The Use of Weighting in Periodicity Searches in All-Sky Monitor Data: Applications to the GLAST LAT

The light curves produced by all-sky monitors, such as the Rossi X-ray Timing Explorer All-Sky Monitor and the Swift Burst Alert Telescope (BAT), generally have non-uniform error bars. In searching for periodic modulation in this type of data using power spectra it can be important to use appropriate weighting of data points to achieve the best sensitivity. It was recently demonstrated that for Swift BAT data a simple weighting scheme can actually sometimes reduce the sensitivity of the power spectrum depending on source brightness. Instead, a modified weighting scheme, based on the Cochran semi-weighted mean, gives improved results independent of source brightness. We investigate the benefits of weighting power spectra in period searches using simulated GLAST LAT observations of gamma-ray binaries.Comment: 2 pages. To appear in the Proceedings of the First International GLAST Symposium, Febuary 5-8, 2007, Stanford University, AIP, Eds. S. Ritz, P. F. Michelson, and C. Meega

The Orbital Period of the Be/Neutron Star Binary RX J0812.4-3114

We present the results of Rossi X-ray Timing Explorer observations of the Be star X-ray binary system RX J0812.4-3114. A light curve obtained with the RXTE All-Sky Monitor shows that the source is currently in an active state with outbursts occurring at approximately 80 day intervals. The source underwent a transition from an inactive state to this regular outburst state early in 1998. An observation of RX J0812.4-3114 was obtained with the RXTE Proportional Counter Array close to the time of a predicted maximum in March 1999 and strong pulsations were detected at a period of 31.88 seconds. This confirms the result of an earlier PCA observation by Reig & Roche which was serendipitously also obtained near the predicted maximum flux of the 80 day period and also near the start of the current active state. We interpret the periodicity in the ASM light curve as indicating the orbital period of RX J0812.4-3114 with outbursts occurring around periastron passage

Looking for Stars and Finding the Moon: Effects of Lunar Gamma-ray Emission on Fermi LAT Light Curves

We are conducting a search for new gamma-ray binaries by making high signal-to-noise light curves of all cataloged Fermi LAT sources and searching for periodic variability using appropriately weighted power spectra. The light curves are created using a variant of aperture photometry where photons are weighted by the probability that they came from the source of interest. From this analysis we find that the light curves of a number of sources near the ecliptic plane are contaminated by gamma-ray emission from the Moon. This shows itself as modulation on the Moon's sidereal period in the power spectra. We demonstrate that this contamination can be removed by excluding times when the Moon was too close to a source. We advocate that this data screening should generally be used when analyzing LAT data from a source located close to the path of the Moon.Comment: 2012 Fermi Symposium proceedings - eConf C12102

Diverse Long-Term Variability of Five Candidate High-Mass X-ray Binaries from Swift Burst Alert Telescope Observations

We present an investigation of long-term modulation in the X-ray light curves of five little-studied candidate high-mass X-ray binaries using the Swift Burst Alert Telescope. IGR J14488-5942 and AX J1700.2-4220 show strong modulation at periods of 49.6 and 44 days, respectively, which are interpreted as orbital periods of Be star systems. For IGR J14488-5942, observations with Swift X-ray Telescope show a hint of pulsations at 33.4 s. For AX J1700.2-4220, 54 s pulsations were previously found with XMM. Swift J1816.7-1613 exhibits complicated behavior. The strongest peak in the power spectrum is at a period near 150 days, but this conflicts with a determination of a period of 118.5 days by La Parola et al. (2014). AX J1820.5-1434 has been proposed to exhibit modulation near 54 days, but the extended BAT observations suggest modulation at slightly longer than double this at approximately 111 days. There appears to be a long-term change in the shape of the modulation near 111 days, which may explain the apparent discrepancy. The X-ray pulsar XTE J1906+090, which was previously proposed to be a Be star system with an orbital period of ~30 days from pulse timing, shows peaks in the power spectrum at 81 and 173 days. The origins of these periods are unclear, although they might be the orbital period and a superorbital period respectively. For all five sources, the long-term variability, together with the combination of orbital and proposed pulse periods, suggests that the sources contain Be star mass donors.Comment: Accepted for publication in The Astrophysical Journal. 15 pages, 27 figures. (v2 corrects citation