Lack of emission lines in the optical spectra of SAX J1808.4–3658 during reflaring of the 2019 outburst^⋆

¹ INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, (LC), Italy
² Università dell’Insubria, Dipartimento di Scienza e Alta Tecnologia, Via Valleggio 11, I-22100 Como, Italy
³ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Rome, Italy
⁴ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monteporzio Catone, (RM), Italy
⁵ Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE

^⋆⋆ Corresponding author; laura.asquini@inaf.it

Received: 21 May 2024
Accepted: 18 October 2024

Abstract

Aims. We present spectroscopy of the accreting X-ray binary and millisecond pulsar SAX J1808.4–3658. These observations are the first to be obtained during a reflaring phase. We collected spectroscopic data during the beginning of the reflaring of the 2019 outburst and compared them to previous datasets taken at different epochs, both of the same outburst and across the years. To this end, we also present spectra of the source taken during quiescence in 2007, one year before the next outburst.

Methods. We made use of data taken by the Very Large Telescope (VLT) X-shooter spectrograph on August 31, 2019, three weeks after the outburst peak. For flux calibration, we used photometric data taken during the same night by the 1m telescopes from the Las Cumbres Observatory network that are located in Chile. We compare our spectra to the quiescent data taken by the VLT-FORS1 spectrograph in September, 2007. We inspected the spectral energy distribution by fitting our data with a multicolored accretion-disk model and sampled the posterior probability density function for the model parameters with a Markov chain Monte Carlo (MCMC) algorithm.

Results. We find the optical spectra of the 2019 outburst to be unusually featureless, with no emission lines present despite the high resolution of the instrument. Fitting the UV-optical spectral energy distribution with a disk plus irradiated star model results in a very large value for the inner disk radius of ∼5130 ± 240 km, which could suggest that the disk was emptied of material during the outburst, possibly accounting for the emission-less spectra. Alternatively, the absence of emission lines could be due to a significant contribution of the jet emission at optical wavelengths.