A brown dwarf donor and an optically thin accretion disc with a complex stream impact region in the period-bouncer candidate BW Sculptoris

We present an analysis of multi-epoch spectroscopic and photometric observations of the WZ Sge-type dwarf nova BW Scl, a period-bouncer candidate. We detected multiple irradiation-induced emission lines from the donor star allowing the radial velocity variations to be measured with high accuracy. Also, using the absorption lines Mgii 4481 and Caii K originated in the photosphere of the accreting white dwarf (WD), we measured the radial velocity semi-amplitude of the WD and its gravitational redshift. We find that the WD has a mass of 0.85$\pm$0.04 M$_\odot$, while the donor is a low-mass object with a mass of 0.051$\pm$0.006 M$_\odot$, well below the hydrogen-burning limit. Using NIR data, we put an upper limit on the effective temperature of the donor to be $\lesssim$1600 K, corresponding to a brown dwarf of T spectral type. The optically thin accretion disc in BW Scl has a very low luminosity $\lesssim$4 $\times 10^{30}$ erg s$^{-1}$ which corresponds to a very low mass accretion rate of $\lesssim$7 $\times 10^{-13}$ M$_\odot$ year$^{-1}$. The outer parts of the disc have a low density allowing the stream to flow down to the inner disc regions. The brightest part of the hotspot is located close to the circularization radius of the disc. The hotspot is optically thick and has a complex, elongated structure. Based on the measured system parameters, we discuss the evolutionary status of the system.Comment: Accepted for publication in MNRAS. The title changed to better reflect scientific content. New Figure 8 (left) was added and the system parameters were refined; general conclusions were unchanged. Supplementary figures (dynamical Doppler maps) available at https://vitaly.neustroev.net/researchfiles/bwscl

FS Aur - a new class of Cataclysmic Variables or the missing link between Intermediate Polars and SW Sex objects?

FS Aur is a known dwarf nova with an orbital period of about 85.7 minutes. It has been assumed to be a member of the SU UMa subclass of cataclysmic variables (CVs), but previous searches for superhumps and superoutburst have been unsuccessful. We conducted a series of photometric and spectroscopic observations of FS Aur during quiescence. We confirmed its short orbital period from radial velocity measurements. However, the long-term photometry revealed an unexpected result: the system also shows a distinct 0.24 mag modulation in the BVR photometric bands with a period of 205.5 min, which is 2.4 times longer than the orbital period. We discuss various possible causes for such a peculiar behavior.Comment: 12 pages, 1 table, 12 figures, accepted for publication in PAS

MAGIC and H.E.S.S. detect VHE gamma rays from the blazar OT081 for the first time: a deep multiwavelength study

https://pos.sissa.it/395/815/pdfPublished versio

A brown dwarf donor and an optically thin accretion disc with a complex stream impact region in the period-bouncer candidate BW Sculptoris

Abstract We present an analysis of multi-epoch spectroscopic and photometric observations of the WZ Sge-type dwarf nova BW Scl, a period-bouncer candidate. We detected multiple irradiation-induced emission lines from the donor star allowing the radial velocity variations to be measured with high accuracy. Also, using the absorption lines Mg II 4481 Å and Ca II K originated in the photosphere of the accreting white dwarf (WD), we measured the radial velocity semi-amplitude of the WD and its gravitational redshift. We find that the WD has a mass of 0.85 ± 0.04 M⊙, while the donor is a low-mass object with a mass of 0.051 ± 0.006 M⊙, well below the hydrogen-burning limit. Using NIR data, we put an upper limit on the effective temperature of the donor to be ≲1600 K, corresponding to a brown dwarf of T spectral type. The optically thin accretion disc in BW Scl has a very low luminosity ≲4 × 10³⁰ erg s⁻¹ which corresponds to a very low-mass accretion rate of ≲7 × 10⁻¹³ M⊙ yr⁻¹. The outer parts of the disc have a low density allowing the stream to flow down to the inner disc regions. The brightest part of the hotspot is located close to the circularization radius of the disc. The hotspot is optically thick and has a complex elongated structure. Based on the measured system parameters, we discuss the evolutionary status of the system

2017 Golden Age of Cataclysmic Variables and Related Objects IV, GOLDEN 2017

Standard evolutionary theory suggests that cataclysmic variables (CVs) evolve from longer to shorter orbital periods. CVs that have passed beyond the period minimum and are evolving back towards longer periods are called the period bouncers. CVs of the WZ Sge-type have been long considered as potential period-bounce candidates. However, only very few of recently discovered WZ Sge-type stars were observed spectroscopically in quiescence due to their faintness. The lack of information on many of WZ Sge-type stars does not allow us to put restrictions on their system parameters and to confirm or deny their period bounce nature. Here we present a novel, simpler yet equally valuable approach based on multicolour broadband photometry to reveal the best period bounce candidates. By adopting such an approach, we performed a pilot study of a sample of WZ Sge-type stars and accreting WDs and have found solid evidence for very low luminosity donor stars in several of them.</p