Claude Chevalier

CCD photometry of low mass X-ray binaries: LMCX-2, 1556-60 and 1957+11

We present preliminary results from V-band CCD photometric observations of several low-mass X-ray binaries with faint optical counterparts obtained at ESO and CFHT from February to September 1984. LMCX-2 exhibits large (0.3–0.5 magnitude) variations consistent with a possible 6.4 hour period. 1556-605 shows 0.2–0.5 magnitude variations suggesting that the orbital period is longer than 7 hours. 1957+115 shows short time scale (1 hour) variations of 0.1 magnitude.

LHG 83: Another Outstanding X-Ray Binary in the LMC

The strongest X-ray point source, LHG 83, discovered in the EINSTEIN survey of the LMC and not being associated with a nearby coronal type stellar emitter or background AGN is identified with a faint blue variable object. Spectrophotometry reveals low mass X-ray binary characteristics at a mean velocity consistent with LMC membership. The He II λ4686 emission exhibits a unique blue shifted component suggesting outflow velocities of several thousand km/s. Optical brightness changes by 0.3 mag in less than one hour are likely to be intrinsic to the source rather than induced by orbital motion. The low X-ray to optical flux ratio is probably due to the fact that the central X-ray source is blocked from direct view by the accretion disk.

Recent Optical Observations of the X-Ray Pulsar 4U 1626-67

High-speed broad-band optical photometry of the X-ray pulsar 4U 1626-67 has been obtained on five nights in May 1978. May 1979 and June 1980 with the ESO 3.6m telescope. These observations yield a pulse period and pulse derivative consistent with the X-ray data. The average pulse, of 4% amplitude, shows a broad, flat maximum and a narrow, deep minimum, much like the HEAO-1 low-energy X-ray pulse. The time-averaged flux is seen to vary by 15-50% on time scales of 10-15 minutes and by 15% on time scales down to 1 minute. Results of a search on 5 June 1980 for the down-shifted frequency component reported by Middleditch et al. (1981) shows no detectable signal at the expected frequency. The 90% confidence upper limit on the power at this frequency is P/Po≤ 1.5, where Po is the local noise power level. Since for the main peak P/Po ≃ 53, this means that the side-lobe was at least a factor of 35 times weaker than the main peak at the time of these observations.

A Search for Optical Couterparts of Globular Cluster X-Ray Sources

The identification of the optical counterpart of the M 15 (NGC 7078) X-ray source has been confirmed photometrically and spectroscopically (Auriere et al. 1984, 1985, 1986; Ilovaisky and Chevalier: 1985; Charles et al. 1985, 1986). It is designated M15:AC211 from the Auriere and Cordoni catalogue (1981).

Discovery of Rapid Optical Variability in GX 339-4

The optical counterpart of the Cygnus X-1-like X-ray source GX339-4 has recently undergone large brightness variations. Although it was fainter than 21 mag in B on March 8, 1981 (Ilovaisky and Chevalier, 1981, IAU Circ n° 3586), it was found to be at V = 15.4 on May 28, 1981 (Motch, Ilovaisky and Chevalier, 1981, IAU Circ. n° 3609). It was not detected in X-rays on 7Apr.1981 (Oda, IAU Circ. n°3594)

The Optical Light Curve of LMC X-4 and the 30-Day X-Ray Period

Analysis of an extensive set of optical photometric observations of the massive X-ray binary LMC X-4 obtained from 1976 through 1981 at the European Southern Observatory, La Silla, Chile, shows a clear modulation of the 1.408-d binary light curve amplitude with the 30.48-d X-ray period discovered by Lang et al. (1981) from HEAO-1 observations. The B filter amplitude varies by a factor of 2 from a lowest value of 0.09 mag during the X-ray OFF portion of the 30.48-d cycle to a highest value of 0.18 mag during the X-ray ON part of the cycle. The depth of the primary minimum at binary phase 0.0 is stable within the 30.48-day cycle but the minimum at binary phase 0.5 deepens during X-ray ON phases.

The 8-Day Modulation in V616 Mon (A0620-00)

Photoelectric photometry of V616 Mon, the optical counterpart of the bright transient X-ray source A0620-00, obtained at the Haute Provence Observatory in October 1975 and at the European Southern Observatory in March 1976 shows evidence of a low-amplitude 8-day intensity modulation. Walter and Duerbeck (1976) have reported a 4-day periodicity soon after outburst. Interpreting their results in terms of a modulation having twice that period, and thus showing two maxima and two minima per cycle, and combining them with our data, we obtain a period of 7.80 ± 0.03 days. This modulation agrees in period and probably in phase with that detected in X-rays in January 1976 (Matilsky et al. 1976). Its amplitude increases with wavelength in March but no such effect is detectable in October. We note a similar lack of correlated color variability in the Duerbeck and Walter data. A recurring eclipse-like feature, which may be phase-locked to the 7.8-day cycle (Robertson et al. 1976), is superposed onto one cycle of the modulation in March. Significant intra-night variability is usually present, amounting to as much as 0.17 mag in two hours. Such variability, also detected at u.v. wavelengths (Wu et al. 1976) may be due to an underlying 1-day periodicity.