Recent RXTE/ASM and ROTSEIIId Observations of EXO 2030+375

TL;DR

This study analyzes RXTE/ASM and SWIFT/BAT data to determine the orbital phases of Type I outbursts in EXO 2030+375, investigates their correlation with optical brightness variations, and examines the impact of Type II outbursts on these phases and amplitudes.

Contribution

It provides new estimates of orbital phases of Type I outbursts and explores their relationship with optical brightness and Type II outbursts, revealing phase shifts and amplitude changes.

Findings

Type I outbursts delayed by 6 days before Type II outburst

Phase shift of 13 days after the giant Type II outburst

Optical brightness decreased by 0.4 mag during amplitude decrease

Abstract

Using the archival RXTE/ASM and SWIFT/BAT observations, the new orbital phases of Type I outbursts of EXO 2030+375 are estimated. A possible correlation between the Type II outburst and optical brightness variations is investigated. In order to estimate the phases of Type I outbursts, we fitted Gaussian profiles to the RXTE/ASM and SWIFT/BAT light curves. The time corresponding to the maximum value of the profiles is treated as the arrival time of Type I outburst. We used differential magnitudes in the time-series analysis of the optical light curve. MIDAS and its suitable packages were used to reduce and analyze the spectra. Prior to the Type II outburst, orbital phases of Type I outbursts were delayed for 6 days after the periastron passage, which is consistent with findings of Wilson et al., (2002, 2005). After the giant Type II outburst, the phase of Type I outbursts underwent a sudden shift of 13 days after the periastron passage. The amplitudes of Type I outbursts were increased between MJD 52500 and 53500. These amplitudes then decreased for 10 orbital cycles until the Type II outburst was triggered. If the change of outburst amplitudes correlated with the mass accretion, then during the decrease of these amplitudes mass should be deposited in a disk around neutron star temporarily. The release of this stored mass may ignite the Type II outburst. We report that the optical light curve became fainter by 0.4 mag during the decrease of amplitude of the Type I outbursts. The observed H$\alpha$ profiles and their equivalent widths during the decay and after the giant outburst are consistent with previous observations of the system.