XMM-Newton Detection and Spectrum of the Second Fastest Spinning Pulsar PSR J0952-0607

With a spin frequency of 707 Hz, PSR J0952-0607 is the second fastest spinning pulsar known. It was discovered in radio by LOFAR in 2017 at an estimated distance of either 0.97 or 1.74 kpc and has a low-mass companion with a 6.42 hr orbital period. We report the discovery of the X-ray counterpart of PSR J0952-0607 using XMM-Newton. The X-ray spectra can be well-fit by a single power law (PL) model (Gamma approximate to 2.5) or by a thermal plus PL model (kT(eff) approximate to 40 eV and Gamma approximate to 1.4). We do not detect evidence of variability, such as that due to orbital modulation from pulsar wind and companion star interaction. Because of its fast spin rate, PSR J0952-0607 is a crucial source for understanding the r-mode instability, which can be an effective mechanism for producing gravitational waves. Using the high end of our measured surface temperature, we infer a neutron star core temperature of similar to 10(7) K, which places PSR J0952-0607 within the window for the r-mode to be unstable unless an effect such as superfluid mutual friction damps the fluid oscillation. The measured luminosity limits the dimensionless r-mode amplitude to be less than similar to 1 x 10(-9).