The ultraluminous X-ray pulsar M82 X-2 is still spinning down

ATel #13583; Matteo Bachetti (INAF-OACagliari), John Tomsick (UC Berkeley), Dominic J Walton (U. Cambridge), Murray Brightman (Caltech), Felix Fuerst (ESAC), Deepto Chakrabarty (MIT), Hannah P Earnshaw (Caltech), Brian W. Grefenstette (Caltech), Kristin C Madsen (Caltech), Fiona A Harrison (Caltech), Marianne Heida (ESO), Gian Luca Israel (INAF-OARoma), Matthew J. Middleton (U. Southampton), Guillermo Rodriguez (INAF-OARoma), Thomas J Maccarone (Texas Tech)
on 24 Mar 2020; 08:30 UT
Credential Certification: Matteo Bachetti (bachetti@oa-cagliari.inaf.it)

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We report on the preliminary results of a 160-ks (80 ks exposure) NuSTAR observation of M82 X-2, executed between 2020 March 10 and March 12 (ObsID 30502021002). We used the pre-processed NuSTAR data from the HEASARC, and corrected the arrival times of the photons to the Solar System Barycenter with the barycorr FTOOL. We did not use a clock correction file (which is not yet available for this recent observation from the NuSTAR SOC), but for the very slow 1.7s pulsation of M82 X-2 an error of ±100 ms is acceptable at the current stage. However, this caveat should be considered when interpreting the numbers below. The total NuSTAR count rate from the two summed detectors is ~0.9 ct/s (0.7 ct/s below 10 keV). This implies a luminosity above 10^40 erg/s, with the caveat that the nearby M82 X-1 can reach very high luminosities (up to ~10^41 erg/s) and cannot be resolved in NuSTAR. The pulsar is clearly detected in this observation (> 10 sigma using the Z^2_2 search). We used the ephemeris from Bachetti+2020 (ApJ, 891, 1), but slightly shifted the orbital phase to account for a possible propagated error on the orbital period. Orbital and spin parameters are highly correlated. The pulse fraction is ~10% between 3 and 10 keV, and increases up to ~25% between 15 and 25 keV. Again, this possibly includes flux from M82 X-1, which is not resolved. In the last four years, the pulsar has further decreased its rotation frequency, and it is now at 0.72193 Hz, with an uncertainty of ~5e-5 Hz dominated by the uncertainty on the orbital parameters. Given the last measured frequency in 2016 of 0.7239 Hz, the pulsar has, on average, spun down by 2x10^-11 Hz/s in the last four years. NuSTAR completed today an additional observation of this source, as part of AO5, that we will promptly analyze as soon as the data are available in the HEASARC. We encourage follow up with other timing-capable missions.

Frequency - Orbital T0 search diagram