NICER Study of Pulsed Thermal X-Rays from Calvera: A Neutron Star Born in the Galactic Halo?
Journal
Date Issued
2021
Author(s)
•
Taverna, Roberto
•
Baldeschi, Luca
•
•
Turolla, Roberto
•
Zane, Silvia
•
Abstract
Calvera (1RXS J141256.0+792204) is an isolated neutron star detected only through its thermal X-ray emission. Its location at high Galactic latitude (b = +37 ) is unusual if Calvera is a relatively young pulsar, as suggested by its spin period (59 ms) and period derivative (3.2 10-15 s s-1). Using the Neutron Star Interior Composition Explorer, we obtained a phase-connected timing solution spanning four years, which allowed us to measure the second derivative of the frequency d? ? =-2.5× 10-23 Hz s-2 and to reveal timing noise consistent with that of normal radio pulsars. A magnetized hydrogen atmosphere model, covering the entire star surface, provides a good description of the phase-resolved spectra and energy-dependent pulsed fraction. However, we found that a temperature map more anisotropic than that produced by a dipole field is required, with a hotter zone concentrated toward the poles. By adding two small polar caps, we found that the surface effective temperature and that of the caps are ~0.1 and ~0.36 keV, respectively. The inferred distance is ~3.3 kpc. We confirmed the presence of an absorption line at 0.7 keV associated with the emission from the whole star surface, difficult to interpret as a cyclotron feature and more likely originating from atomic transitions. We searched for pulsed ?-ray emission by folding seven years of Fermi-LAT data using the X-ray ephemeris, but no evidence for pulsations was found. Our results favor the hypothesis that Calvera is a normal rotation-powered pulsar, with the only peculiarity of being born at a large height above the Galactic disk.
Volume
922
Issue
2
Start page
253
Issn Identifier
0004-637X
Rights
open.access
File(s)![Thumbnail Image]()
Loading...
Name
6-Mereghetti_2021_ApJ_922_253.pdf
Description
PDF editoriale
Size
954.25 KB
Format
Adobe PDF
Checksum (MD5)
b4f8f78ea3e3043c870f8cba4ebd794e