Upper limits on X-ray emission from two rotating radio transients

Abstract

X-ray emission from the enigmatic rotating radio transients (RRATs) offers a vital clue to understanding these objects and how they relate to the greater neutron star population. An X-ray counterpart to RRAT J1819−1458 is known, and its properties are similar to those of other middle-aged (0.1 Myr) neutron stars. We have searched for X-ray emission with Chandra/Advanced CCD Imaging Spectrometer at the positions of two RRATs with arcsecond (or better) localization, J0847−4316 and J1846−0257. Despite deep searches (especially for RRAT J1846−0257) we did not detect any emission with 0.3–8 keV count-rate limits of 1 and 0.068 counts ks[superscript −1], respectively, at 3σ confidence. Assuming thermal emission similar to that seen from RRAT J1819−1458 (a blackbody with radius ≈20 km), we derive effective temperature limits of 77 and 91 eV for the nominal values of the distances and column densities to both sources, although both of those quantities are highly uncertain and correlated. If we instead fix the temperature of the emission (a blackbody with kT= 0.14 keV), we derive unabsorbed luminosity limits in the 0.3–8 keV range of 1 × 10[superscript 32] and 3 × 1032 erg s[superscript −1]. These limits are considerably below the luminosity of RRAT J1819−1458(4 × 10[superscript 33] erg s[superscript −1]), suggesting that RRATs J0847−4316 and J1846−0257 have cooled beyond the point of visibility (plausible given the differences in characteristic age). However, as we have not detected X-ray emission, it may also be that the emission from RRATs J0847−4316 and J1846−0257 has a different character from that of RRAT J1819−1458. The two non-detections may prove a counterpoint to RRAT J1819−1458, but more detections are certainly needed before we can begin to derive general X-ray emission properties for the RRAT populations.