The OGLE Light Curve of SXP 46.6

ATel #16310; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 27 Oct 2023; 17:59 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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Corbet et al. (1998 IAU Circ. 6803) discovered X-ray pulsations with a period of 46.63 s in ASCA observations of 1WGA J0053.8-7226. Using RXTE data, Laycock et al. (2005, ApJS, 161, 96) found recurrent X-ray outbursts with P~139 d, which they suggested might be the orbital period. A refined value of P=137.4 d was given by Galache et al. 2005 (ATel #674). Schmidtke et al. 2007 (ATel #1316) and McGowan et al. (2008, MNRAS, 384, 821) used OGLE-III data to study the behavior of star `B' (a known Be star) in the finding chart of Buckley et al. (2001, MNRAS, 320, 281). Small outbursts coincident with the X-ray events were found, confirming the optical identification. Following the convention of Coe et al. (2005, MNRAS, 356, 502), the system is known as SXP 46.6. The outbursts likely arise from the interaction between the pulsar and the disk that surrounds the B-type star. The OGLE-III outbursts were strongest in Seasons 1-3 (with double-peaked structure) while being absent or weak (with a single-peak profile) in Seasons 4-8 (see summary by Schmidtke et al. 2013, MNRAS, 431, 252).

New I data from OGLE-IV, spanning the interval 2010-2022, are available from the XROM web page (https://ogle.astrouw.edu.pl/ogle4/xrom/xrom.html). A composite light curve that shows all OGLE observations (phases III and IV) is in the top panel of the accompanying figure. A small shift in magnitude between phases may be present. We interpret the 0.25-mag dimming during Seasons 4 and 5 of OGLE-III, which corresponds with the disappearance of optical outbursts, as a collapse of the circumstellar disk. Unfortunately, none of the H-alpha spectra described by McGowan et al. were taken when the source was in this faint state. As the disk reformed and brightened, optical outbursts resumed. They continue to be present in all seasons of OGLE-IV. These outbursts are superimposed on an underlying brightness that varies between I=14.55 and 14.65, a much smaller range compared to that in OGLE-III. A plot of observations from only 2022 (after the pandemic shutdown) is shown in the figure. These data were taken at very high cadence (up to 21 points per night) but cover a time interval much less than a full observing season.

Long-term trends in individual seasons of the 2010-2020 data were removed by subtracting a quadratic fit to non-outburst measurements. The resulting observations are denoted as I* magnitudes. A phase dispersion minimization plot of the detrended data is shown in the figure. The best period is 137.65+/-0.08 d, with maximum light at JD 2455464+/-2. A plot of I* data folded on this ephemeris is shown in the bottom panel. The light curve has one single-peaked outburst each orbit (in contrast to the double-peaked outburst present in earliest OGLE-III data), with an amplitude of 0.06 mag. The outburst is slightly asymmetrical (faster rise and slower decline) and lasts 0.25 of the orbital period. The 5 highest points near maximum light come from 4 different seasons. These are likely to be artifacts of data detrending.

We searched the high-cadence observations from 2022 for non-radial pulsations (NRPs) of the Be star. Using flattened data from the declining portion of the outburst (233 points from 31 nights), a periodogram was calculated for trial periods in the range 0-5 d. No meaningful signal was found. The strongest power is for a P=3.545 d sine wave, having a full amplitude of 0.0050 mag and r.m.s. of fit of 0.0047 mag. A search of 119 non-outburst data points from Season 1 of OGLE-IV also revealed no NRP signal.

In summary, we have conducted the first extensive study of OGLE-IV observations for SXP 46.6. The orbital period, P=137.65 d, is consistent with studies of OGLE-III data. When folded on this period, the light curve shows a small, single-peaked outburst with an asymmetrical profile. No significant NRPs were found.

The OGLE Light Curve of SXP 46.6