Outbursts Large and Small from EXO 2030+375

TL;DR

This paper reports the first giant outburst of the X-ray pulsar EXO 2030+375 observed in 2006, including discovery of a cyclotron feature, detailed flux and frequency analysis, and new orbital parameters over a 16-year span.

Contribution

It provides the first detailed observations of a giant outburst from EXO 2030+375, including cyclotron feature detection and updated orbital analysis.

Findings

Detection of a cyclotron feature at ~11 keV during the outburst

Pulsations observed at all orbital phases during the outburst

Orbital phase shifts of normal outbursts post-giant outburst

Abstract

During the summer of 2006, the accreting X-ray pulsar EXO 2030+375 underwent its first giant outburst since its discovery in 1985. Our observations include the first ever of the rise of a giant outburst of EXO 2030+375. EXO 2030+375 was monitored daily with the Rossi X-ray Timing Explorer (RXTE) from 2006 June through 2007 May. During the giant outburst, we discovered evidence for a cyclotron feature at ~11 keV. This feature was confidently detected for about 90 days, during the brighter portion of the outburst. Daily observations of the next five EXO 2030+375 orbits detected pulsations at all orbital phases and normal outbursts shifted to a later orbital phase than before the giant outburst. An accretion disk appears to be present in both the normal and giant outbursts, suggesting that the long-term behavior is a product of the state of the Be star disk and the accretion disk. Here we will present flux and frequency histories from our detailed RXTE observations of the giant outburst and the normal outbursts that surrounded it. A new orbital analysis is presented that includes observations from 1991 through 2007 August.