The Cosmic Ray Distribution in Sagittarius B

Abstract

The HESS instrument has observed a diffuse flux of ~ TeV gamma rays from a large solid angle around the Galactic center (GC). This emission is correlated with the distribution of gas in the region suggesting that the gamma rays originate in collisions between cosmic ray hadrons (CRHs) and ambient matter. Of particular interest, HESS has detected gamma rays from the Sagittarius (Sgr) B Molecular Cloud Complex. Prompted by the suggestion of a hadronic origin for the gamma rays, we have examined archival 330 and 74 MHz Very Large Array radio data and 843 MHz Sydney University Molonglo Sky Survey data covering Sgr B, looking for synchrotron emission from secondary electrons and positrons (expected to be created in the same interactions that supply the observed gamma rays). Intriguingly, we have uncovered non-thermal emission, but at a level exceeding expectation. Adding to the overall picture, recent observations by the Atacama Pathfinder Experiment telescope show that the cosmic ray ionization rate is ten times greater in the Sgr B2 region of Sgr B than the local value. Lastly, Sgr B2 is also a very bright X-ray source. We examine scenarios for the spectra of CRHs and/or primary electrons that would reconcile all these different data. We determine that (i) a hard (~ E^-2.2), high-energy (> TeV) population CRHs is unavoidably required by the HESS gamma ray data and (ii) the remaining broad-band, non-thermal phenomenology is explained either by a rather steep (~ E^-2.9) spectrum of primary electrons or a (~ E^-2.7) population of CRHs. No single, power-law population of either leptons or hadrons can explain the totality of broadband, non-thermal Sgr B phenomenology.Comment: 45 pages, 5 figures; Minor revisions - version accepted for publication in Ap

A lower limit of 50 microgauss for the magnetic field near the Galactic Centre

“…Alternatively, this phenomenology is also consistent with a scenario 66 that sees cosmic rays accelerated diffusively in relatively tenuous GC interstellar medium phases before interacting primarily in the periphery of molecular clouds, possibly because neither primary electrons nor primary ions (that might generate synchrotron-radiating secondary electrons) can penetrate into dense cloud cores within the particles' loss times. Such a scenario is supported by recent work we have undertaken concerning the Sagittarius B Giant Molecular Cloud 58,67 .…”

“…Fitting of synchrotron emission from cooled electron population Our calculational procedure -described at length elsewhere 58 -is performed within the 'thick target' regime wherein electrons are taken to cool in situ rather than escape via diffusion. That we are in this regime is guaranteed by the fact that we do observe a break; free escape would imply an electron spectrum which is unmodified by injection.…”

Fermi-LAT observations of the Sagittarius B complex

“…For the purposes of our analysis we chose -as in Crocker et al (2007) -to define the Sgr B region as 0.4…”

Cosmic ray models of the ridge-like excess of gamma rays in the Galactic Centre

“…4, in the two flare case, the escape time is greater than the electron/positron energy loss time for all energies. Based on this comparison, we neglect the diffusive transport in our estimation and can thus solve the diffusion equation in the thick target limit (e.g., Crocker et al 2007):…”