CHANDRA OBSERVATIONS OF 1RXS J141256.0+792204 (CALVERA)

We report the results of a 30 ks Chandra/ACIS-S observation of the isolated compact object (ICO) 1RXS J141256.0+792204 (Calvera). The X-ray spectrum is adequately described by an absorbed neutron star hydrogen atmosphere model with kT∞eff = 88.3 ± 0.8 eV and radiation radius R∞/d = 4.1 ± 0.1 km kpc−1. The best-fit blackbody spectrum yields parameters consistent with previous measurements; although the fit itself is not statistically acceptable, systematic uncertainties in the pile-up correction may contribute to this. We find marginal evidence for a narrow spectral feature in the X-ray spectrum between 0.3 and 1.0 keV. In one interpretation, we find evidence at 81% confidence for an absorption edge at E = 0.64+0.08-0.06 keV with equivalent width EW ≈ 70 eV; if this feature is real, it is reminiscent of features seen in the isolated neutron stars RX J1605.3+3249, RX J0720.4 − 3125, and 1RXS J130848.6+212708 (RBS 1223). In an alternative approach, we find evidence at 88% confidence for an unresolved emission line at energy E = 0.53 ± 0.02 keV, with equivalent width EW ≈ 28 eV; the interpretation of this feature, if real, is uncertain. We search for coherent pulsations up to the Nyquist frequency νNyq = 1.13 Hz and set an upper limit of 8.0% rms on the strength of any such modulation. We derive an improved position for the source and set the most rigorous limits to date on any associated extended emission on arcsecond scales. Our analysis confirms the basic picture of Calvera as the first ICO in the ROSAT/Bright Source Catalog discovered in six years, the hottest such object known, and an intriguing target for multiwavelength study.