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1. Eclipse Timing the Milky Way’s Gravitational Potential

   https://doi.org/10.3847/2041-8213/ac5c43  

   Chakrabarti, Sukanya ; Stevens, Daniel J. ; Wright, Jason ; Rafikov, Roman R. ; Chang, Philip ; Beatty, Thomas ; Huber, Daniel ( April 2022 , The Astrophysical Journal Letters)

   We show that a small but measurable shift in the eclipse midpoint time of eclipsing binary (EBs) stars of ∼0.1 s over a decade baseline can be used to directly measure the Galactic acceleration of stars in the Milky Way at ∼kiloparsec distances from the Sun. We consider contributions to the period drift rate from dynamical mechanisms other than the Galaxy’s gravitational field and show that the Galactic acceleration can be reliably measured using a sample of Kepler EBs with orbital and stellar parameters from the literature. The contribution from tidal decay we estimate here is an upper limit assuming the stars are not tidally synchronized. We find there are about 200 detached EBs that have estimated timing precision better than 0.5 s, and for which other dynamical effects are subdominant to the Galactic signal. We illustrate the method with a prototypical, precisely timed EB using an archival Kepler light curve and a modern synthetic HST light curve (which provides a decade baseline). This novel method establishes a realistic possibility to constrain dark matter substructure and the Galactic potential using eclipse timing to measure Galactic accelerations, along with other emerging new methods, including pulsar timing and extreme-precision radial velocitymore »observations. This acceleration signal grows quadratically with time. Therefore, given baselines established in the near future for distant EBs, we can expect to measure the period drift in the future with space missions like JWST and the Roman Space Telescope.

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2. Ultra-short-period massive black hole binary candidates in LSST as LISA ‘verification binaries’

   https://doi.org/10.1093/mnras/stab1856  

   Xin, Chengcheng ; Haiman, Zoltán ( July 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The Legacy Survey of Space and Time (LSST) by the Vera C. Rubin Observatory is expected to discover tens of millions of quasars. A significant fraction of these could be powered by coalescing massive black hole (MBH) binaries, since many quasars are believed to be triggered by mergers. We show that under plausible assumptions about the luminosity functions, lifetimes, and binary fractions of quasars, we expect the full LSST quasar catalogue to contain between 20 and 100 million compact MBH binaries with masses M = 105–9M⊙, redshifts z = 0–6, and orbital periods P = 1–70 d. Their light-curves are expected to be distinctly periodic, which can be confidently distinguished from stochastic red-noise variability, because LSST will cover dozens, or even hundreds of cycles. A very small subset of 10–150 ultracompact (P ≲ 1 d) binary quasars among these will, over ∼5–15 yr, evolve into the mHz gravitational-wave frequency band and can be detected by LISA. They can therefore be regarded as ‘LISA verification binaries’, analogous to short-period Galactic compact-object binaries. The practical question is how to find these handful of ‘needles in the haystack’ among the large number of quasars: this will likely require a tailored co-adding analysis optimized for thismore »purpose.« less
3. PSR J2222−0137: I. Improved physical parameters for the system

   https://doi.org/10.1051/0004-6361/202141450  

   Guo, Y. J. ; Freire, P. C. ; Guillemot, L. ; Kramer, M. ; Zhu, W. W. ; Wex, N. ; McKee, J. W. ; Deller, A. ; Ding, H. ; Kaplan, D. L. ; et al ( October 2021 , Astronomy & Astrophysics)

   Context. The PSR J2222−0137 binary system has a set of features that make it a unique laboratory for tests of gravity theories. Aims. To fully exploit the system’s potential for these tests, we aim to improve the measurements of its physical parameters, spin and orbital orientation, and post-Keplerian parameters, which quantify the observed relativistic effects. Methods. We describe an improved analysis of archival very long baseline interferometry (VLBI) data, which uses a coordinate convention in full agreement with that used in timing. We have also obtained much improved polarimetry of the pulsar with the Five hundred meter Aperture Spherical Telescope (FAST). We provide an improved analysis of significantly extended timing datasets taken with the Effelsberg, Nançay, and Lovell radio telescopes; this also includes previous timing data from the Green Bank Telescope. Results. From the VLBI analysis, we have obtained a new estimate of the position angle of the ascending node, Ω = 189 −18 +19 deg (all uncertainties are 68% confidence limits), and a new reference position for the pulsar with an improved and more conservative uncertainty estimate. The FAST polarimetric results, and in particular the detection of an interpulse, yield much improved estimates for the spin geometry of themore »pulsar, in particular an inclination of the spin axis of the pulsar of ∼84 deg. From the timing, we obtain a new ∼1% test of general relativity (GR) from the agreement of the Shapiro delay parameters and the rate of advance of periastron. Assuming GR in a self-consistent analysis of all effects, we obtain much improved masses: 1.831(10)  M ⊙ for the pulsar and 1.319(4)  M ⊙ for the white dwarf companion; the total mass, 3.150(14)  M ⊙ , confirms this as the most massive double degenerate binary known in the Galaxy. This analysis also yields the orbital orientation; in particular, the orbital inclination is 85.27(4) deg – indicating a close alignment between the spin of the pulsar and the orbital angular momentum – and Ω = 187.7(5.7) deg, which matches our new VLBI estimate. Finally, the timing also yields a precise measurement of the variation in the orbital period, Ṗ b = 0.251(8) × 10 −12 ss −1 ; this is consistent with the expected variation in the Doppler factor plus the orbital decay caused by the emission of gravitational waves predicted by GR. This agreement introduces stringent constraints on the emission of dipolar gravitational waves.« less
4. PNV J00444033+4113068: Early superhumps with 0.7 mag amplitude and non-red color

   https://doi.org/10.1093/pasj/psac068  

   Tampo, Yusuke ; Isogai, Keisuke ; Kojiguchi, Naoto ; Uemura, Makoto ; Kato, Taichi ; Tordai, Tamás ; Vanmunster, Tonny ; Itoh, Hiroshi ; Dubovsky, Pavol A. ; Medulka, Tomáš ; et al ( September 2022 , Publications of the Astronomical Society of Japan)

   In the first days of WZ Sge-type dwarf nova (DN) outbursts, the 2 : 1 resonance induces a spiral arm structure in the accretion disk, which is observed as early superhumps in optical light curves. We reports on our optical observations of an eclipsing WZ Sge-type DN PNV J00444033+4113068 during its 2021 superoutburst using the 3.8 m Seimei telescope and through the Variable Star Network collaboration. The eclipse analysis showed that its orbital period was 0.055425534(1) d. Our observations confirmed early superhumps with an amplitude of 0.7 mag, the largest amplitude among known WZ Sge-type DNe. More interestingly, its early superhumps became the reddest around their secondary minimum, whereas other WZ Sge-type DNe show the reddest color around the early superhump maximum. The spectrum around the peak of the outburst showed two double-peaked emission lines of He ii 4686 Å and Hα with a peak separation of ≥700 km s−1, supporting a very high-inclination system. With the early superhump mapping, the unique profile and color of the early superhump are successfully reproduced by an accretion disk with a vertically extended double arm structure. Therefore, a large amplitude and a unique color behavior of the early superhumps in PNV J00444033+4113068 can be explained by the 2 : 1 resonance model along with other WZ Sge-type DNe.
5. The EREBOS project: Investigating the effect of substellar and low-mass stellar companions on late stellar evolution: Survey, target selection, and atmospheric parameters

   https://doi.org/10.1051/0004-6361/201936019  

   Schaffenroth, V. ; Barlow, B. N. ; Geier, S. ; Vučković, M. ; Kilkenny, D. ; Wolz, M. ; Kupfer, T. ; Heber, U. ; Drechsel, H. ; Kimeswenger, S. ; et al ( October 2019 , Astronomy & Astrophysics)

   Eclipsing post-common-envelope binaries are highly important for resolving the poorly understood, very short-lived common-envelope phase of stellar evolution. Most hot subdwarfs (sdO/Bs) are the bare helium-burning cores of red giants that have lost almost all of their hydrogen envelope. This mass loss is often triggered by common-envelope interactions with close stellar or even substellar companions. Cool companions to hot subdwarf stars such as late-type stars and brown dwarfs are detectable from characteristic light-curve variations – reflection effects and often eclipses. In the recently published catalog of eclipsing binaries in the Galactic Bulge and in the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey, we discovered 125 new eclipsing systems showing a reflection effect seen by visual inspection of the light curves and using a machine-learning algorithm, in addition to the 36 systems previously discovered by the Optical Gravitational Lesing Experiment (OGLE) team. The Eclipsing Reflection Effect Binaries from Optical Surveys (EREBOS) project aims at analyzing all newly discovered eclipsing binaries of the HW Vir type (hot subdwarf + close, cool companion) based on a spectroscopic and photometric follow up to derive the mass distribution of the companions, constrain the fraction of substellar companions, and determine the minimum mass needed to stripmore »off the red-giant envelope. To constrain the nature of the primary we derived the absolute magnitude and the reduced proper motion of all our targets with the help of the parallaxes and proper motions measured by the Gaia mission and compared those to the Gaia white-dwarf candidate catalog. It was possible to derive the nature of a subset of our targets, for which observed spectra are available, by measuring the atmospheric parameter of the primary, confirming that less than 10% of our systems are not sdO/Bs with cool companions but are white dwarfs or central stars of planetary nebula. This large sample of eclipsing hot subdwarfs with cool companions allowed us to derive a significant period distribution for hot subdwarfs with cool companions for the first time showing that the period distribution is much broader than previously thought and is ideally suited to finding the lowest-mass companions to hot subdwarf stars. The comparison with related binary populations shows that the period distribution of HW Vir systems is very similar to WD+dM systems and central stars of planetary nebula with cool companions. In the future, several new photometric surveys will be carried out, which will further increase the sample of this project, providing the potential to test many aspects of common-envelope theory and binary evolution.« less