Extended , steep-spectrum radio synchrotron sources are pre-dominantly found in massive galaxy clusters as opposed to groups .
LOFAR Two-Metre Sky Survey images have revealed a diffuse , ultra-steep spectrum radio source in the low-mass cluster Abell 1931 .
The source has a fairly irregular morphology with a largest linear size of about 550 kpc .
The source is only seen in LOFAR observations at 143 MHz and GMRT observations at 325 MHz .
The spectral index of the total source between 143 MHz and 325 MHz is \alpha _ { 143 } ^ { 325 } = -2.86 \pm 0.36 .
The source remains invisible in Very Large Array ( 1–2 GHz ) observations as expected given the spectral index .
Chandra X-ray observations of the cluster revealed a bolometric luminosity of L _ { X } = ( 1.65 \pm 0.39 ) \times 10 ^ { 43 } erg s ^ { -1 } and a temperature of 2.92 _ { -0.87 } ^ { +1.89 } keV which implies a mass of around \sim 10 ^ { 14 } M _ { \odot } .
We conclude that the source is a remnant radio galaxy that has shut off around 200 Myr ago .
The brightest cluster galaxy , a radio-loud elliptical galaxy , could be the source for this extinct source .
Unlike remnant sources studied in the literature , our source has a steep spectrum at low radio frequencies .
Studying such remnant radio galaxies at low radio frequencies is important for understanding the scarcity of such sources and their role in feedback processes .