We report the discovery of a white dwarf exhibiting deep, irregularly shaped transits, indicative of circumstellar planetary debris. Using Zwicky Transient Facility DR2 photometry of ZTF J013906.17+524536.89 and follow-up observations from the Las Cumbres Observatory, we identify multiple transit events that recur every ≈107.2 days, much longer than the 4.5-4.9 hr orbital periods observed in WD 1145+017, the only other white dwarf known with transiting planetary debris. The transits vary in both depth and duration, lasting 15-25 days and reaching 20%-45% dips in flux. Optical spectra reveal strong Balmer lines, identifying the white dwarf as a DA with =  T 10, 530 140 K eff and ( ) =  g log 7.86 0.06. A Ca IIK absorption feature is present in all spectra both in and out of transit. Spectra obtained during one night at roughly 15% transit depth show increased Ca IIK absorption with a model atmospheric fit suggesting [Ca/H]=−4.6±0.3, whereas spectra taken on three nights out of transit have [Ca/H] of −5.5, −5.3, and −4.9 with similar uncertainties. While the Ca IIK line strength varies by only 2σ, we consider a predominantly interstellar origin for Ca absorption unlikely. We suggest a larger column density of circumstellar metallic gas along the line of site or increased accretion of material onto the white dwarf's surface are responsible for the Ca absorption, but further spectroscopic studies are required. In addition, high-speed time series photometry out of transit reveals variability with periods of 900 and 1030 s, consistent with ZZ Ceti pulsations.

We are monitoring established and putative redback millisecond pulsars (MSPs) in time-series photometry, repeatedly covering their 5-6 hr orbital light curves in r ′ or R. On timescales of months, PSR J1048+2339 and XMMU J083850.38−282756.8 exhibit similar variability of ≈ 0.3 mag on the heated side of the companion star. However, the heating light curve is rarely symmetric, suggesting that the intrabinary shock generated by the pulsar wind is skewed in addition to being variable, or that changing magnetic fields intrinsic to the companion channel the pulsar wind. In addition to this variable heating, there are long-lived flaring states that increase the brightness by an additional 0.5 mag, with variability on ≈ 10 minute timescales. These flares also appear to originate on the heated side of the companion, while the "night"-side brightness remains relatively stable. Somewhat less active, PSR J1628−3205 has an optical light curve that is dominated by tidal distortion (ellipsoidal modulation), although it too shows evidence of variable and asymmetric heating due to shifting magnetic fields or migrating star spots. These effects frustrate any effort to derive system parameters such as inclination angle and Roche-lobe filling factor from optical light curves of redback MSPs. We also report on two Chandra X-ray observations of PSR J1048+2339 that show strong orbital modulation, possibly due to beaming along the intrabinary shock, and a third observation that is dominated by flaring. The peak flare luminosity in the 0.3-8 keV band is ≈ 12% of the pulsar's spin-down power, which may require magnetic reconnection. None of these three systems has yet shown a transition back to an accreting state.

We conducted time-resolved optical spectroscopy and/or time-series photometry of 15 cataclysmic binaries that were discovered in hard X-ray surveys by the Swift Burst Alert Telescope (BAT) and the International Gamma-Ray Astrophysics Laboratory (INTEGRAL), with the goal of measuring their orbital periods and searching for spin periods. Four of the objects in this study are new optical identifications: Swift J0535.2+2830, Swift J2006.4+3645, IGR J21095+4322, and Swift J2116.5+5336. Coherent pulsations are detected from three objects for the first time, Swift J0535.2+2830 (1523 s), 2PBC J1911.4+1412 (747 s), and 1SWXRT J230642.7+550817 (464 s), indicating that they are intermediate polars (IPs). We find two new eclipsing systems in time-series photometry: 2PBC J0658.0−1746, a polar with a period of 2.38 hr, and Swift J2116.5+5336, a disk system that has an eclipse period of 6.56 hr. Exact or approximate spectroscopic orbital periods are found for six additional targets. Of note is the long 4.637-day orbit for Swift J0623.9−0939, which is revealed by the radial velocities of the photospheric absorption lines of the secondary star. We also discover a 12.76 hr orbital period for RX J2015.6+3711, which confirms that the previously detected 2.00 hr X-ray period from this star is the spin period of an IP, as inferred by Coti Zelati et al. These results support the conclusion that hard X-ray selection favors magnetic CVs, with IPs outnumbering polars.

Dynamic optical networking has promising potential to support the rapidly changing traffic demands in metro and long-haul networks. However, the improvement in dynamicity is hindered by wavelength-dependent power excursions in gain-controlled erbium doped fiber amplifiers (EDFA) when channels change rapidly. We introduce a general approach that leverages machine learning (ML) to characterize and mitigate the power excursions of EDFA systems with different equipment and scales. An ML engine is developed and experimentally validated to show accurate predictions of the power dynamics in cascaded EDFAs. Recommended channel provisioning based on the ML predictions achieves within 1% error of the lowest possible power excursion over 94% of the time. We also showcase significant mitigation of EDFA power excursions in super-channel provisioning when compared to the first-fit wavelength assignment algorithm.