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°) 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 ν ̈ = − 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 γ-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.

Possible discovery of Calvera's supernova remnant

“…Models for the Galactic electron distribution predict maximum dispersion measures (DMs) of 42.2 pc cm −3 at 10.1 kpc (NE2001; Cordes & Lazio 2002) and 35.5 pc cm −3 at 15.2 kpc (YMW16, Yao et al 2017) The observation was folded with the Halpern & Gotthelf (2015) X-ray timing ephemeris using dspsr (van Straten & Bailes 2011) to 10 s sub-integrations. Though the observation epoch falls outside of the timing span of both the Halpern & Gotthelf (2015) and Mereghetti et al (2021) timing ephemerides, extrapolating these to the observation epoch yields consistent folding periods (∆P = −17.6 ± 6.9 ns). Using psrchive (Hotan et al 2004) tools, radio frequency interference (RFI) was masked and the folded observation was subsequently dedispersed to 401 trial dispersion measures (DMs) between 0 and 80 pc cm −3 at 0.2 pc cm −3 steps.…”

“…Initially identified as a candidate thermally emitting isolated neutron star by Rutledge et al (2008), selected on its soft X-ray spectrum and high X-ray to optical flux ratio from the ROSAT All-Sky Survey (Voges et al 1999), it was nicknamed "Calvera" due to its similarities with the 7 nearby X-ray dim isolated neutron stars (XDINS), colloquially named the "Magnificent Seven" (Haberl 2007). However, its high high black body temperature set it apart from the XDINS, a distinction further confirmed by the discovery of X-ray pulsations at a 59 ms spin period (Zane et al 2011), much shorter than the several second spin periods of the "Magnificent Seven", and by a distance estimate of 3.3 kpc (Mereghetti et al 2021), much larger than the few hundred parsec distances of the other XDINS. X-ray timing yields dipolar spindown properties ( Ė = 6.1 × 10 35 erg s −1 , τ c = 2.9 × 10 5 yr, B = 4.4 × 10 11 G) that are more in line with those of young radio pulsars (Halpern et al 2013;Bogdanov et al 2019;Mereghetti et al 2021), disfavouring the classification as a nearby millisecond pulsar or a central compact object (CCO).…”

“…However, its high high black body temperature set it apart from the XDINS, a distinction further confirmed by the discovery of X-ray pulsations at a 59 ms spin period (Zane et al 2011), much shorter than the several second spin periods of the "Magnificent Seven", and by a distance estimate of 3.3 kpc (Mereghetti et al 2021), much larger than the few hundred parsec distances of the other XDINS. X-ray timing yields dipolar spindown properties ( Ė = 6.1 × 10 35 erg s −1 , τ c = 2.9 × 10 5 yr, B = 4.4 × 10 11 G) that are more in line with those of young radio pulsars (Halpern et al 2013;Bogdanov et al 2019;Mereghetti et al 2021), disfavouring the classification as a nearby millisecond pulsar or a central compact object (CCO). On the other hand, the absence of radio pulsations (Hessels et al 2007;Zane et al 2011) or gamma-ray pulsations (Halpern 2011;Halpern et al 2013;Mereghetti et al 2021) are somewhat inconsistent with those of young radio pulsars.…”

“…X-ray timing yields dipolar spindown properties ( Ė = 6.1 × 10 35 erg s −1 , τ c = 2.9 × 10 5 yr, B = 4.4 × 10 11 G) that are more in line with those of young radio pulsars (Halpern et al 2013;Bogdanov et al 2019;Mereghetti et al 2021), disfavouring the classification as a nearby millisecond pulsar or a central compact object (CCO). On the other hand, the absence of radio pulsations (Hessels et al 2007;Zane et al 2011) or gamma-ray pulsations (Halpern 2011;Halpern et al 2013;Mereghetti et al 2021) are somewhat inconsistent with those of young radio pulsars.…”

Thermal and non-thermal X-ray emission from the rotation-powered radio/$γ$-ray pulsar PSR J1740+1000

“…In all these sources, the thermal component of the SED is well fitted by two blackbody components. In the left panel of Figure 7 we plot in black the ratios of emitting radii and temperatures of PSR J1740+1000 and of the Three Musketeers, the younger Vela pulsar (Manzali et al 2007) and 1RXS J141256.0+792204 ("Calvera"), for which the lack of non-thermal emission is probably due to unfavourable orientation and/or large distance (Mereghetti et al 2021). It is very striking to observe that, for all these pulsars except Geminga, the ratio 𝑅 hot /𝑅 cold is in the range 0.05 − 0.2.…”