NICER study of pulsed thermal X-rays from Calvera: a neutron star born in the Galactic halo?

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 ($b=+37^{\circ }$) is unusual if Calvera is a relatively young pulsar, as suggested by its spin period (59 ms) and period derivative ($3.2\times 10^{-15}$ Hz s${}^{-1}$). Using the Neutron Star Interior Composition Explorer (NICER), we obtained a phase-connected timing solution spanning four years which allowed us to measure the second derivative of the frequency $\ddot{\nu }=-2.5\times 10^{-23}$ 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 find that a temperature map more anisotropic than that produced by a dipole field is required, with a hotter zone concentrated towards the poles. By adding two small polar caps, we find 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 confirm the presence of an absorption line at 0.7 keV associated to 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 $\gamma$-ray emission by folding seven years of Fermi-LAT data using the X-ray ephemeris, but no evidence for pulsations was found. Our results favour 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.

Accepted for publication in ApJ, 12 pages, 6 figures, 4 tables

References in corpus (11)

• Assessing the Role of Spin Noise in the Precision Timing of Millisecond Pulsars
• An Empirical Background Model for the NICER X-ray Timing Instrument
• Evolution of a buried magnetic field in the central compact object neutron stars
• Isolated neutron stars: Magnetic fields, distances, and spectra
• CCOs and the hidden magnetic field scenario
• Central Compact Objects in Supernova Remnants
• Discovery of an Isolated Compact Object at High Galactic Latitude
• Diagnostics of timing noise in middle aged pulsars
• Neutron Star Interior Composition Explorer X-ray Timing of the Radio and Gamma-ray Quiet Pulsars PSR J1412+7922 AND PSR J1849-0001
• A Strong Upper Limit on the Pulsed Radio Luminosity of the Compact Object 1RXS J141256.0+792204
• Deep optical observations of unusual neutron star Calvera with the GTC

Cited by in corpus (10)

• Timing six energetic rotation-powered X-ray pulsars, including the fast-spinning young PSR J0058-7218 and Big Glitcher PSR J0537-6910
• Thermal and non-thermal X-ray emission from the rotation-powered radio/$\gamma$-ray pulsar PSR J1740+1000
• Search for continuous gravitational waves from known pulsars in the first part of the fourth LIGO-Virgo-KAGRA observing run
• Fermi-LAT detection of G118.4+37.0: a supernova remnant in the Galactic halo seen around the Calvera pulsar
• Natal kicks of compact objects
• New upper limits on low-frequency radio emission from isolated neutron stars with LOFAR
• Detection of the extended $\gamma$-ray emission from the high Galactic latitude Calvera's SNR candidate
• Multi-wavelength study of the high Galactic latitude supernova remnant candidate G118.4+37.0 associated with the Calvera pulsar
• Supernova explosions of runaway stars and young neutron stars above the Galactic plane
• XMM-Newton observations of PSR J0554+3107: pulsing thermal emission from a cooling high-mass neutron star