavelength constraints. If the primary electron spectrum extends to ∼PeV energies and has a proton/electron injection ratio similar to the Galactic value, we find that synchrotron emission contributes 2%-20% of their unresolved, diffuse hard X-ray emission. However, there is great uncertainty in this conclusion because of the limited information on the CR electron spectrum at these high energies. IC emission is likewise a minority of the unresolved X-ray emission in these starbursts, from 0.1% in the Galactic center to 10% in Arp 220's nuclei, with the main uncertainty being the starbursts' magnetic field. We also model generic starbursts, including submillimeter galaxies, in the context of the FIR-X-ray relation, finding that anywhere between 0% and 16% of the total hard X-ray emission is synchrotron for different parameters, and up to 2% in the densest starbursts assuming an E −2.2 injection spectrum and a diffusive escape time of 10 Myr (E/3 GeV) −1/2 (h/100 pc). Neutrino observations by IceCube and TeV γ -ray data from HESS, VERITAS, and CTA can further constrain the synchrotron X-ray emission of starbursts. Our models do not constrain the possibility of hard, second components of primary e ± from sources like pulsars in starbursts, which could enhance the synchrotron X-ray emission further.

Cosmic rays and magnetic fields in the core and halo of the starburst M82: implications for galactic wind physics

“…The ISRF energy density (from reprocessed starlight) is calculated under the assumption that the starlight is produced by an optically-thin disk with a stellar density that exponentially decays with r with a scale-length of r core (φ). The ISRF model is normalized to an energy density of 1000 eV cm −3 at the center of the galaxy, which is comparable to Lacki & Thompson (2013) and Yoast-Hull et al (2013). Moreover, this value is consistent with a model where half (Melo et al 2002) of the total infrared luminosity of M82, ∼10 10.7 L (Sanders et al 2003), is isotropically emitted from a thin galactic disk (i.e.…”

“…We note that Domingo-Santamaría & Torres (2005) and Inoue (2011) calculated the gamma-ray optical depth due to the ISRF in M82 and a similar starburst galaxy, NGC 253, and found absorption was only important for gammaray emission above ∼10 TeV. Lacki & Thompson (2013) did a similar analysis and calculated the tertiary CRe spectrum and their diffuse emission. They find the tertiary CRe contribution to the the integrated gamma-ray spectrum negligible and the contribution to the integrated synchrotron spectrum only important in the X-ray band, which we do not analyze in this paper.…”

Spectral Energy Distribution and Radio Halo of NGC 253 at Low Radio Frequencies

“…Based on the assumptions of equipartition, Heesen et al (2009aHeesen et al ( , 2011 find the magnetic field within the halo to be very high, 7-12 μG, reaching as much as 160±20 μG in the central regions and 46±10 μG in the starburst outflow. The magnetic field in the central regions is strong enough for synchrotron emission to contribute a few percent to the total X-ray emission (Lacki & Thompson 2013). As discussed in the previous section, we also find that an external free-free absorbing screen is not a statistically preferred model.…”

Testing the hadronuclear origin of PeV neutrinos observed with IceCube

“…Furthermore, synchrotron cooling of cascade e AE may convert the energy into x rays and low-energy rays, for which the diffuse IGB is not constraining. In contrast, pp sources considered here are transparent up to * 10-100 TeV energies [26,27]. We arrive at the following implications for pp scenarios: (a) The spectral index should be hard, À & 2:1-2:2, consistent with the present IceCube data.…”