NICER detection of pulsations from RX J0529.8-6556

Following the SRG/eROSITA discovery of a strong outburst from the LMC Be/X-ray binary RX J0529.8-6556 (Haberl et al., ATEL #13828) we triggered NICER ToO observations to search for pulsations and obtain high-quality spectra. NICER observations started on 2020 June 12, and a total of ~10 ks were obtained within ∽1.5 days (MJD 59012.6-59014.1).

NICER quick look data were filtered to obtain higher S/N using standard tools and options. The average NICER count rate was 9.2 c/s (0.3-10 keV), and preliminary background estimates yield a background contribution of ∽10%. Pulsations are visible by eye in the NICER lightcurve. We searched for pulsations in the barycentric corrected data using epoch folding technique.

The epoch folding periodogram revealed multiple peaks between 0.014 and 0.015 Hz that were caused by observing gaps. The strongest signal was derived for a period of 68.68±0.01 s. This value is consistent with the value of 69.5±2.5 s derived by archival ROSAT data and reported by Haberl et al. 1997.

However, analysis of an XMM-Newton observation performed on 2000 October 7 (MJD 51824) had revealed a pulse period of 69.232±0.002 s (Haberl et al. 2003). Thus the new measurement by NICER indicates a secular spin up rate of about -8.8×10-10 s/s over the last 20 years (not accounting for any orbital Doppler effect).

The pulse profile obtained by NICER data is double peaked with a strong main peak and a weaker secondary peak separated by ∽0.375 in phase. Compared to archival ROSAT and XMM-Newton data, the pulse profile is more complex, and the pulsed fraction has decreased. However, the overall shape is consistent within uncertainties.

The pulsed fraction, defined as PF= (FMax - FMin)/(FMax + FMin), was found to be 0.52±0.02 by using 32 phase bins for the pulse profile (0.3-10.0 keV). The 0.3-10.0 keV spectrum can be fitted by an absorbed power-law model with a photon index of 0.89±0.03 and an absorption column density of 2.8±0.5×1020 cm-2 , yielding an X-ray flux of 4.1×10-11 erg cm-2 s-1 and an absorption corrected LX of 1.2×1037 erg s-1 (distance of 50 kpc).

The residuals of the best fit X-ray spectrum shows some structure that resembles a soft-excess. This could be attributed to diffuse emission from the LMC or contamination from nearby sources. However, any soft X-ray spectral component that is used to improve the fit, does not account for more than 10% of the total flux.

Acknowledgements: We thank the NICER team for promptly scheduling our ToO observations. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.