LOFAR Detection of a Low-Power Radio Halo in the Galaxy Cluster Abell
  990
  Radio halos are extended ($\sim{\rm Mpc}$), steep-spectrum sources found in
the central region of dynamically disturbed clusters of galaxies. Only a
handful of radio halos have been reported to reside in galaxy clusters with a
mass $M_{500}\lesssim5\times10^{14}\,M_\odot$. In this paper we present a LOFAR
144 MHz detection of a radio halo in the galaxy cluster Abell 990 with a mass
of $M_{500}=(4.9\pm0.3)\times10^{14}\,M_\odot$. The halo has a projected size
of $\sim$700$\,{\rm kpc}$ and a flux density of $20.2\pm2.2\,{\rm mJy}$ or a
radio power of $1.2\pm0.1\times10^{24}\,{\rm W\,Hz}^{-1}$ at the cluster
redshift ($z=0.144$) which makes it one of the two halos with the lowest radio
power detected to date. Our analysis of the emission from the cluster with
Chandra archival data using dynamical indicators shows that the cluster is not
undergoing a major merger but is a slightly disturbed system with a mean
temperature of $5\,{\rm keV}$. The low X-ray luminosity of
$L_{X}=(3.66\pm0.08)\times10^{44}\,{\rm ergs\,s}^{-1}$ in the 0.1--2.4 keV band
implies that the cluster is one of the least luminous systems known to host a
radio halo. Our detection of the radio halo in Abell 990 opens the possibility
of detecting many more halos in poorly-explored less-massive clusters with
low-frequency telescopes such as LOFAR, MWA (Phase II) and uGMRT.
