PSR J1023+0038: phase-resolved optical spectroscopy and continued X-ray activity

ATel #5868; M. Linares, J. Casares, P. Rodriguez-Gil, T. Shahbaz (IAC & ULL, Spain)
on 10 Feb 2014; 15:05 UT
Credential Certification: Manuel Linares (linares@mit.edu)

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After the state change of the binary millisecond pulsar PSR J1023+0038 in mid 2013 (ATels #5513, #5514, #5515, #5516, #5534), we performed orbital phase resolved optical spectroscopy in order to study the evolution of the emission lines and constrain the properties of the accretion disk during the ongoing active period. We obtained 22 low-resolution (ACAM, spectral resolution ~ 400 km/s) spectra of the system using the 4-meter William Herschel Telescope on La Palma on 2013-12-16 starting at 02:34 UTC, covering about 85% of the (4.8 hr) binary orbit.

We clearly detect broad (1300-3600 km/s FWHM) double-peaked H-alpha, H-beta, H-gamma and HeI (4472,4921,5016-5048,5876,6678 A) emission lines, indicating that the accretion disk is still present 55 and 43 days after the previous detections (Halpern et al. 2013, ATel #5514;Takata et al. 2013, arXiv:1312.0605). For the average H-alpha line we measure an equivalent width EW = 23.7 +/- 0.3 A and FWHM = 1430 +/- 20 km/s. All FWHM values reported here have been corrected for instrumental resolution.

From time-resolved single-Gaussian fits to the H-alpha emission line profile, we find a clear orbital modulation of the line's centroid wavelength, EW and FWHM. The EW has a sharp maximum near orbital phase 0, and a broad minimum at phases 0.3-0.7 (using ephemerides from Archibald et al. 2013, arXiv:1311.5161, but defining phase=0 as the companion/secondary star at inferior conjunction). This probably reflects variations in the continuum light, caused by irradiation of the inner face of the companion star. The line centroid appears redshifted by about 50 km/s near phase 0.9, and blueshifted by about 230 km/s near phase 0.4. Given the asymmetric line profile, these velocity shifts are likely due to a brightness asymmetry in the disk seen at different orbital phases.

The average X-ray luminosity measured by Swift-XRT on 2013-12-17, one day after our spectroscopic observations was Lx=[1.5+/-0.1]E33 erg/s (0.5-10 keV; 1.3 kpc) and the latest available Swift observation taken on 2014-02-09 shows PSR J1023+0038 at a similar luminosity Lx=[1.6+/-0.3]E33 erg/s, i.e., still ~20 times higher than the quiescent level (Kong 2013, ATel #5515).

We thank the WHT and Swift directors and teams for making these observations possible.