Long duration radio transients lacking optical counterparts are possibly Galactic Neutron Stars

摘要

(abridged) Recently, a new class of radio transients in the 5-GHz band wasdetected by Bower et al. We present new deep near-Infrared (IR) observations ofthe field containing these transients, and find no counterparts down to alimiting magnitude of K=20.4 mag. We argue that the bright (>1 Jy) radiotransients recently reported by Kida et al. are consistent with beingadditional examples of the Bower et al. transients. We refer to these groups ofevents as "long-duration radio transients". The main characteristics of thispopulation are: time scales longer than 30 minute but shorter than severaldays; rate, ~10^3 deg^-2 yr^-1; progenitors sky surface density of >60 deg^-2(95% C.L.) at Galactic latitude ~40 deg; 1.4-5 GHz spectral slopes, f_\nu ~\nu^alpha, with alpha>0; and most notably the lack of any counterparts inquiescence in any wavelength. We rule out an association with many types ofobjects. Galactic brown-dwarfs or some sort of exotic explosions remainplausible options. We argue that an attractive progenitor candidate for theseradio transients is the class of Galactic isolated old neutron stars (NS). Weconfront this hypothesis with Monte-Carlo simulations of the space distributionof old NSs, and find satisfactory agreement for the large areal density.Furthermore, the lack of quiescent counterparts is explained quite naturally.In this framework we find: the mean distance to events in the Bower et al.sample is of order kpc; the typical distance to the Kida et al. transients areconstrained to be between 30 pc and 900 pc (95% C.L.); these events shouldrepeat with a time scale of order several months; and sub-mJy level burstsshould exhibit Galactic latitude dependence. We discuss possible mechanismsgiving rise to the observed radio emission.