Clues to the origin of Fermi Bubbles from OVIII/OVII line ratio
  We constrain the origin of Fermi Bubbles using 2D hydrodynamical simulations
of both star formation driven and black hole accretion driven wind models. We
compare our results with recent observations of OVIII to OVII line ratio within
and near Fermi Bubbles. Our results suggest that independent of the driving
mechanisms, a low luminosity ($\mathcal{L} \sim 0.7-1\times 10^{41}$ erg
s$^{-1}$) energy injection best reproduces the observed line ratio for which
the shock temperature is $\approx 3\times 10^6$ K. Assuming the Galactic halo
temperature to be $2\times 10^6$K, we estimate the shock velocity to be $\sim
300$ km s$^{-1}$ for a weak shock. The corresponding estimated age of the Fermi
bubbles is $\sim 15-25$ Myr. Such an event can be produced either by a star
formation rate of $\sim 0.5$ M$_\odot$ yr$^{-1}$ at the Galactic centre or a
very low luminosity jet/accretion wind arising from the central black hole. Our
analysis rules out any activity that generates an average mechanical luminosity
$\gtrsim 10^{41}$ \ergps as a possible origin of the Fermi Bubbles.
