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P.A. Charles, R.I. Hynes (Southampton), J. Casares, G. Israelian, P. Rodrigues, T. Shahbaz, C. Zurita (IAC), T. Abbott (NOT), P. Hakala (Turku), A.R. King (Leicester)
The soft X-ray transient (SXT) XTE J1859+226 has been optically monitored from Tenerife using the IAC-80 and OGS 1m telescopes since its outburst on 1999 Oct 10 (see IAUC 7274, 7279). This R-band CCD photometry revealed a smooth decline of 0.017 mag/day for most of the next 8 months in parallel with that seen by the RXTE ASM, until two large (~2.5 mag) dips occurred, each lasting about a week and separated by a week. This was followed by a mini-outburst (reaching R~18) on 2000 July 4.
Fast CCD photometry with the NOT 2.5m telescope on La Palma during this mini-outburst then revealed QPO flaring activity with a 21.5 min period at a time when J1859+226 had dimmed by ~30%. The flaring activity ceased when the source returned to its predip level. The QPO timescale is very close to that reported by Hynes et al (IAUC 7294) early in the outburst. A second night of fast photometry revealed no flaring activity, but the presence of a smooth modulation (amplitude 0.1 mag) on a timescale of ~6 hours.
This behaviour will be compared with other SXTs, particularly A0620-00 (where a similar dip was seen at 160 days after outburst), J0422+32 and Nova Mus 1991. The nature and origin of the modulations will be discussed, in particular the possibility of an instability at the L1 point leading to oscillations on the ~20 min timescale. It is possible that such behaviour is ubiquitous amongst SXTs, however, these observations demonstrate the importance of high speed, high sensitivity monitoring programs throughout the outburst light curve, and these are rarely possible.