Nonradial Pulsations in [MA93]798 (Possible Optical Counterpart of SXP6.88/SXP4693)

ATel #7831; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 23 Jul 2015; 21:16 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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The emission-line star [MA93]798 (Meyssonnier & Azzopardi, 1993, A&ASS, 102, 451) has been suggested as the optical counterpart for SXP4693 (Laycock et al. 2010, ApJ, 716, 1217) and one of several possible counterparts for SXP6.88 (McBride et al. 2007, MNRAS, 382, 743). Antoniou et al. (2009, ApJ, 707, 1080) confirmed the presence of Halpha emission and classified the star as B1e. However, X-ray pulsations in both SXP6.88 and SXP4693 have been seen with low significance at only one epoch, suggesting uncertainty in their assignments as Be/X-ray pulsars.

McBride et al. analyzed 10 years of OGLE-II/III data for [MA93]798, searching for periods >1.5 d. They identified a signal near P=2.7 d (and alias at 1.6 d), which they proposed might be the orbital period. We note their periodogram shows considerable structure at both values, which implies the true period is not unique. Using MACHO and OGLE-II data, Schmidtke and Cowley (ATel #2998) and Sarraj et al. (2012, IBVS, 6030) pointed out the optical variations are consistent with a changing period near 0.730 d, which could come from nonradial pulsations (NRPs) of the Be star. This interpretation was further supported in a study of the full OGLE-II/III data set by Schmidtke et al. (2013, MNRAS, 431, 252), who found NRPs with periods that varied by ~1% (0.7261 to 0.7328 d). They demonstrated that periods <1 d can be reliably detected in undersampled synoptic data and period changes in NRPs can artificially enhance the signal strength of longer alias periods. Since new data from 5 additional years of OGLE-IV observing are now available through the XROM web site (Udalski 2008, Acta Astron., 58, 187), we have reinvestigated the optical behavior of [MA93]798.

The I-band light curve spans 17 yr and slowly varies in mean brightness (I~15.2) with small scatter superimposed. Data for each season were detrended by subtracting a low-order polynomial fit, and L-S periodograms were calculated for P=0.5 to 1000 d. These are shown in http://www.public.asu.edu/~atpcs/SXP/MA93-798_Periodograms.pdf . Significant power near 2.7, 1.6, 0.7 and 0.6 d (which are aliases of each other) is present in most individual seasons as well as the combined data from all seasons. In general the peaks at P~2.7 and P~0.7 d are the highest and have comparable power. However, the periods and strengths of these peaks can change considerably from season to season (for example, the large shifts in peaks between OGLE-III seasons 1 and 4). The periodogram of combined data shows considerable structure, which implies the fundamental period must be changing. A best-fit sinewave was calculated for each peak labeled in the figure. The period for the signal near 2.7 d varies by ~3.4% while that for the 0.7-d signal varies by only 0.9%. The amplitudes of all sinewave fits are small, generally <0.018 mag. The changing period and sinusoidal shape make it unlikely that the 2.7-d periodicity is caused by orbital motion. Instead, the photometric variations most likely arise from ~0.7-d NRPs of the Be star. This conclusion is consistent with the small season-to-season period changes and accounts for the complex structure seen in the combined periodogram.

We note that the amplitudes and periods of the NRPs in [MA93]798 may be related. In general NRPs with shorter periods have higher amplitudes. This tendency is shown in http://www.public.asu.edu/~atpcs/SXP/MA93-798_Amplitude_vs_Period.pdf , where the solid line is a formal fit of the correlation.