Possible Calvera supernova remnant. (a) Radio emission from the diffuse ring at 144 MHz in the LOFAR HBA LoTSS imaging mosaic (1°25 × 1°25 field of view, 20″ resolution). The location of Calvera is indicated with a circle, with the arrow representing the Halpern & Gotthelf (2015) proper motion from 10kyr in the past to 10kyr into the future. The dashed lines indicate the propagated 1σ positional uncertainty due to proper motion. The cross sign indicates the estimated center of the diffuse ring, with the dotted circles representing the inner and outer radii. The dashed circle (4′ radius) denotes the location of the diffuse X-ray emission seen by Zane et al. (2011) from 1RXS J140818.1+792113, with the square indicating the location of Ha emission. (b) The diffuse ring in the Westerbork Synthesis Radio Telescope WENSS survey imaging at 325 MHz and (c) the Very Large Array NVSS survey at 1.4 GHz. Both panels have a 1°25 × 1°25 field of view and an identical pointing and orientation to panel a. (d) The 0.1-2.4 keV X-ray image from RASS (Voges et al. 1999), with the ring center and inner outer radii of the radio ring denoted by the cross and dotted circles. The dashed circle encircles the emission from 1RXS J140818.1+792113. (e) A 2′ × 2′ section of the 6″ resolution LoTSS imaging mosaic at the location of Calvera, indicated with the dotted circle (10″ radius).