Abstract Cataclysmic variables can experience short optical brightenings, which are commonly attributed to phenomena such as dwarf novae outbursts, micronovae, donor flares, or magnetic gating ...bursts. Since these events exhibit similar observational characteristics, their identification has often been ambiguous. In particular, magnetic gating bursts and micronovae have been suggested as alternative interpretations of the same phenomena. Here we show that the timescales and energies separate the optical brightenings into separate clusters consistent with their different classifications. This suggests that micronovae and magnetic gating bursts are in fact separate phenomena. Based on our findings, we develop diagnostic diagrams that can distinguish between these bursts/flares based on their properties. We demonstrate the effectiveness of this approach on observations of a newly identified intermediate polar, CTCV J0333-4451, which we classify as a magnetic gating system. CTCV J0333-4451 is the third highest spin-to-orbital period ratio intermediate polar with magnetic gating, suggesting that these bursts are common among these rare systems.
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We study the stellar kinematic properties and spatial distribution of the association Cyg OB3 using precise astrometric data from Gaia DR2. All known O- and B-type stars in Cyg OB3 region ...with positions, parallaxes, and proper motions available are included, comprising a total of 41 stars. The majority of stars are found to be concentrated at a heliocentric distance of 2.0 ± 0.3 kpc. The mean peculiar velocity of the sample after removing Galactic rotation and solar motion is ∼22 km s−1, dominated by the velocity component towards the Galactic centre. The relative position and velocity of the black hole X-ray binary Cyg X-1 with respect to the association suggest that Cyg OB3 is most likely its parent association. The peculiar kinematic properties of some of the stars are revealed and are suggestive of past stellar encounters. The sample includes a previously known runaway star HD 227018, and its high peculiar velocity of ∼50 km s−1 is confirmed with Gaia. We estimated the velocities of stars relative to the association and the star HD 225577 exhibits peculiar velocity smaller than its velocity relative to the association. The star has lower value of proper motion than the rest of the sample. The results suggest a slowly expanding nature of the association, which is supported by the small relative speeds (<20 km s−1) with respect to the association for a majority of the sample stars.
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The black hole X-ray transient MAXI J1820 + 070 (= ASSASN-18ey) discovered in 2018 March was one of the optically brightest ever seen, which has resulted in very detailed optical outburst ...light curves being obtained. We combine them here with X-ray and radio light curves to show the major geometric changes the source undergoes. We present a detailed temporal analysis that reveals the presence of remarkably high amplitude (>0.5 mag) modulations, which evolve from the superhump (16.87 h) period towards the presumed orbital (16.45 h) period. These modulations appear ∼87 d after the outburst began, and follow the Swift/BAT hard X-ray light curve, which peaks 4 d before the radio flare and jet ejection, when the source undergoes a rapid hard to soft state transition. The optical modulation then moves closer to the orbital period, with a light-curve peak that drifts slowly in orbital phase from ∼0.8 to ∼0.3 during the soft state. We propose that the unprecedentedly large amplitude modulation requires a warp in the disc in order to provide a large enough radiating area, and for the warp to be irradiation driven. Its sudden turn-on implies a change in the inner disc geometry that raises the hard X-ray-emitting component to a height where it can illuminate the warped outer disc regions.
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Swift J1357.2 − 0933 is a black-hole candidate X-ray transient, which underwent its third outburst in 2019, during which several multiwavelength observations were carried out. Here, we ...report results from the Neil Gehrels Swift and NICER observatories and radio data from AMI. For the first time, millihertz quasi-periodic X-ray oscillations with frequencies varying between ∼1 and 5 $\rm {mHz}$ were found in NICER observations and a similar feature was also detected in one Swift–xrt data set. Our spectral analysis indicate that the maximum value of the measured X-ray flux is much lower compared to the peak values observed during the 2011 and 2017 outbursts. This value is ∼100 times lower than found with MAXI on MJD 58558 much (∼68 d) earlier in the outburst, suggesting that the Swift and NICER fluxes belong to the declining phase of the 2019 outburst. An additional soft component was detected in the xrt observation with the highest flux level, but at a relatively low $L_{\rm X} \sim 3{\times }10^{34}~(d/{\rm 6~kpc)}^2\rm {erg}~\rm {s}^{-1}$ , and which we fitted with a disc component at a temperature of ∼0.17 keV. The optical/UV magnitudes obtained from Swift–uvot showed a correlation with X-ray observations, indicating X-ray reprocessing to be the plausible origin of the optical and UV emission. However, the source was not significantly detected in the radio band. There are currently a number of models that could explain this millihertz-frequency X-ray variability; not least of which involves an X-ray component to the curious dips that, so far, have only been observed in the optical.
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We report on observations of black hole Swift J1357.2–0933, a member of the modest population of very faint X-ray transients. This source has previously shown intense dips in the optical ...light curve, a phenomena that has been linked to the existence of a ‘unique toroidal structure’ in the inner region of the disc, seen at a high inclination. Our observations, carried out by the Neil Gehrels Swift and NuSTAR X-ray observatories, do not show the presence of intense dips in the optical light curves. We find that the X-ray light curves do not show any features that would straightforwardly support an edge-on configuration or high inclination configuration of the orbit. This is similar to what was seen in the X-ray observations of the source during its 2011 outburst. Moreover, the broad-band spectra were well described with an absorbed power-law model without any signatures of cut-off at energies above 10 keV, or any reflection from the disc or the putative torus. Thus, the X-ray data do not support the unique ‘obscuring torus’ scenario proposed for J1357. We also performed a multiwavelength study using the data of X-ray telescope and Ultraviolet/Optical Telescope aboard Swift, taken during the ∼4.5 months duration of the 2017 outburst. This is consistent with what was previously inferred for this source. We found a correlation between the simultaneous X-ray and ultraviolet/optical data and our study suggests that most of the reprocessed flux must be coming out in the ultraviolet.
ABSTRACT We report on spin variations in the intermediate polar and cataclysmic variable CC Scl, as seen by the Transiting Exoplanet Survey Satellite (TESS). By studying both the spin period and its ...harmonic, we find that the spin has varied since it was first observed in 2011. We find the latest spin value for the source to be 389.473(6) s, equivalent to 0.00450779(7) d, 0.02 s shorter than the first value measured. A linear fit to these and intermediate data give a rate of change of spin (Ṗ) ∼ −4.26(2.66)× 10−11 and a characteristic time-scale τ ∼ 2.90 × 105 yr, in line with other known intermediate polars with varying spin. The spin profile of this source also matches theoretical spin profiles of high-inclination intermediate polars, and furthermore, appears to have changed in shape over a period of three years. Such ‘spin-up’ in an intermediate polar is considered to be from mass accretion on to the white dwarf (the primary), and we note the presence of dwarf nova eruptions in this source as being a possible catalyst of the variations.
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We present high-speed optical photometry from SAAO and SALT on the black hole LMXB MAXI J1820+070 (ASSASN-18ey), some of it simultaneous with NICER, Swift, and Insight-HXMT X-ray coverage. ...We detect optical quasi-periodic oscillations (QPOs) that move to higher frequencies as the outburst progresses, tracking both the frequency and evolution of similar X-ray QPOs previously reported. Correlated X-ray/optical data reveal a complex pattern of lags, including an anticorrelation and a sub-second lag that evolve over the first few weeks of outburst. They also show correlated components separated by a lag equal to the QPO period roughly centered on zero lag, implying that the inter-band variability is strongly and consistently affected by these QPOs at a constant phase lag of roughly ±π. The synchronization of X-ray and optical QPOs indicates that they must be produced in regions physically very close to each other; we, thus, propose that they can be explained by a precessing jet model, based on analogies with V404 Cyg and MAXI J1348−630.
Abstract Time-resolved SALT spectra of the short-period, dipping X-ray transient, Swift J1357.2−0933, during its 2017 outburst has revealed broad Balmer and He ii λ4686 absorption features, ...blueshifted by ∼600 km s−1. Remarkably these features are also variable on the ∼500 s dipping period, indicating their likely association with structure in the inner accretion disc. We interpret this as arising in a dense, hot (≳30 000 K) outflowing wind seen at very high inclination, and draw comparisons with other accretion disc corona sources. We argue against previous distance estimates of 1.5 kpc and favour a value ≳6 kpc, implying an X-ray luminosity LX ≳ 4 × 1036 erg s−1. Hence it is not a very faint X-ray transient. Our preliminary 1D Monte Carlo radiative transfer and photoionization calculations support this interpretation, as they imply a high intrinsic LX, a column density NH ≳ 1024 cm−2, and a low covering factor for the wind. Our study shows that Swift J1357.2−0933 is truly remarkable amongst the cohort of luminous, Galactic X-ray binaries, showing the first example of He ii λ4686 absorption, the first (and only) variable dip period and is possibly the first black hole ‘accretion disc corona’ candidate.
Studying the rapid variability of many astronomical objects is key to understanding the underlying processes at play. However, a combination of limited telescope availability, viewing constraints, ...and the unpredictable nature of many sources mean that obtaining data well-suited to this task can be tricky, especially when it comes to simultaneous multiwavelength observations. Researchers can often find themselves tuning observational parameters in real-time, or may realise later that their observation did not achieve their goals. Here, we present CorrSim, a program to aid planning of multiwavelength coordinated observations. CorrSim takes a model of a system (i.e. Power Spectra, Coherence, and Lags), and returns a simulated multiwavelength observation, including effects of noise, telescope parameters, and finite sampling. The goals of this are: (i) To simulate a potential observation (to inform decisions about its feasibility); (ii) To investigate how different Fourier models affect a system's variability (e.g. how altering the frequency-dependent lags between bands can affect data products like cross-correlation functions); and (iii) To simulate existing data and investigate its trustworthiness. We outline the methodology behind CorrSim, show how a variety of parameters (e.g. noise sources, observation length, and telescope choice) can affect data, and present examples of the software in action.
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