MAGIC observations of the diffuse gamma-ray emission in the vicinity of the Galactic Centre

June 2nd, 2020 / Comments Off on MAGIC observations of the diffuse gamma-ray emission in the vicinity of the Galactic Centre / by Vitaly

Another MAGIC paper entitled “MAGIC observations of the diffuse gamma-ray emission in the vicinity of the Galactic Centre” has been accepted for publication in A&A.
arXiv:2006.00623

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The broadening of emission lines in TCP J21040470+4631129 occurs during a day at the end of rapid fading from the superoutburst

April 20th, 2020 / Comments Off on The broadening of emission lines in TCP J21040470+4631129 occurs during a day at the end of rapid fading from the superoutburst / by Vitaly

ATel #13646 (17 Apr 2020): V. Neustroev (U. Oulu), F. Sims (ARAS), K. L. Page (U. Leicester), T. Tordai (MCSE), A. V. Moiseev (SAO RAS), D. Boyd (ARAS), J. V. Echevarria (IA UNAM), B. T. Gaensicke (U. Warwick), C. Knigge (U. Southampton), T. R. Marsh (U. Warwick), R. Michel (IA UNAM), A. Oksanen (Hankasalmi Obs), J. P. Osborne (U. Leicester), S. Zharikov (IA UNAM)

We report the results of our optical and X-ray observations of a new superoutburst of the bright WZ Sge-type dwarf nova TCP J21040470+4631129 (hereafter TCP2104), discovered on July 12, 2019. At the maximum light, it was one of the brightest objects of this type ever observed (V~8.6 mag). However, the pre-outburst V magnitude of this object was ~18.1 (ATel #13122), thus the total amplitude of the outburst was ~9.5 mag. After the main superoutburst of a total duration of 22 days, the object has never returned to its original low state but instead experienced two smaller superoutbursts (V~10.8-11 mag at the maximums) and three rebrightenings (for previous reports, see ATel #12936, #12947, #13009, #13122, #13297).

On 2020 March 31, D. Denisenko reported that TCP2104 went into a new outburst (vsnet-alert 24120) which, after the detection of superhumps, appeared to be the third small superoutburst. Our first spectrum of TCP2104 was taken with the 6-m telescope of the SAO RAS within 14.5 hours after the observation by D. Denisenko. Using other smaller telescopes, we were also able to obtain spectra during all but one day of this superoutburst, and also during the rapid fading stage. Our photometric time-resolved observations were performed using 30-cm class telescopes.

The new superoutburst closely resembles the two previous small superoutbursts. At the maximum, the transient reached V~10.8 mag, it then slowly declined for ~9 days. Rapid fading started when the transient has reached V~11.85 mag – the exact same level as in all previous outbursts. As of 2020 April 16, TCP2104 is at V~15.3 mag.

We also obtained a series of Halpha images using a narrow-band filter, attempting to detect a nebulosity in emission (Hernandez Santisteban et al., 2019, 486, 2631). No sign of such a nebulosity was visible.

These new spectroscopic observations allowed us to trace a broadening of emission lines, which was first reported in ATel #13297. We found that the lines were narrow from the very beginning of the outburst to an initial part of the rapid fading stage (the FWHM of the Halpha line was ~450 km/s). However, during a single day at the end of rapid fading the lines became very broad (~2000 km/s, see Figure).

We also continued monitoring TCP2104 with Swift, getting a relatively dense coverage of this entire superoutburst. Before the first optical signature of an outburst, the XRT (0.3-10 keV) count rate of the transient was ~0.08+/-0.01 cts/s, consistent with (though slightly lower than) the inter-outburst observations obtained on 2019 December 24-30 after the previous superoutburst (0.10+/-0.01 cts/s). At the beginning of the optical outburst, during ~6.4 h the count-rate decreased sharply to 0.012+/-0.004 cts/s. 16 hours later the transient was found at 0.032+/-0.006 cts/s, staying at this level during the rest of the optical outburst. After the superoutburst ended, the count-rate returned to the pre-outburst level (see Figure).

We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for their ongoing support.

Light curves and spectra (Followup Reports page for TCP2104)

 

 

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Two Nature papers report the detection of the highest-energy photons from a GRB

November 24th, 2019 / Comments Off on Two Nature papers report the detection of the highest-energy photons from a GRB / by Vitaly

Gamma-ray bursts (GRBs) are the most violent explosions in the Universe suddenly appearing in the sky, about once per day. They are thought to result from the collapse of massive stars or the merging of neutron stars in distant galaxies.

The two papers published in Nature on 2019 November 21 report the first ever detection of very-high-energy photons from a GRB in the range of 0.2–1 teraelectronvolts, the highest energy photons measured from these objects. This ground-breaking achievement by the MAGIC telescopes provides critical new insight for understanding the physical processes at work in GRBs, which are still mysterious.

For more info about GRBs, afterglows, synchrotron emission, and the MAGIC telescopes, see the press release by MAGIC

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Dramatic broadening of emission lines in TCP J21040470+4631129

November 20th, 2019 / Comments Off on Dramatic broadening of emission lines in TCP J21040470+4631129 / by Vitaly

ATel #13297 (19 Nov 2019): V. Neustroev, A. E. Watkins, P. E. Kvist, E. P. Halsio, A. E. N. Ruokanen, M. M. Anetjarvi (U. Oulu), T. Tordai (MCSE), K. L. Page, J. P. Osborne (U. Leicester), G. Sjoberg (AAVSO), D. Boyd (ARAS), T. R. Marsh, B. T. Gaensicke (U. Warwick), C. Knigge (U. Southampton), S. Zharikov (UNAM), R. P. V. Rautio, T. A. Rikkola; L. Poranen; E. Sarkar (U. Oulu), N. P. M. Kuin (UCL-MSSL)

We report on new optical and X-ray observations of the WZ Sge-type dwarf nova TCP J21040470+4631129 (hereafter TCP2104) discovered on 2019 July 12. The object experienced two superoutbursts and three rebrightenings, and now is slowly declining (ATel #12936, #12947, #13009, #13122). Our new optical spectroscopic data were obtained on November 6, 50 days after the most recent rebrightening observed on September 15-17 (ATel #13122). We used the ALFOSC spectrograph at the Nordic Optical Telescope (NOT) on La Palma.

This observation shows a remarkable change in TCP2104’s emission line profiles. All the Balmer and He I lines are now much broader than they were during the superoutbursts and rebrightenings. For example, the FWHM of the Halpha line is now ~1550 km/s, 3-4 times larger than it was during the superoutburst (350-500 km/s). In the blue part of the spectrum, the Balmer emission lines are now superposed on the broad absorption lines of the white dwarf.

Time-resolved optical photometric observations show that after the last rebrightening TCP2104 has been declining at a rate of ~0.006 mag/day, and has been showing double-wave periodic modulations with an amplitude of ~0.08 mag. Their period of 77.09+/-0.01 min is consistent with the spectroscopic orbital period of 77.07+/-0.02 min (ATel #13009). TCP2104 is currently at the level of V~15.25 mag.

On November 3, we obtained another observation of TCP2104 with Swift. The observation showed no significant change in both the X-rays or UV compared to previous observations. The XRT count rate was 0.088 +/- 0.011 count/sec, while the UV magnitudes were: w2 = 12.64 +/-0.02, m2 = 12.75 +/- 0.03, and w1 = 12.97 +/- 0.02.

The data presented here were obtained in part with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOTSA. We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for their ongoing support.

NOT spectra
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The disappearance and reappearance of optical emission lines and the drop in a Swift/XRT count rate during the recent rebrightening of TCP J21040470+4631129

October 12th, 2019 / Comments Off on The disappearance and reappearance of optical emission lines and the drop in a Swift/XRT count rate during the recent rebrightening of TCP J21040470+4631129 / by Vitaly

ATel #13122 (23 Sep 2019): V. Neustroev (U. Oulu), D. Boyd, F. Sims (ARAS), K. L. Page (U. Leicester), T. Tordai (MCSE), S. M. Brincat, C. Galdies, G. Sjoberg (AAVSO), S. Zharikov (UNAM), J. P. Osborne (U. Leicester), N. P. M. Kuin (UCL-MSSL), T. R. Marsh, B. T. Gaensicke (U. Warwick), C. Knigge (U. Southampton)

We report the results of our continuing optical and X-ray monitoring of the bright WZ Sge-type dwarf nova TCP J21040470+4631129 (hereafter TCP2104) discovered on July 12, 2019 (for previous reports, see ATel #12947, #13009). Our optical photometric observations are mostly performed using 30-cm class telescopes, while spectroscopic data are obtained with the 2.1-m telescope at the OAN-SPM, the 2.5-m Isaac Newton Telescope on La Palma, and other smaller telescopes.

The observations show that the object is exhibiting remarkable, very unusual behaviour. After the end of the plateau stage on August 3, TCP2104 experienced 2 short rebrightenings on August 8-10 and 14-16, after which the object underwent the second superoutburst with a relatively short plateau stage of about 9 days (from August 25 to September 3). After the second plateau ended, TCP2104 showed another short rebrightening on September 15-17. We note that the fading after this rebrightening has again decelerated suggesting that another rebrightening is possible. We also point out that superhump modulations which were observed from the very beginning of the superoutburst have never completely disappeared (see also ATel #12947 and #13009), and they are still present in a light curve after the end of the most recent rebrightening (their amplitude is about 0.1 mag). Using the observations obtained between September 18-21, we measured a period of modulations to be P=77.15+/-0.3 min, which is consistent with the orbital period Porb=77.07+/-0.02 min (ATel #13009).

TCP2104 is currently at the level of V~14.7 mag, which is still about 1.3 mag brighter than just before the discovery (according to ASAS-SN). However, the Pan-STARRS1 (PS1) catalog lists much lower fluxes; at a mean epoch of 2014 August 23 the mean V magnitude was measured to be ~18.1 mag (converted from PS1 magnitudes). We note, however, the PS1 detection tables indicate that between 2010 and 2014 TCP2104 has gradually brightened by ~0.5 mag.

Our extensive optical spectroscopic monitoring showed a notable difference between the spectra obtained during the first and the second plateau stages. During the second plateau, the flux in the emission lines was much reduced, and especially higher-order Balmer lines were significantly weakened. But the most dramatic spectral changes were observed during the recent rebrightening. The spectrum obtained on September 15.84, just before the rebrightening maximum, shows only Balmer absorption lines, while all the emission lines completely disappeared. However, another spectrum obtained 9 hours later (September 16.23) shows very strong emission lines again.

We also continued monitoring TCP2104 with Swift. The observations obtained between rebrightenings showed a very stable XRT count rate at the level of 0.11+/-0.02 cts/s. However, during rebrightenings it has dropped by about 10 times.

We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for their ongoing support.

Light curves and spectra

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The orbital period of the bright WZ Sge-type dwarf nova TCP J21040470+4631129

August 9th, 2019 / Comments Off on The orbital period of the bright WZ Sge-type dwarf nova TCP J21040470+4631129 / by Vitaly

ATel #13009: V. Neustroev (U. Oulu), D. Boyd, P. Berardi (ARAS), S. Zharikov, A. L. Medina (IA UNAM), K. L. Page, J. P. Osborne (U. Leicester), N. P. M. Kuin (UCL-MSSL), C. Knigge (U. Southampton), T. R. Marsh, B. T. Gaensicke (U. Warwick), L. Franco, F. Teyssier (ARAS), G. Sjoberg (AAVSO)

We report optical and X-ray follow-up observations of the bright WZ Sge-type dwarf nova TCP J21040470+4631129 discovered on July 12, 2019 (ATel #12936, #12947). Our optical spectroscopic data were obtained with the 2.1-m telescope at the OAN-SPM, the 2.5-m Isaac Newton Telescope on La Palma, and other smaller telescopes, while photometric observations were mostly performed using 30-cm class telescopes.

Our observations show that during about three weeks after the discovery, the transient monotonically declined to the level of ~11.7 mag. On August 3, it started to fade more rapidly reaching ~13.5V on August 7. It was still more than 4 mag brighter than in quiescence (ATel #12936). However, on August 8.8 we found that TCP J21040470+4631129 rebrightened again to the level of ~11.9V confirming thus the observations of Y. Sato and H. Nishimura (vsnet-alert 23465). Such a behavior resembles that of another WZ Sge-type dwarf nova, the solid period-bounce candidate SSS J122221.7-311525 (Kato et al., 2013, PASJ, 65, L11; Neustroev et al., 2017, MNRAS, 467, 597). Time-resolved photometric observations obtained during the first days of the superoutburst revealed the presence of early superhumps with the period of 77.04+/-0.58 min (ATel #12947), suggesting a very short orbital period. Normal, full-grown superhumps appeared on July 23 (vsnet-alert 23422). Their period is not stable; using the observations obtained between July 23-29, we measured a period of normal superhumps to be Psh=78.9+/-0.4 min.

Between July 13 and 31, we obtained 18 nights of time-resolved spectroscopic observations. In general, the appearance of spectra was similar to that reported by F. Teyssier (ATel #12936). We note, however, that the He II 4686 line and the Bowen blend were quite strong during the first days of the superoutburst peaking at 30% and 15% above the continuum, respectively, but then almost disappeared on July 19-20. Although the emission lines of TCP J21040470+4631129 are relatively narrow, the spectral resolution of our time-resolved data (R=1000-6000) is enough to trace the variability of line profiles. For consistency, we extracted two sets of quantities from the H-alpha line – the radial velocities measured using the double-Gaussian method, and the line asymmetry measured as the ratio between the areas of the blue and red parts of the line – and calculated periodograms for them. Both sets of quantities revealed the presence of the orbital variability with the period of Porb=77.07+/-0.02 min, which is consistent with the period of early superhumps.

Such a short orbital period places TCP J21040470+4631129 at the very short-period end of the orbital period distribution of cataclysmic variables where the main-sequence donor stars transform into brown dwarf-like objects. However, it seems that TCP J21040470+4631129 is still at the start of this transition. Indeed, the fractional superhump period excess eps = (Psh-Porb)/Porb is ~2.3%. Employing the empirical relation between eps and the mass ratio q (Patterson et al. 2005, PASP, 117, 1204), we find q to be ~0.1. Such a large q suggests that the donor star is still on the main sequence.

We also continued monitoring TCP J21040470+4631129 with Swift. Between July 29 and August 5, Swift/XRT obtained four additional 1 ks observations during which an XRT count rate of the transient was at the level of 0.025+/-0.004 cts/s, that is about 3 times lower than during the previous observation on July 17 (ATel #12947).

We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for their ongoing support.

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Swift X-ray detection and ground-based observations of the new WZ Sge-type dwarf nova TCP J21040470+4631129

July 19th, 2019 / Comments Off on Swift X-ray detection and ground-based observations of the new WZ Sge-type dwarf nova TCP J21040470+4631129 / by Vitaly

ATel #12947: K. Sokolovsky (MSU), V. Neustroev (U. Oulu), K. L. Page (U. Leicester), J. Leahy-McGregor, M. Bundas, H. Gallamore, C. Wicklund, B. McIntyre, C. Conner, E. Zobel, E. Aydi, L. Chomiuk, A. Kawash, J, Strader (MSU), K. Mukai (NASA/GSFC), N. P. M. Kuin (UCL-MSSL), J. P. Osborne (U. Leicester), C. Knigge (U. Southampton), D. Denisenko (SAI)

The 9.2 mag optical transient TCP J21040470+4631129 was discovered on 2019-07-12.49 UT by H. Nishimura on images obtained with a DSLR camera and a 200 mm telephoto lens. It was spectroscopically confirmed on 2019-07-12.9 as a dwarf nova outburst (ATel #12936). Swift observed TCP J21040470+4631129 for 5.1 ks between 2019-07-13 and 2019-07-17. During the first observation, Swift/XRT detected a relatively bright X-ray source at the position of the dwarf nova, with an XRT count rate of 0.64 +/- 0.03 cts/s. In the following days, the X-ray flux declined before stabilizing at an average level of 0.09 +/- 0.02 cts/s.

The total X-ray spectrum can be approximated by a combination of two optically thin emission components (the APEC model in XSPEC) having kT = 11.5 +13.8/-4.5 keV and 0.19 +/- 0.02 keV, respectively, and negligible absorption (n_H1 < 7e19 cm^-2). We note, however, that the spectra, extracted separately from the first and remaining observations are statistically different. While the lower APEC temperature component is consistent in both spectra (kT~0.2 keV), the hotter APEC component has cooled from >23.6 keV to 5.4 +4.6/-1.5 keV. The peak 0.3-10 keV flux of 1.5e-11 ergs/cm^2/s corresponds to the X-ray luminosity of 2.1e31 ergs/s at 109 pc (see below). The object is still too bright for UVOT photometry or spectroscopy.

We also observed TCP J21040470+4631129 with the 0.6 m telescope of the Michigan State University Campus Observatory (MPC code 766) on 2019-07-13, 15, 16 and 17 for ~7 h each night. The object declined from V=8.59 +/-0.01 on 2019-07-13.1145 to V=10.25 +/-0.01 on 2019-07-18.1111. The period search (after detrending) revealed the presence of superhumps with the period of 0.0535 +/-0.0003 d, close to the value reported by T. Vanmunster in vsnet-alert 23388, and shorter than the periods suggested by T. Kato in vsnet-alert 23396. The peak-to-peak amplitude of the superhumps is about 0.04 mag.

We used 10 best-seeing images to measure the position of TCP J21040470+4631129 relative to UCAC3 (Zacharias et al. 2010 AJ, 139, 2184) stars in the field: 21:04:04.691 +46:31:13.68 J2000 +/-0.05″; thus confirming the identification of Gaia DR2 2163612727665972096 (g=17.77, parallax 9.13 +/-0.12 mas) with the transient. The large outburst amplitude combined with the presence of superhumps suggests that TCP J21040470+4631129 is a WZ Sge-type dwarf nova. A dramatic increase in X-ray luminosity accompanying the optical outburst has been observed in other WZ Sge systems: GW Lib (Byckling et al. 2009, MNRAS, 399, 1576), SSS J122221.7-311525 (Neustroev et al. 2018, A&A, 611, 13), V805 Aur (ATel #4954), ASASSN-18fs (Parikh & Wijnands 2018, RNAAS, 2d, 201), TCP J06373299-0935420 (ATel #12531), ASASSN-19hl (ATel #12629).

Further Swift observations are planned to monitor this object. We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for rapid scheduling of these ToO observations and for their ongoing support.

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Multicolour Optical Photometry of Swift J1357.2-0933 just prior to the 2019 outburst rise

June 17th, 2019 / Comments Off on Multicolour Optical Photometry of Swift J1357.2-0933 just prior to the 2019 outburst rise / by Vitaly

ATel #12815: Vitaly Neustroev (University of Oulu), Elena Neustroeva (Oulu)

The black hole transient Swift J1357.2-0933 has recently been reported to be in a new outburst, less than 2 years after the previous one in 2017 (ATel #10297). The new outburst was first detected in the optical by the Zwicky Transient Facility (ATel #12796), and later confirmed in X-rays by NICER observations (ATel #12801). Russell et al. (ATel #12803) have reported the results of long-term optical monitoring of the transient. They found that Swift J1357.2-0933 has been in quiescence on 2019 March 19, but on March 21 its flux increase was detected. Thus, Russell et al. concluded that the optical outburst has started on MJD 58562.44 +/- 1.06.

On 2019 March 19 (MJD 58561.24+/-0.01), we observed the field of Swift J1357.2-0933 with the Nordic Optical Telescope (NOT) equipped with the STANcam imager. Observations were performed with the Bessel B, V, R, I, and sdss z’ filters. The observations gave the following magnitudes:
B=22.05+/-0.38, V=21.23+/-0.14, R=20.59+/-0.09, I=20.11+/-0.09, sdss z’=21.0+/-0.5.

Both the obtained spectral energy distribution (SED) and all the magnitudes are consistent with those found in the PanSTARRS catalog, allowing us to conclude that at the time of our observations (MJD 58561.24), the transient has not yet shown any sign of the forthcoming outburst.

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A paper on the long cycle variability of the Algol OGLE-LMC-DPV-065 has been accepted in MNRAS

May 13th, 2019 / Comments Off on A paper on the long cycle variability of the Algol OGLE-LMC-DPV-065 has been accepted in MNRAS / by Vitaly

The paper entitled “On the long cycle variability of the Algol OGLE-LMC-DPV-065 and its stellar, orbital and disk parameters” has been accepted for publication in MNRAS.
Preprint: ArXiv:1905.04231.

Mennickent R. E., …, Neustroev V. V., et al.
Abstract:
OGLE-LMC-DPV-065 is an interacting binary whose double-hump long photometric cycle remains hitherto unexplained. We analyze photometric time series available in archive datasets spanning 124 years and present the analysis of new high-resolution spectra. A refined orbital period is found of 10\fd0316267 ± 0\fd0000056 without any evidence of variability. In spite of this constancy, small but significant changes in timings of the secondary eclipse are detected. We show that the long period continuously decreases from 350 to 218 days during 13 years, then remains almost constant for about 10 years. Our study of radial velocities indicates a circular orbit for the binary and yields a mass ratio of 0.203 ± 0.001. From the analysis of the orbital light curve we find that the system contains 13.8 and 2.81 \msun\ stars of radii 8.8 and 12.6 \rsun\ and absolute bolometric magnitudes -6.4 and -3.0, respectively. The orbit semi-major axis is 49.9 \rsun\ and the stellar temperatures are 25460 K and 9825 K. We find evidence for an optically and geometrically thick disk around the hotter star. According to our model, the disk has a radius of 25 \rsun, central and outer vertical thickness of 1.6 \rsun\ and 3.5 \rsun, and temperature of 9380 K at its outer edge. Two shock regions located at roughly opposite parts of the outer disk rim can explain the light curves asymmetries. The system is a member of the double periodic variables and its relatively high-mass and long photometric cycle make it similar in some aspects to β Lyrae.

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A paper on a high-proper motion cataclysmic variable imbedded in a bow-shock nebula has been accepted in MNRAS

April 1st, 2019 / Comments Off on A paper on a high-proper motion cataclysmic variable imbedded in a bow-shock nebula has been accepted in MNRAS / by Vitaly

The paper entitled “From outburst to quiescence: spectroscopic evolution of V1838 Aql imbedded in a bow-shock nebula” has been accepted for publication in MNRAS.
Preprint: ArXiv:1811.02349.

Hernandez Santisteban J. V., Echevarria J., Zharikov S., Neustroev V. V., et al.
Abstract:
We analyse new optical spectroscopic, direct-image and X-ray observations of the recently discovered a high proper motion cataclysmic variable V1838 Aql. The data were obtained during its 2013 superoutburst and its subsequent quiescent state. An extended emission around the source was observed up to 30 days after the peak of the superoutburst, interpreted it as a bow–shock formed by a quasi-continuous outflow from the source in quiescence. The head of the bow–shock is coincident with the high-proper motion vector of the source (v⊥=123±5 km/s) at a distance of d=202±7 pc. The object was detected as a weak X-ray source (0.015±0.002 counts/s) in the plateau of the superoutburst, and its flux lowered by two times in quiescence (0.007±0.002 counts/s). Spectroscopic observations in quiescence we confirmed the orbital period value Porb=0.0545±0.0026 days, consistent with early-superhump estimates, and the following orbital parameters: γ=−21±3 km/s and K1=53±3 km/s. The white dwarf is revealed as the system approaches quiescence, which enables us to infer the effective temperature of the primary Teff=11600±400K. The donor temperature is estimated ≲2200K and suggestive of a system approaching the period minimum. Doppler maps in quiescence show the presence of the hot spot in HeI line at the expected accretion disc-stream shock position and an unusual structure of the accretion disc in Hα.

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