Accretion disks around compact objects are expected to enter an unstable phase at high luminosity
. One instability may occur when the radiation pressure generated by accretion modifies the disk ...viscosity, resulting in the cyclic depletion and refilling of the inner disk on short timescales
. Such a scenario, however, has only been quantitatively verified for a single stellar-mass black hole
. Although there are hints of these cycles in a few isolated cases
, their apparent absence in the variable emission of most bright accreting neutron stars and black holes has been a continuing puzzle
. Here we report the presence of the same multiwavelength instability around an accreting neutron star. Moreover, we show that the variability across the electromagnetic spectrum-from radio to X-ray-of both black holes and neutron stars at high accretion rates can be explained consistently if the accretion disks are unstable, producing relativistic ejections during transitions that deplete or refill the inner disk. Such a new association allows us to identify the main physical components responsible for the fast multiwavelength variability of highly accreting compact objects.
ABSTRACT
We present two contiguous nights of simultaneous time-resolved Gran Telescopio Canarias spectroscopy and William Herschel Telescope photometry of the black hole X-ray transient XTE ...J1859+226, obtained in 2017 July during quiescence. Cross-correlation of the individual spectra against a late K-type spectral template enabled us to constrain the orbital period to 0.276 ± 0.003 d and the radial velocity semi-amplitude of the donor star to K2 = 550 ± 59 km s−1. An ellipsoidal modulation is detected in the photometric r- and i-band light curves, although it is strongly contaminated by flickering activity. By exploiting correlations between the properties of the double-peaked H α emission-line profile and the binary parameters, we derived an orbital inclination of 66.6 ± 4.3 deg, a refined K2 = 562 ± 40 km s−1 and mass ratio q = M2/M1 = 0.07 ± 0.01. From these values, we obtained an updated black hole mass of M1 = 7.8 ± 1.9 M⊙. An independent mass estimate based on X-ray timing agrees well with our value, which gives further support for the outburst quasi-periodic oscillation triplet being explained by the relativistic precession model. We also obtained a companion star mass M2 = 0.55 ± 0.16 M⊙, which is consistent with its K5-K7 V spectral type.
Accreting white dwarfs are known to show signatures of wind-type outflows in the ultraviolet. However, at optical wavelengths, wind detections have only been reported for a few sources. We present ...GTC-10.4 m optical spectroscopy of four accreting white dwarfs (BZ Cam, V751 Cyg, MV Lyr, and V425 Cas) observed during luminous epochs, when their optical emission is expected to be dominated by the accretion disc. Our analysis focuses on four emission lines: H
α
and He
I
λ
5876,
λ
6678, and
λ
7065. Line profiles are complex and variable on short (minutes) and long (days to weeks) timescales, with transient absorption and emission components. Among them, we detect strong blueshifted absorptions at ≳1000 km s
−1
. These high-velocity components, present only in the blue wing of the emission lines, are observed in all four sources and could be associated with accretion disc winds. For MV Lyr and V425 Cas, these would represent the first detection of optical outflows in these objects, while in the cases of BZ Cam and V751 Cyg, the presence of outflows has been previously reported. This study suggests that, in addition to ultraviolet winds, optical outflows might also be common in accreting white dwarfs. We discuss the observational properties of these winds and their possible similarity to those detected in accreting black holes and neutrons stars.
ABSTRACT
We present optical spectroscopy obtained with the GTC, VLT, and SALT telescopes during the decline of the 2016–2017 outburst of the black hole candidate GRS 1716−249 (Nova Oph 1993). Our ...18-epoch data set spans 6 months and reveals that the observational properties of the main emission lines are very variable, even on time-scales of a few hours. Several epochs are characterized by P-Cyg (as well as flat-top and asymmetric) profiles in the Hα, Hβ, and He ii (λ4686) emission lines, implying the presence of an accretion disc wind, which is likely hot and dense. The wind’s terminal velocity (∼2000 km s−1) is similar to that observed in other black hole X-ray transients. These lines also show transient and sharp red-shifted absorptions, taking the form of inverted P-Cyg profiles. We argue that these profiles can be explained by the presence of infalling material at ∼1300 km s−1. We propose a failed wind scenario to explain this inflow and discuss other alternatives, such as obscuration produced by an accretion-related structure (e.g. the gas stream) in a high inclination system.
Swift J1357.2-0933 is a black hole transient that is of particular interest due to the optical recurrent dips found during its first two outbursts (in 2011 and 2017), which lack an obvious X-ray ...equivalent. We present a study based on fast optical photometry during its two most recent outbursts, in 2019 and 2021. Our observations reveal that the optical dips were present in every observed outburst of the source, although they were shallower and showed longer recurrence periods in the two most recent and fainter events. We performed a global study of the dips properties in the four outbursts and found that they do not follow a common temporal evolution. In addition, we discovered a correlation with the X-ray and optical fluxes, with the dips being more profound and showing shorter recurrence periods for brighter stages. This trend seems to extend even to the faintest, quiescent states of the source. We discuss these results in the context of the possible connection between optical dips and outflows found in previous works.
The Earth's albedo is a fundamental climate parameter for understanding the radiation budget of the atmosphere. It has been traditionally measured not only from space platforms but also from the ...ground for 16 years from Big Bear Solar Observatory by observing the Moon. The photometric ratio of the dark (earthshine) to the bright (moonshine) sides of the Moon is used to determine nightly anomalies in the terrestrial albedo, with the aim of quantifying sustained monthly, annual, and/or decadal changes. We find two modest decadal scale cycles in the albedo, but with no significant net change over the 16 years of accumulated data. Within the evolution of the two cycles, we find periods of sustained annual increases, followed by comparable sustained decreases in albedo. The evolution of the earthshine albedo is in remarkable agreement with that from the Clouds and the Earth's Radiant Energy System instruments, although each method measures different slices of the Earth's Bond albedo.
Key Points
We presente a new 16 year long global albedo record (a fundamental climate parameter) taken using the earthshine methodolgy
The Earth's reflectance presents decadal variability, but overall no long‐term trend is identified
The new data seem to agree well with the only other available albedo data set, the one from CERES instrumentation
We present the discovery of an optical accretion disk wind in the X-ray transient Swift J1858.6-0814. Our 90-spectrum data set, taken with the 10.4 m Gran Telescopio Canarias telescope over eight ...different epochs and across five months, reveals the presence of conspicuous P-Cyg profiles in He i at 5876 and H . These features are detected throughout the entire campaign, albeit their intensity and main observational properties are observed to vary on timescales as short as 5 minutes. In particular, we observe significant variations in the wind velocity, between a few hundreds and ∼2400 . In agreement with previous reports, our observations are characterized by the presence of frequent flares, although the relation between the continuum flux variability and the presence/absence of wind features is not evident. The reported high activity of the system at radio waves indicates that the optical wind of Swift J1858.6-0814 is contemporaneous with the radio jet, as is the case for the handful of X-ray binary transients that have shown so far optical P-Cyg profiles. Finally, we compare our results with those of other sources showing optical accretion disk winds, with emphasis on V404 Cyg and V4641 Sgr, since they also display strong and variable optical wind features as well as similar flaring behavior.
Abstract
After more than 26 years in quiescence, the black hole transient V404 Cyg went into a luminous outburst in June 2015, and additional activity was detected in late December of the same year. ...Here, we present an optical spectroscopic follow-up of the December mini-outburst, together with X-ray, optical and radio monitoring that spanned more than a month. Strong flares with gradually increasing intensity are detected in the three spectral ranges during the ∼10 days following the Swift trigger. Our optical spectra reveal the presence of a fast outflowing wind, as implied by the detection of a P-Cyg profile (He i–5876 Å) with a terminal velocity of ∼ 2500 km s−1 . Nebular-like spectra – with an Hα equivalent width of ∼500 Å – are also observed. All these features are similar to those seen during the main June 2015 outburst. Thus, the fast optical wind simultaneous with the radio jet is most likely present in every V404 Cyg outburst. Finally, we report on the detection of a strong radio flare in late January 2016, when X-ray and optical monitoring had stopped due to Sun constraints.
ABSTRACT
We present a new method to derive binary inclinations in quiescent black hole (BH) X-ray transients (XRTs), based on the depth of the trough (T) from double-peaked H α emission profiles ...arising in accretion discs. We find that the inclination angle (i) is linearly correlated with T in phase-averaged spectra with sufficient orbital coverage (≳50 per cent) and spectral resolution, following i(deg) = 93.5 × T + 23.7. The correlation is caused by a combination of line opacity and local broadening, where a leading (excess broadening) component scales with the deprojected velocity of the outer disc. Interestingly, such scaling allows to estimate the fundamental ratio M1/Porb by simply resolving the intrinsic width of the double-peak profile. We apply the T–i correlation to derive binary inclinations for GRO J0422+32 and Swift J1357−0933, two BH XRTs where strong flickering activity has hindered determining their values through ellipsoidal fits to photometric light curves. Remarkably, the inclination derived for GRO J0422+32 (i = 55.6 ± 4.1○) implies a BH mass of $2.7^{+0.7}_{-0.5}$ M⊙ thus placing it within the gap that separates BHs from neutron stars. This result proves that low-mass BHs exist in nature and strongly suggests that the so-called ‘mass gap’ is mainly produced by low number statistics and possibly observational biases. On the other hand, we find that Swift J1357−0933 contains a $10.9^{+1.7}_{-1.6}$ M⊙ BH, seen nearly edge on ($i=87.4^{+2.6}_{-5.6}$ deg). Such extreme inclination, however, should be treated with caution since it relies on extrapolating the T–i correlation beyond i ≳ 75○, where it has not yet been tested.
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