Optical andJ, K-photometry of the quiescent black hole X-ray nova A0620−00 in the passive and active states

Abstract

Photometric observations of the low-mass X-ray binary system A0620-00=V616 Mon are performed in the optical (unfiltered light, λ eff ≈ 6400Å) and the near-infrared J and K-bands. The mean system flux, the orbital light curve shape and the flickering amplitude dependences on wavelength are examined in the λλ6400 − 22000Å range for two activity stages of the system which remains in quiescence.In 2015-16 the A0620-00 system was observed to be in passive stage (according to the terminology of Cantrell et al., 2008) exhibiting the regular orbital light curve with a relatively low flickering level.In less than 230 days in 2016-17 the system switched into active stage: its mean brightness increased by ∼ 0.2 − 0.3 mag in all the studied bands, the orbital light curve experienced drastic changes while the flickering amplitude increased more than twice both in the optical and in the infrared.The regular orbital light curves of A0620-00 were modelled within two frameworks, namely assuming existence of "cold" spots on the optical companion surface and without those. These models allow for a good fit of observed orbital light curves both in passive and in active stages. A close correlation between the luminosity of accretion disc, "hotline" and the flickering amplitude is established.The absolute fluxes having been calibrated and corrected for the interstellar extinction, the dependence of the mean square flickering amplitude (in fluxes) is computed as a function of wavelength in the λλ6400 − 22000Å range.In active stage, the observed flickering amplitude decreases as a function of wavelength over the whole studied range and may be adequately represented by a single dependence ∆F fl ∼ λ −2 which corresponds to the thermal free-free emission of optically thin high-temperature plasma.In passive stage, the ∆F fl value behaves like ∆F fl ∼ λ −4 in the short wavelength range of λλ6400 − 12500Å that corresponds to the thermal radiation of optically thick high-temperature plasma. In the long-wave domain λλ12500 − 22000Å the flickering amplitude dependence is flat: ∆F fl ∼ const which may imply existence of a synchrotron component of the relativistic jets emission.The ∆F fl dependence on the mean system flux in K-filter is shallower than corresponding relations for the filter J and for the optical range.The above mentioned flickering features let us propose that the mechanism responsible for flickering includes at least two components: thermal and, apparently, synchrotron, that agrees with the recent discovery of the variable linear polarization of the infrared system emission (Russell et al., 2016).

ALMA observations of A0620–00: fresh clues on the nature of quiescent black hole X-ray binary jets

“…This "flickering" component is stronger at longer wavelengths, with a fractional rms amplitude of ∼ 15-24 per cent at optical wavelengths, rising to ∼ 42 per cent at (K-band) NIR wavelengths (Dinçer et al 2018). The spectral index of the variable component has been inferred by various authors to be steeply red at optical-NIR wavelengths (α = −1.4 to −0.7; Shahbaz et al 2004;Cantrell et al 2010;Dinçer et al 2018), and flat/slightly inverted at NIR (α ∼ 0.3 to 0.4; Cherepashchuk et al 2019) and mid-IR (α = 0.2 to 0.3 at 3.6-8.0 µm; Maitra et al 2011) wavelengths. Noting that the extrapolation of this non-stellar mid-IR spectrum of A0620-00 down to GHz frequencies was consistent with the radio flux measured by the VLA in 2005, Russell et al (2013a) concluded that the optically thick-to-thin jet break lies in the optical frequency range: 1.3 ± 0.5 × 10 14 Hz.…”

“…Figure 2. A0620-00's orbital-phased SMARTS V-band magnitudes, calculated using the updated ephemeris by Cherepashchuk et al (2019). Nearly simultaneous epochs with other multi-wavelength measurements quoted in the body of this paper are highlighted with different colors, and compared against the passive state "envelope" defined by Cantrell et al (2008), in black, vs. active state data from Cantrell et al, as well as Dinçer et al (2018), in sand and light pink.…”

“…In the active state, the optical-NIR flux is 0.1-0.4 magnitudes brighter and variable, with prominent variations even on short timescales (down to seconds; e.g., Hynes et al 2003;Neilsen, Steeghs & Vrtilek 2008). A0620-00, which has been in quiescence since the decline from the 1975 outburst that led to its discovery (Elvis et al 1975), can spend months or years in these different quiescent states, being in the passive state in 1997-2003, the active state in 2004-2007 and 2013, the passive state in 2015-2016, changing to the active state by 2016 November (Shugarov et al 2016;van Grunsven et al 2017;Dinçer et al 2018;Cherepashchuk et al 2019). Figure 2 illustrates the phase-folded, V-band variability of A0620-00 during (or close to) the different campaigns and observations discussed below, vis-a-vis the well-identified passive and active states, shown by the black and sand/pink circles, respectively.…”

Optical, J, and K light curves of XTE J1118+480 = KV UMa: the mass of the black hole and the spectrum of the non-stellar component

“…It should be emphasised that in the optical light and in the IR range a considerable growing of the average brightness of the system was not accompanied by the increase of the flickering. In this feature KV UMa differed from the low mass X-ray nova A0620-00 (see, e.g., Shugarov et al 2016;Cherepashchuk et al 2019).…”

“…Recently it was realised that even in quiescence there are non-stationary processes in X-ray novae with black holes. They are manifested in the fact that some X-ray novae in quiescence show passive and active states of the optical variability (Cantrell et al 2008(Cantrell et al , 2010Cherepashchuk et al 2019). In the passive state the amplitude of the irregular variability (flickering) is relatively low, the orbital light curve has a regular shape.…”