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

TL;DR

This study presents optical spectroscopy of SAX J1808.4-3658 during a reflaring phase, revealing unusually featureless spectra and suggesting disk depletion or jet emission as possible reasons for the lack of emission lines.

Contribution

First spectroscopic observations of SAX J1808.4-3658 during a reflaring phase, comparing spectral features and energy distribution with previous data to understand accretion disk behavior.

Findings

Spectra during reflaring are featureless with no emission lines.

The inner disk radius is estimated to be around 5130 km, indicating possible disk depletion.

Absence of emission lines may be due to jet emission contribution.

Abstract

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 reflaring of the 2019 outburst and we compare them to previous datasets, taken at different epochs both of the same outburst and across the years. In order to do so, we also present spectra of the source taken during quiescence in 2007, one year before the next outburst. 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 multi-colour accretion disk model and sampled the posterior probability density function for the model parameters with a Markov-Chain Monte Carlo algorithm. 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 $\sim 5130 \pm 240$ km, which could suggest that the disk has been 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.