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Pyrzas S.,University of Warwick | Gansicke B.T.,University of Warwick | Brady S.,AAVSO | Parsons S.G.,University of Warwick | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We identify SDSS J121010.1+334722.9 as an eclipsing post-common-envelope binary, with an orbital period of Porb = 2.988 h, containing a very cool, low-mass, DAZ white dwarf and a low-mass main-sequence star of spectral type M5. A model atmosphere analysis of the metal absorption lines detected in the blue part of the optical spectrum, along with the Galaxy Evolution Explorer near-ultraviolet flux, yields a white dwarf temperature of Teff,WD = 6000±200K and a metallicity value of log [Z/H]=-2.0 ± 0.3. The Na I λλ8183.27, 8194.81 absorption doublet is used to measure the radial velocity of the secondary star, Ksec = 251.7 ± 2.0kms-1, and Fe I absorption lines in the blue part of the spectrum provide the radial velocity of the white dwarf, KWD = 95.3 ± 2.1kms-1, yielding a mass ratio of q = 0.379 ± 0.009. Light-curve model fitting, using the Markov chain Monte Carlo method, gives the inclination angle as i = (79.°05-79.°36) ± 0.°15, and the stellar masses as MWD = 0.415 ± 0.010M⊙ and Msec = 0.158 ±0.006M⊙. Systematic uncertainties in the absolute calibration of the photometric data influence the determination of the stellar radii. The radius of the white dwarf is found to be RWD = (0.0157-0.0161) ± 0.0003R⊙ and the volume-averaged radius of the tidally distorted secondary is Rsec,vol.aver. = (0.197-0.203) ± 0.003R⊙. The white dwarf in SDSS J121010.1+334722.9 is a very strong He-core candidate. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Munari U.,National institute for astrophysics | Henden A.,AAVSO | Frigo A.,Astronomical Observatory
New Astronomy | Year: 2014

The Aquarius stream has been recently discovered in the course of the RAVE Survey. It is a chemically coherent structure, originating from the tidal disruption of a 12 Gyr, [Fe/H] = -1.0 globular cluster. We have surveyed a ∼ 284 deg2 area of the sky containing the 15 known members of the Aquarius stream looking for RR Lyr variables. RR Lyr variables are primary distance indicators and discovering some of them firmly associated with the Aquarius stream would provide a 3D representation of its Galactic orbit and would probe the spatial structure of the Galactic gravitational potential. During September and October 2012, we have obtained on-purpose, epoch photometry in the Landolt B,V and Sloan g,r,i bands with the APASS South telescopes located at Cerro Tololo. Our data are uniformly complete to V = 15.2 mag over the whole surveyed area, the faintest recorded stars reaching V = 18 mag. We have found 71 RR Lyr variables, and a firm pulsation period was derived for 53 of them. Our census of RR Lyr variables is complete to a distance of 8 kpc from the Sun. For all objects we provide distances and light- and color-curves, mean values and amplitudes in all five BVgri passbands, finding charts and accurate local photometric sequences. About half of the RR Lyr variables we have discovered were previously known, but we provide the first multi-band photometric data. They were in fact mostly discovered as by-products of white-light patrol searches for optical counterparts to gamma-ray bursters or potentially hazardous asteroids. © 2013 Elsevier B.V. All rights reserved.

News Article | January 18, 2016

The star KIC 8462852 has dazzled space nerds around the world due to wild speculation that alien megastructures might be causing its funky light fluctuations. But over the last few months, SETI astronomers have scanned it for unusual radio or laser signals that might indicate technological sophistication, and turned up nothing. That doesn’t rule out the ET hypothesis, but it doesn’t exactly bolster it either. Indeed, as astronomers have emphasized from the beginning, KIC 8462852’s odd behavior is much more likely to be caused by natural phenomenon than massive alien construction projects. To that point, the leading theory has been that the star is periodically occulted by a huge cluster of comets that block out light from our perspective—sometimes as much as 20 percent. But Bradley Schaefer, a prolific astrophysicist and professor based at Louisiana State University, has thrown a wrench into the comet hypothesis with a paper published last week on arxiv. It is not just Schaefer’s conclusions, but his methods, that make this new research so interesting. By sifting through Harvard College Observatory’s collection of historic glass photographic plates—an archive that dates back to the 1880s and consists of about 500,000 samples—Schaefer was able to reconstruct the history of KIC 8462852’s light fluctuations as far back as the 1890s. “For some science questions, we care about knowing the long-term activity of a star, and this can be measured by looking at the star's brightness over many images going far back in time,” Schaefer told me over email. “The idea is to create what we call a 'light curve', which is just really a time history of the star's brightness.” In recent decades, most astronomers have increasingly relied on charge coupled devices (CCDs) to image the night sky, which has allowed for much more sensitivity, higher resolution, and easier processing procedures. The original study on KIC 8462852 that sparked so much interest in its dimming periods was based on data acquired by the Kepler space telescope, which relies on an array of 42 CCDs. As such, it provided a recent summary of the star’s behavior rather than a long view. But when it comes to studying light curves over extended periods like decades, or even centuries, the traditional photographic plate reigns supreme. “For timescales longer than a few years, CCDs fail completely, and we can only use old archival sky photographs,” Schaefer said. “In the case of KIC 8462852, I got 1,232 good plates and accurate measures of the star's magnitude (i.e., brightness).” You know how DJs dig through vinyl record stores for good hooks and samples? This is basically an astronomical riff on the same idea, and the plates revealed a deeper, untold story about this flagrantly weird star. As it turns out, KIC 8462852 is not only subject to extreme light fluctuations in the short term, but a longer gradual dimming that has been beautifully recorded in the glass annals housed at Harvard. Based on the rate at which the star’s light curve is waning, Schaefer calculated that it would take a glut of about 648,000 massive comets with diameters of at least 200 kilometres to account for the observed fluctuations. That is an extremely unlikely scenario that is almost as far-fetched as the alien gambit. “This fading is what has the deep implications, like refuting many of the proposed models, including the comet-family model, as well as the extreme speculations,” Schaefer said. So if this perplexing light curve is not likely to be sculpted by megastructures or comet swarms, then what the flux is going on with KIC 8462852? The only way to find out is more observation—especially during one of the star’s now famous periods of occultation. “Likely the best try is to get a spectrum of the star during a dip,” Schaefer told me. “From the spectrum, we might see absorption lines from any gas associated with the 'occulter,' we might see a reddening that would point to the occulter being mainly dust, or we might see a color neutral dip that would point to a solid body. Thus, a spectrum would tell us the nature of the occulter, and this would greatly narrow down models.” “But dips in the KIC 8462852 light curve are uncommon, so we have to await the next big dip,” he added. Fortunately, astronomer Tabby Boyajian, the lead author of the original paper about KIC 8462852, is on the case. She is heading up a team of researchers from the American Association of Variable Star Observers (AAVSO) who are watching the star’s every move. “These observers are worldwide and they are great, and they are of professional quality,” Schaefer said. “Tabby also has lined up big scopes to fast switch over to her star whenever a dip is found. Alas, so far, there have been no dips. But they will come sometime, and we'll get a spectrum in a dip, and learn the nature of the occulter.” Until then, it’s enough to marvel at how KIC 8462852 continues to casually defy explanation after months of attempts to account for its unprecedented behavior. Mad props to this utter stellar weirdo.

News Article | February 22, 2016

People on Earth look forward to sky events such as a solar eclipse, where the moon fully or partially blocks the sun. The solar eclipses experienced here last only for a few minutes and are nothing compared to a stellar eclipse that was discovered by scientists to last for three and a half years. Every 69 years, in a binary star system about 10,000 light-years from Earth, the sun disappears in a near-total eclipse. The binary system, referred to as TYC 2505-672-1, is now known for two reasons. Among all known binary systems, it has the longest time between eclipses and the longest duration stellar eclipse. In the past, the title holder was Epsilon Aurigae. This giant binary star is eclipsed by its partner star every 27 years for a period of 640 to 730 days. At 2,200 light-years from Earth, it is nearer than TYC 2505-672-1. Aside from its proximity, its brightness also allows astronomers to study it in greater detail. An international team of astronomers from Vanderbilt University and Harvard University made the discovery. Scientists from Ohio State University, Lehigh University, Pennsylvania State University, Las Cumbres Observatory Global Telescope Network and the American Association of Variable Star Observers (AAVSO) assisted with the study. Scanned photographic plates from the Digital Access to a Sky Century at Harvard (DASCH) program were also crucial in the discovery of the bizarre properties of the system. The researchers examined 1,432 images of TYC 2505-672-1 captured by astronomers from Harvard between 1890 and 1989 as well as nearly 9,000 images of the system taken in the last eight years. When the researchers analyzed these together with the AAVSO network's photos of the most recent eclipse, they were able to piece everything together. The researchers found that TYC 2505-672-1 consists of a pair of red giant stars. One of the stars is surrounded by an "extended disk of opaque material" that causes the eclipse to last for years. As for the 69-year eclipse interval, the researchers' calculations show that the red giants must be orbiting at a great distance. "Here we have a rare opportunity to study a phenomenon that plays out over many decades and provides a window into the types of environments around stars that could represent planetary building blocks at the very end of a star system's life," said Keivan Stassun, professor at Vanderbilt and co-author of the study. The study has been accepted for publication in the Astronomical Journal. On July 16, 2186, people on Earth will experience the longest total solar eclipse in 12,000 years. Scientists predict there will be more than seven minutes of darkness when the moon passes between the planet and the sun. If the Earth will experience a solar eclipse that will last for three and a half years, it would be enough to kill all life on Earth. The planet will be too cold and dark that everyone and everything that depends on the sun for survival would die. However, in stars, it's a different story. Years of stellar eclipse are possible in a binary star system.

Henden A.,AAVSO | Munari U.,National institute for astrophysics
Contributions of the Astronomical Observatory Skalnate Pleso | Year: 2014

The APASS photometric survey covers the whole sky, from Pole to Pole, and has measured in Landolt B, V and Sloan g',r',i' bands all stars in the range 10.0≤V ≤17.0 over about four distinct epochs between 2009 and 2013. The photometry is accurate to 0.02 mag and the astrometry to 0.17 arcsec. At the time of writing 8 incremental Data Releases have been issued covering ≥50 million stars. The final survey products will be ready by the end of 2014 and will include 100 million stars. Extension to brighter stars (7.5≤V ≤10.0) and additional bands (u', z' and Y) is underway.

Zacharias N.,Us Naval Observatory | Finch C.T.,Us Naval Observatory | Girard T.M.,Yale University | Henden A.,AAVSO | And 3 more authors.
Astronomical Journal | Year: 2013

The fourth United States Naval Observatory (USNO) CCD Astrograph Catalog, UCAC4, was released in 2012 August (double-sided DVD and CDS data center Vizier catalog I/322). It is the final release in this series and contains over 113 million objects; over 105 million of them with proper motions (PMs). UCAC4 is an updated version of UCAC3 with about the same number of stars also covering all-sky. Bugs were fixed, Schmidt plate survey data were avoided, and precise five-band photometry was added for about half the stars. Astrograph observations have been supplemented for bright stars by FK6, Hipparcos, and Tycho-2 data to compile a UCAC4 star catalog complete from the brightest stars to about magnitude R = 16. Epoch 1998-2004 positions are obtained from observations with the 20 cm aperture USNO Astrograph's "red lens," equipped with a 4k by 4k CCD. Mean positions and PMs are derived by combining these observations with over 140 ground- and space-based catalogs, including Hipparcos/Tycho and the AC2000.2, as well as unpublished measures of over 5000 plates from other astrographs. For most of the faint stars in the southern hemisphere, the first epoch plates from the Southern Proper Motion program form the basis for PMs, while the Northern Proper Motion first epoch plates serve the same purpose for the rest of the sky. These data are supplemented by 2MASS near-IR photometry for about 110 million stars and five-band (B, V, g, r, i) APASS data for over 51 million stars. Thus the published UCAC4, as were UCAC3 and UCAC2, is a compiled catalog with the UCAC observational program being a major component. The positional accuracy of stars in UCAC4 at mean epoch is about 15-100 mas per coordinate, depending on magnitude, while the formal errors in PMs range from about 1 to 10 mas yr-1 depending on magnitude and observing history. Systematic errors in PMs are estimated to be about 1-4 mas yr-1. © 2013. The American Astronomical Society. All rights reserved.

Henden A.A.,AAVSO
Proceedings of the International Astronomical Union | Year: 2011

Citizen Science is the act of collecting or analyzing data by enlisting the help of volunteers who may have no specific scientific training. The workshop discussed how Citizen Science fits into time-domain astronomy, what the roles of such volunteers might be, and how amateur astronomers can help in the new era of surveys. © 2012 International Astronomical Union.

Henden A.A.,AAVSO
Proceedings of the International Astronomical Union | Year: 2011

The AAVSO is initiating a new survey of the sky. It will cover the entire visible sky, both north and south, on a daily basis, in two colours, and with a limiting magnitude of V = 17. This will be a perfect complement to LSST, but will be available years earlier and will continue into the indefinite future. The photometry will be publicly available within 24 hours through our website. Some details of the hardware and operations are described. © 2012 International Astronomical Union.

Munari U.,National institute for astrophysics | Henden A.,AAVSO | Belligoli R.,ANS Collaboration | Castellani F.,ANS Collaboration | And 3 more authors.
New Astronomy | Year: 2013

Accurate and densely populated BVRCIC lightcurves of supernovae SN 2011fe in M101, SN 2012aw in M95 and SN 2012cg in NGC 4424 are presented and discussed. The SN 2011fe lightcurves span a total range of 342 days, from 17 days pre- to 325 days post-maximum. The observations of both SN 2012aw and SN 2012cg were stopped by solar conjunction, when the objects were still bright. The lightcurve for SN 2012aw covers 92 days, that of SN 2012cg spans 44 days. Time and brightness of maxima are measured, and from the lightcurve shapes and decline rates the absolute magnitudes are obtained, and the derived distances are compared to that of the parent galaxies. The color evolution and the bolometric lightcurves are evaluated in comparison with those of other well observed supernovae, showing no significant deviations. © 2012 Elsevier B.V. All rights reserved.

Immediately after the detection by Swift/BAT on June 15.77197 ut, the VSNET collaboration team31 started a worldwide photometric campaign of V404 Cyg. There was also an independent detection by CCD (charge coupled device) photometry on June 16.169 ut32. Time-resolved CCD photometry was carried out at 27 sites using 36 telescopes with apertures of dozens of centimetres (Extended Data Table 2). We also used the public AAVSO data33. We corrected for bias and flat-fielding in the usual manner, and performed standard aperture photometry. The observers, except for TAOS34, used standard filters (B, V, R , I ; we write R and I for R and I in the main text and figures for brevity) and measured magnitudes of V404 Cyg relative to local comparison stars whose magnitudes were measured by A. Henden (sequence 15167RN) from the AAVSO Variable Star Database35. We applied small zero-point corrections to some observers’ measurements. When filtered observations were unavailable, we used unfiltered data to construct the light curve. The exposure times were mostly 2–30 s, with some exceptional cases of 120 s in B band, giving typical time resolution of a few seconds. All of the observation times were converted to BJD. For the Swift/XRT light curves (Fig. 3 and Extended Data Fig. 2), we extracted source events from a region with a 30-pixel radius centred on V404 Cyg. To avoid pile-up effects, we further excluded an inner circular region if the maximum count rate of the XRT raw light curves, binned in 10 s intervals, exceeded 200 counts s−1. The inner radii are set to be 10 and 20 pixels at the maximum raw rate of 1,000 counts s−1 and 2,000 counts s−1, respectively, and those for intermediate count rates were determined via linear interpolation between the two points. The presented light curves were corrected for photon losses due to this exclusion by using the xrtlccorr tool. In addition, from Fig. 3a, c and d, we can see a time delay in the start of a dip in optical light, relative to that in X-rays. The delay time was ~1 min, which is similar to the reported value of 0–50 s (ref. 36). This was determined by cross-correlating the U-band and X-ray (0.3–10 keV) light curves obtained with Swift/UltraViolet and Optical Telescope (UVOT) and Swift/XRT on ut 2015 June 2136. The observations were carried out when the source showed little rapid optical flickering and no extreme flares, and thus the nature of the lag may be different from that in our observations. We also note that the apparent difference between the Swift/UVOT and the ground-based times36 is caused by the drift of the clock on board the satellite, to which we have applied the necessary corrections. In order to examine the possibility that absorption by gas in the line-of-sight causes the observed violent flux variations in the optical and X-ray bands (Fig. 3), we studied intensity-sliced X-ray spectra. A striking example is shown in Extended Data Fig. 3a. The period shown corresponds to that in Fig. 3a when both the X-ray and optical fluxes exhibited a sudden intensity drop towards the latter part of the period. We divided it into five intervals (T1 to T5; Extended Data Fig. 3a), and generated spectra through the tools xrtpipeline and xrtproducts in standard pipeline processing. We excluded the central 60-arcsecond strip from this Windowed Timing (WT) mode data, to avoid the heavy pile-up effect when the raw count rate exceeds ~150 counts s−1. We compared the vF spectra of the five intervals, where the spectra are fitted by a single power-law model multiplied by photoelectric absorption (phabs × pegpwrlw; in the standard X-ray spectral fitting package XSPEC). The absorbed X-ray flux ranges by two orders of magnitude, from 2.1 × 10−9 erg s−1 cm−2 in T5 to 3.0 × 10−7 erg s−1 cm−2 in T3. However, the best-fit column density and photon index were relatively stable over the five intervals, ~(2–6) × 10−21 cm−2 and ~1.0–1.5, respectively. Since the X-ray spectrum does not show a noticeable rise in column density when the X-ray flux sharply dropped, and since there is no stronger iron edge in the latter part of the observation, absorption cannot be the primary cause of the time variation in our data sets that cover the X-ray and optical bands simultaneously. In Extended Data Table 3 we show the list of X-ray binaries that have shown violent short-term variations either in X-rays or in optical wavelengths. IGR J17091−3624 is known as the second black hole X-ray binary whose X-ray light curves showed a variety of patterns, resembling those of GRS 1915 + 10518. The variations observed in the 2011 outburst of this object were classified as ρ (‘heartbeat’), ν (similar to class ρ but with secondary peak after the dips), α (‘rounded-bumps’), β/λ (repetitive short-term oscillations after low-quiet period) and μ (ref. 18). The Rapid Burster (RB or MXB 1730−335), a low-mass X-ray binary (LMXB) containing a neutron star (NS), was discovered by Small Astronomy Satellite (SAS-3) observations37. This object has been recently reported to show cyclic long X-ray bursts with periods of a few seconds resembling class ρ (‘heartbeat’) variations and those with periods of 100–200 s resembling class θ (“M”-shaped light curves) variations of GRS 1915 + 10524. The emission of the Rapid Burster did not reach the Eddington luminosity during these variations38. V4641 Sgr was originally discovered as a variable star39 and was long confused with a different variable star, GM Sgr40. V4641 Sgr is famous for its short and bright outburst in 1999, which reached a optical magnitude of at least 8.8 mag (refs 41, 42, 43, 44). V4641 Sgr showed short-term variations in optical wavelengths during the 2002, 2003 and 2004 outbursts14, 45, 46, 47. It was the first case in which short-term and large-amplitude variations in the optical range during an outburst were detected. V4641 Sgr is classified as a LMXB, and has a long orbital period. Its mass-accretion rate is less than the Eddington rate (except for the 1999 outburst44, 48). These properties are similar to those of V404 Cyg. However, while the short-term variations of V4641 Sgr seemed to be random, those of V404 Cyg showed repetitive patterns; this is the greatest difference between these two objects. There has been a suggestion that V4641 Sgr is a ‘microblazar’49 because the jets observed during the outburst in 1999 were proposed to have the largest bulk Lorentz factor among known galactic sources43. There are also other X-ray transients showing short-term optical variations (for example, XTE J1118+480 and GX 339−4). However, these two sources are quasi-periodic oscillations (QPOs), characterized by very short periods. The periods are much shorter than those of repetitive patterns (tens of seconds to a few hours) that we discuss in this Letter. Furthermore, the amplitudes of their variations are significantly smaller than those observed in V4641 Sgr4, 50 on timescales longer than tens of seconds. Following the method in ref. 15, we estimated the mass stored in the disk at the onset of the outburst. By integrating the X-ray light curve of Swift/BAT and assuming the spectral model C in table 1 in ref. 15, we obtained a value of 5.0 × 1025 g assuming a radiative efficiency of 10% and a distance of 2.4 ± 0.2 kpc (ref. 8). The mass during the 1989 outburst has been updated to 3.0 × 1025 g by using this updated distance. The stored mass in the 2015 outburst was approximately the same as that in the 1989 one. As discussed in ref. 15, these masses are far smaller than the mass of a fully built-up disk, estimated to be 2.0 × 1028 g, if these outbursts were starting at the outermost region. We compare the published optical light curves of the 1989 and 1938 outbursts51, 52 with our data from the 2015 outburst (Extended Data Fig. 4). We can see that these outbursts have different durations. The 1938 outburst was apparently longer than the others, and it may have had different properties from the 1989 and 2015 ones. The fading rates of the 1989 and 2015 outbursts are significantly larger than those of classical X-ray transients6, or of FRED (fast rise and exponential decline)-type outbursts, such as 0.028 mag d−1 in V518 Per = GRO J0422+32 (ref. 53) and 0.015 mag d−1 in V616 Mon = A0620−00 (ref. 54). This supports the hypothesis that the outbursts in 1989 and 2015 are different from typical outbursts of classical X-ray transients and that the stored disk mass was a factor of ~103 smaller in the 1989 and 2015 outbursts than the mass of a fully built up disk. We performed power spectral analyses on BJD 2,457,193, BJD 2,457,196 and BJD 2,457,200. We used the continuous and regularly sampled high-cadence data set obtained by LCO (Extended Data Table 1) with exposure times of 5 s (on BJD 2,457,193) and 2 s (others). The durations of these observations are 1.4, 3.1 and 2.2 h, respectively. Considering the read-out times of 1 s, the Nyquist frequencies of these observations are 0.08 and 0.17 Hz, respectively. The power spectral densities (PSDs) were calculated using powspec software in the FTOOLS Xronos package on magnitude measurements. We did not apply de-trending of the light curve since the durations of the individual observations were shorter than the timescale of the global variation of the outburst. The power spectra are well expressed by a power law (P ∝ f −Γ ) with an index Γ of 1.9 ± 0.1, 1.8 ± 0.1, and 2.3 ± 0.1 on BJD 2,457,193, 2,457,196 and 2,457,200, respectively (Extended Data Fig. 5). Interpretation of the physical origins on the basis of these variations is difficult, because a power law index of ~2 in the PSDs is often observed in natural phenomena. In this region (f < 0.01 Hz), the power originating in the optical variations of V404 Cyg is significantly higher than that of white noise estimated from the observations. We next summarize the other reports on short-term variations of V404 Cyg during the present outburst. On BJD 2,457,191, this object was observed using the Argos photometer on the 2.1m Otto Struve Telescope at McDonald Observatory with an exposure time of 2 s55. They reported that the power spectrum was dominated by steep red noise. Observations on BJD 2,457,193 and BJD 2,457,194 were also performed using the ULTRACAM attached with the 4.2m William Herschel Telescope on La Palma observatory with a high time resolution (466.8 ms)56. They reported that the variations were dominated by timescales longer than tens of seconds. Although large amplitude flares (0.3–0.4 mag) on timescales shorter than 1 s were reported57, these flares may be of different origin. For the variations with timescales longer than 100 s, our results agree with these reports55, 56. The timescale τ of heating/cooling waves in dwarf novae and X-ray transients58 is a function of the mass of the central object (M ) and radius (r) with the form , where α is the viscosity parameter59. Here, we estimate the disk radius of V404 Cyg assuming that the timescale of the final fading reflected a dwarf nova-type cooling wave. Using the Kepler data of V344 Lyr and V1504 Cyg, we measured a fading rate of 1.5 mag d−1 of the normal outbursts immediately preceding superoutbursts. During the outbursts in V344 Lyr and V1504 Cyg60, the disk radius is expected to be very close to the 3:1 resonance radius. Adopting a typical mass of a white dwarf in a cataclysmic variable (M  = 0.83M ; ref. 61), we estimated the disk radius of V404 Cyg to be 7.8 × 1010 cm for a black hole mass of 9M . This is much smaller than the radius (1.2 × 1012 cm) expected for a fully built-up disk15. Extended Data Fig. 6a shows the multi-wavelength SED on BJD 2,457,199.431 to 2,457,199.446, when the source was simultaneously observed in the X-ray, ultraviolet (UV) and optical bands. The optical fluxes in the V and I bands are taken from our photometric data averaged over the period. Note that R -band data are also available but not used here, because of the contamination by the continuum strong Hα line62, 63, 64. The X-ray spectrum is extracted from simultaneous Swift/XRT data (ObsID 00031403058) which were taken in the WT mode. The data are processed through the pipeline processing tool xrtpipeline. The events detected within 20 pixels around the source position are removed to mitigate pile-up effects. The U-band flux is obtained from the Swift/UVOT images with the same ObsID as the XRT, through the standard tool uvot2pha provided by the Swift team. A circular region centred at the source position with a radius of 5 arcsec is adopted as the source extraction region of the UVOT data. The optical, UV and X-ray data are corrected for interstellar extinction/absorption by assuming A (interstellar extinction in the V band) = 4 (ref. 65) and using the extinction curve in ref. 66 and the N (hydrogen column density) versus E(B−V) relation in ref. 67. Radio data are from the RATAN-600 observation performed in the same period68. The multi-wavelength SED can be reproduced with the diskir model69, 70, which accounts for the emission from the accretion disk, including the effects of Comptonization in the inner disk and reprocessing in the outer disk. We find that partial covering X-ray absorption (using the pcfabs model implemented in the spectral analysis software XSPEC) improves the quality of the fit significantly. The inner-disk temperature is estimated to be 0.12 ± 0.01 keV, and the electron temperature and photon index of the Comptonization component, the ratio between the luminosity of the Compton tail and disk blackbody (L /L ), and the fraction of the bolometric flux thermalized in the outer disk (f ), are 17.5 ± 0.8 keV, 1.78 ± 0.03, 1.17 ± 0.03, and , respectively (the errors in this section represent 90% confidence ranges for one parameter). The inner radius (R ) is estimated to be (1.5–5.4) × 108 cm, and the outer radius (R ) is (2.5 ± 0.3) × 1012 cm. The derived value of R is comparable to or even larger than the binary separation (~2.2 × 1012 cm). However, it could be smaller due to uncertainties in interstellar/circumbinary extinction71 and/or the contribution of jet emission. For instance, if A is 0.4 mag larger than the assumed value (4.0), R becomes (1.9 ± 0.2) × 1012 cm. The maximum achievable radius of a stable disk for a q (mass ratio) = 0.06 object (Extended Data Table 3) is around 0.62A (radius of the 2:1 resonance) to ~0.7A (tidal limit), where A is the binary separation72. Considering the uncertainties, the result of our analysis (>~ 0.77A) is compatible with this maximum radius. Our result appears to favour a large A value. For the partial covering absorber, the best-fit value of the column density is cm−2 and that of the covering fraction is 64 ± 4%. The radio SED can be approximated by a power-law with a photon index of ~1, as in other black hole binaries in the low/hard state73. This profile is likely to be generated by the optically-thick synchrotron emission from compact jets74. Because an optically-thick synchrotron spectrum often extends up to the millimetre to near-infrared bands75, 76, 77, it may contribute to the optical fluxes, in particular at longer wavelengths. The blackbody emission from the companion, a K3III-type star7 with a radius of ~3 R and a temperature of ~4,320 K, contributes to the SED negligibly. Extended Data Figure 6b plots the simultaneous SED on BJD 2,457,191.519 to 2,457,191.524, which is ~2 orders of magnitude fainter in the X-ray band than that shown in the left panel. The X-ray, UV and optical data are taken from the Swift data (ObsID 00031403038) and our photometric measurements in the same manner as described above. This SED can be reproduced with the irradiated disk model as well, with somewhat smaller photon index and inner-disk temperature (<0.07 keV), and a larger than those on BJD 2,457,199.431 to 2,457,199.446. The bolometric luminosity L of V404 Cyg is evaluated based on the hard X-rays above ~15 keV where the intrinsic spectrum is less affected by an absorption. We processed the Swift/BAT archival survey data via batsurvey in the HEAsoft package to derive count rates with individual exposures of ~300 s. Even within this short exposure, photon statistics are good during bright states (>0.05 counts s−1). Assuming a Crab-like spectrum (1 Crab ≈ 0.039 counts s−1), the BAT count rates R (counts s−1) are then converted into 15–50 keV flux (F ) and luminosity (L ) using F  = 3.6 × 10−7R (erg s−1 cm−1) and a fiducial distance of 2.4 kpc, respectively. In Fig. 4, we show L after multiplying by a conversion factor L /L  = 7 determined from SED modelling (previous section). We find that this bolometric correction factor lies within the range 2.5–10 by fitting 19 X-ray(XRT)-optical simultaneous SED in different periods between BJD 2,457,192.019 and 2,457,201.011. Since the BAT survey data are rather sparse, in order to catch shorter-term variations, we further overlaid the INTEGRAL IBIS/ISGRI monitoring in the 25–60 keV band available at ref. 78, assuming a conversion parameter of 1 Crab rate to be 172.1 counts s−1 and a bolometric correction factor of L /L  = 9.97. The luminosity was highly variable during the outburst, changing by five orders of magnitude. While V404 Cyg sometimes reaches the Eddington luminosity (L ) at the peak of multiple sporadic flares, it also repeatedly dropped below 1–10% of L (Fig. 4). At earlier phases of this outburst, the characteristic oscillation already occurred during a lower luminosity state, as discussed in the main text. No statistical methods were used to predetermine sample size.

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