Gamma-ray and radio background constraints on cosmic rays in Milky Way circumgalactic medium. (arXiv:2007.11015v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Jana_R/0/1/0/all/0/1">Ranita Jana</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Roy_M/0/1/0/all/0/1">Manami Roy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nath_B/0/1/0/all/0/1">Biman B. Nath</a>
We study the interaction of cosmic rays (CRs) with the diffuse circumgalactic
gas of Milky Way (MW) galaxy that results in hadronic $gamma-$ray emission and
radio synchrotron emission. We aim to constrain the CR population in our
circumgalactic medium (CGM) with the help of observed isotropic $gamma$-ray
background (IGRB) and radio continuum. We modify different models of CGM gas in
hydrostatic equilibrium discussed in literature by including a cosmic ray
population, parametrized by $eta equiv P_{rm CR}/P_{rm th}$. For the
simplest isothermal model, the IGRB only allows $eta lesssim 1$. On the other
hand, the precipitation model, in which the cooling of the CGM gas is regulated
with an optimum ratio of cooling time to free-fall time, the IGRB allows $eta
lesssim 230$. There is a small forbidden range near $etasim 1$ that can be
circumvented by changing boundary conditions. We also discuss the importance of
anisotropy in $gamma$-ray emission from CGM because of the Solar system’s
off-center position in the MW, and this consideration may pose problems for
models of CR dominated CGM even when flux limit is satisfied. The radio
continuum limits $eta lesssim 400$ for precipitation model and does not
constrain isothermal model, however these constraints are mitigated by
synchrotron loss time being comparable to CR diffusion time scales. These
bounds are relevant for current numerical simulations that indicate a
significant CR population in CGM of galaxies of MW mass.
We study the interaction of cosmic rays (CRs) with the diffuse circumgalactic
gas of Milky Way (MW) galaxy that results in hadronic $gamma-$ray emission and
radio synchrotron emission. We aim to constrain the CR population in our
circumgalactic medium (CGM) with the help of observed isotropic $gamma$-ray
background (IGRB) and radio continuum. We modify different models of CGM gas in
hydrostatic equilibrium discussed in literature by including a cosmic ray
population, parametrized by $eta equiv P_{rm CR}/P_{rm th}$. For the
simplest isothermal model, the IGRB only allows $eta lesssim 1$. On the other
hand, the precipitation model, in which the cooling of the CGM gas is regulated
with an optimum ratio of cooling time to free-fall time, the IGRB allows $eta
lesssim 230$. There is a small forbidden range near $etasim 1$ that can be
circumvented by changing boundary conditions. We also discuss the importance of
anisotropy in $gamma$-ray emission from CGM because of the Solar system’s
off-center position in the MW, and this consideration may pose problems for
models of CR dominated CGM even when flux limit is satisfied. The radio
continuum limits $eta lesssim 400$ for precipitation model and does not
constrain isothermal model, however these constraints are mitigated by
synchrotron loss time being comparable to CR diffusion time scales. These
bounds are relevant for current numerical simulations that indicate a
significant CR population in CGM of galaxies of MW mass.
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