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Radio halos are extended ($\sim{\rm Mpc}$), steep-spectrum sources found inthe central region of dynamically disturbed clusters of galaxies. Only ahandful of radio halos have been reported to reside in galaxy clusters with amass $M_{500}\lesssim5\times10^{14}\,M_\odot$. In this paper we present a LOFAR144 MHz detection of a radio halo in the galaxy cluster Abell 990 with a massof $M_{500}=(4.9\pm0.3)\times10^{14}\,M_\odot$. The halo has a projected sizeof $\sim$700$\,{\rm kpc}$ and a flux density of $20.2\pm2.2\,{\rm mJy}$ or aradio power of $1.2\pm0.1\times10^{24}\,{\rm W\,Hz}^{-1}$ at the clusterredshift ($z=0.144$) which makes it one of the two halos with the lowest radiopower detected to date. Our analysis of the emission from the cluster withChandra archival data using dynamical indicators shows that the cluster is notundergoing a major merger but is a slightly disturbed system with a meantemperature 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 bandimplies that the cluster is one of the least luminous systems known to host aradio halo. Our detection of the radio halo in Abell 990 opens the possibilityof detecting many more halos in poorly-explored less-massive clusters withlow-frequency telescopes such as LOFAR, MWA (Phase II) and uGMRT.

LOFAR Detection of a Low-Power Radio Halo in the Galaxy Cluster Abell 990