December 14th, 2017 / Comments Off on There is a link between the X-ray behaviour and superhump evolution during superoutbursts of WZ Sge-type stars / by Vitaly
The second paper on a study of SSS J122221.7−311525, now in X-rays, has been accepted for publication in A&A. It is entitled “Superhumps linked to X-ray emission. The superoutbursts of SSS J122221.7-311525 and GW Lib”.
Preprint: ArXiv:1712.03515.
Neustroev V. V., Page K. L., Kuulkers E., Osborne J. P., Beardmore A. P., Knigge C., Marsh T., Suleimanov V. F., Zharikov S. V.
Abstract:
We present more than 4 years of Swift X-ray observations of the 2013 superoutburst, subsequent decline and quiescence of the WZ Sge-type dwarf nova SSS J122221.7-311525 (SSS122222) from 6 days after discovery. Only a handful of WZ Sge-type dwarf novae have been observed in X-rays, and until recently GW Lib was the only binary of this type with complete coverage of an X-ray light curve throughout a superoutburst. We collected extensive X-ray data of a second such system to understand the extent to which the unexpected properties of GW Lib are common to the WZ Sge class. We analysed the X-ray light curve and compared it with the behaviour of superhumps which were detected in the optical light curve. We also performed spectral analysis of the data. The results were compared with the properties of GW Lib, for which new X-ray observations were also obtained. SSS122222 was variable and around five times brighter in 0.3-10 keV X-rays during the superoutburst than in quiescence, mainly because of a significant strengthening of a high-energy component of the X-ray spectrum. The post-outburst decline of the X-ray flux lasted at least 500 d. The data show no evidence of the expected optically thick boundary layer in the system during the outburst. SSS122222 also exhibited a sudden X-ray flux change in the middle of the superoutburst, which occurred exactly at the time of the superhump stage transition. A similar X-ray behaviour was also detected in GW Lib. This result demonstrates a relationship between the outer disc and the white dwarf boundary layer for the first time, and suggests that models for accretion discs in high mass ratio accreting binaries are currently incomplete. The very long decline to X-ray quiescence is also in strong contrast to the expectation of low viscosity in the disc after outburst.
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December 11th, 2017 / Comments Off on A paper on ASASSN-16eg has been published / by Vitaly
A paper entitled “ASASSN-16eg: New candidate for a long-period WZ Sge-type dwarf nova” has been published in PASJ.
Preprint: arXiv:1708.09206.
Abstract:
We report on our photometric observations of the 2016 superoutburst of ASASSN-16eg. This object showed a WZ Sge-type superoutburst with prominent early superhumps with a period of 0.075478(8) d and a post-superoutburst rebrightening. During the superoutburst plateau, it showed ordinary superhumps with a period of 0.077880(3) d and a period derivative of 10.6(1.1) × 10−5 in stage B. The orbital period (Porb), which is almost identical with the period of the early superhumps, is exceptionally long for a WZ Sge-type dwarf nova. The mass ratio (q = M2/M1) estimated from the period of developing (stage A) superhumps is 0.166(2), which is also too large for a WZ Sge-type dwarf nova. This suggests that the 2:1 resonance can be reached in such high-q systems, contrary to our expectation. Such conditions are considered to be achieved if the mass-transfer rate is much lower than those in typical SU UMa-type dwarf novae that have comparable orbital periods to ASASSN-16eg, and a resultant accumulation of a large amount of matter on the disk is realized at the onset of an outburst. We examined other candidates for long-period WZ Sge-type dwarf novae for their supercycles, which are considered to reflect the mass-transfer rate, and found that V1251 Cyg and RZ Leo have longer supercycles than those of other WZ Sge-type dwarf novae. This result indicates that these long-period objects including ASASSN-16eg have a low mass-transfer rate in comparison to other WZ Sge-type dwarf novae.
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December 4th, 2017 / Comments Off on Autumn trips / by Vitaly
This autumn has been busy for me!
In September, I participated in the workshop “The Golden Age of Cataclysmic Variables and Related Objects – IV.” in Palermo (Italy) and have given two talks: “Systematic search for post period-minimum cataclysmic variables: A short review.” and “Voracious vortexes and accretion disc radii in cataclysmic variables: a short review.”.
In October, I have traveled to Chile for observations in the La Silla Observatory. I also visited ESO Headquarters in Santiago and Universidad de Concepción, where I have given two identical talks “Revealing new candidates for post period-minimum cataclysmic variables among WZ Sge-type stars”.
In November, I have visited Tuorla Observatory to participate in “Nordic Optical Telescope observing course”, during which I lectured students to prepare and perform observations with the NOT telescope.
Posted in: Astronomy, Conferences, Observations, Work
Tags: Conferences, News
August 25th, 2017 / Comments Off on Optical spectroscopy and photometry of the black hole transient SWIFT J1753.5-0127 at the faintest ever recorded level / by Vitaly
ATel #10664: Vitaly Neustroev (FINCA, University of Turku, University of Oulu), Sergey Zharikov (IA UNAM), Antonio Cabrera-Lavers (GTC)
The black hole transient SWIFT J1753.5-0127 had been in outburst for more than 11 years, from 2005 to 2016. It started to decline in 2016 September (ATel #9708) and reached quiescence in 2016 November (ATel #9735, #9739, #9741, #9758, #9765). The reported optical magnitude at this time was about V=21.2 and the source showed very red colours, e.g. V-I=1.4 mag (ATel #9741). However, in 2017 February and April, SWIFT J1753.5-0127 exhibited two rebrightenings (ATel #10075, #10081, #10097, #10110, #10114, #10288, #10325). In particular, on May 1 the source reached V = 17.9 and then started slowly declining (ATel #10562). Zhang et al. (2017, ATel #10562) reported a dramatic drop of flux between 2017 May 26 and June 4, and measured an average quiescent magnitude during June 2 to July 5 of V = 22.17+/-0.25.
We observed SWIFT J1753.5-0127 on 2017 June 16, July 17 and Aug 19 using OSIRIS on the Gran Telescopio CANARIAS (GTC). The acquisition images, which were taken with the Sloan filter g’ during centering of the target on the slit, were used for photometric measurements. On June 16 and July 17 we found consistent g’ magnitudes to be 22.44+/-0.12 and 22.48+/-0.14, respectively. However, on Aug 19 we took three images, separated by a few minutes, in which the target was significantly fainter. The first two images give the exact same g’ magnitude of 22.64+/-0.08, whereas the image taken 3 min later gives g’=22.90+/-0.08, indicating a possible short-term variability.
The spectra exhibit a very red continuum and a strong, very broad and double-peaked emission line of H_alpha (EW ~90 Angstrom, peak-to-peak separation ~2800 km/s, FWHM ~3800 km/s). No other emission or absorption lines are clearly seen.
In order to compare the current V-I colour index with previous observations, we used our flux-calibrated spectra. For the August observations, we estimated V-I to be ~2 mag. It is redder than was measured in 2016 November (ATel #9741).
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For more details about SWIFT J1753.5-0127 see here.
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Tags: ATel, News
June 21st, 2017 / Comments Off on A paper on accretion discs in cataclysmic variables / by Vitaly
A paper entitled “The structure of accretion flows in nova-like cataclysmic variables: RW Sextantis and 1RXS J064434.5+334451” has been accepted for publication in MNRAS.
Preprint: ArXiv:1706.06236.
Hernandez, M. S.; Zharikov, S.; Neustroev V.; Tovmassian, G.
Abstract:
New time-resolved optical spectroscopic echelle observations of the nova-like cataclysmic variable RW Sextantis were obtained, with the aim to study the properties of emission features in the system. The profile of the H_alpha emission line can be clearly divided into two (‘narrow’ and ‘wide’) components. Similar emission profiles are observed in another nova-like system, 1RXS J064434.5+33445, for which we also reanalysed the spectral data and redetermined the system parameters. The source of the ‘narrow’, low-velocity component is the irradiated face of the secondary star. We disentangled and removed the ‘narrow’ component from the H_alpha profile to study the origin and structure of the region emitting the wide component. We found that the ‘wide’ component is not related to the white dwarf or the wind from the central part of the accretion disc, but is emanated from the outer side of the disc. Inspection of literature on similar systems indicates that this feature is common for some other long-period nova-like variables. We propose that the source of the ‘wide’ component is an extended, low-velocity region in the outskirts of the opposite side of the accretion disc, with respect to the collision point of the accretion stream and the disc.
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March 7th, 2017 / Comments Off on Two new MAGIC papers accepted for publication in A&A / by Vitaly
Two new papers have been accepted for publication in A&A. Both papers have been sent to ArXiv:
“Very-high-energy gamma-ray observations of the Type Ia Supernova SN 2014J with the MAGIC telescopes”:
ArXiv:1702.07677
“Multiwavelength observations of a VHE gamma-ray flare from PKS 1510-089 in 2015”:
ArXiv:1610.09416
Posted in: Astronomy, MAGIC, Publications, Work
Tags: News
February 26th, 2017 / Comments Off on MAGIC paper on the low-luminosity blazar 1ES 1741+196 / by Vitaly
Our new MAGIC paper entitled “MAGIC detection of very high energy gamma-ray emission from the low-luminosity blazar 1ES 1741+196” has been accepted for publication in MNRAS.
Preprint: ArXiv:1702.06795.
Abstract:
We present the first detection of the nearby (z=0.084) low-luminosity BL Lac object 1ES 1741+196 in the very high energy (VHE: E>100 GeV) band. This object lies in a triplet of interacting galaxies. Early predictions had suggested 1ES 1741+196 to be, along with several other high-frequency BL Lac sources, within the reach of MAGIC detectability. Its detection by MAGIC, later confirmed by VERITAS, helps to expand the small population of known TeV BL Lacs. The source was observed with the MAGIC telescopes between 2010 April and 2011 May, collecting 46 h of good quality data. These observations led to the detection of the source at 6.0 σ confidence level, with a steady flux F(>100GeV)=(6.4±1.7stat±2.6syst)⋅10−12 ph cm−2 s−1 and a differential spectral photon index Γ=2.4±0.2stat±0.2syst in the range of ∼80 GeV – 3 TeV. To study the broad-band spectral energy distribution (SED) simultaneous with MAGIC observations, we use KVA, Swift/UVOT and XRT, and Fermi/LAT data. One-zone synchrotron-self-Compton (SSC) modeling of the SED of 1ES 1741+196 suggests values for the SSC parameters that are quite common among known TeV BL Lacs except for a relatively low Doppler factor and slope of electron energy distribution. A thermal feature seen in the SED is well matched by a giant elliptical’s template. This appears to be the signature of thermal emission from the host galaxy, which is clearly resolved in optical observations.
Posted in: Astronomy, MAGIC, Publications, Work
Tags: News
January 15th, 2017 / Comments Off on SSS J122221.7-311525: the highly-evolved post period-minimum dwarf nova / by Vitaly
A paper entitled “The remarkable outburst of the highly-evolved post period-minimum dwarf nova SSS J122221.7-311525” has been accepted for publication in MNRAS.
Preprint: ArXiv:1701.03134.
Neustroev V., et al.
Abstract:
We report extensive 3-yr multiwavelength observations of the WZ Sge-type dwarf nova SSS J122221.7-311525 during its unusual double superoutburst, the following decline and in quiescence. The second segment of the superoutburst had a long duration of 33 days and a very gentle decline with a rate of 0.02 mag/d, and it displayed an extended post-outburst decline lasting at least 500 days. Simultaneously with the start of the rapid fading from the superoutburst plateau, the system showed the appearance of a strong NIR excess resulting in very red colours, which reached extreme values (B-I~1.4) about 20 days later. The colours then became bluer again, but it took at least 250 days to acquire a stable level. Superhumps were clearly visible in the light curve from our very first time-resolved observations until at least 420 days after the rapid fading from the superoutburst. The spectroscopic and photometric data revealed an orbital period of 109.80 min and a fractional superhump period excess <0.8%, indicating a very low mass ratio q<0.045. With such a small mass ratio the donor mass should be below the hydrogen-burning minimum mass limit. The observed IR flux in quiescence is indeed much lower than is expected from a CV with a near-main sequence donor star. This strongly suggests a brown-dwarf-like nature for the donor and that SSS J122221.7-311525 has already evolved away from the period minimum towards longer periods, with the donor now extremely dim.
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November 14th, 2016 / Comments Off on Detection of a very red source at the position of SWIFT J1753.5-0127 / by Vitaly
ATel #9741: V.V. Neustroev (FINCA, University of Turku & University of Oulu), E. Gafton (NOT), V.-J. Haanpaa (Oulu), J. Harmanen (Turku), T. Kangas (Turku), L. Kauppinen (Oulu), G. Sjoberg (AAVSO), A. Venhola (Oulu & Kapteyn Institute, Groningen)
The black hole transient SWIFT J1753.5-0127 had been in outburst since 2005, but in September 2016 it started to decline to quiescence (Russell et al., ATel #9708). We observed this source on 2016 August 24-25 and September 9 with the 0.35-m Celestron C14 robotic telescope and an SBIG ST-10XME CCD camera with Johnson-Cousins BVRI Astrodon Photometric filters. The averaged B and I magnitudes did not change between these nights (B=17.72+-0.06, I=16.42+-0.03), indicating that a fading had started after 2016 September 9, although the R magnitude dropped by 0.15 mag (R=16.83+-0.03 on August 24-25 and 16.98+-0.02 on September 9).
On 2016 November 8, we observed the field of SWIFT J1753.5-0127 with the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager and spectrograph. Observations were performed with the V, R and I filters. At the position of the transient we found a very red source with the following magnitudes:
V=21.25+-0.03, R=20.66+-0.03, I=19.83+-0.02.
The observed V-I colour index (1.42 mag) is significantly redder than it was during a steady state in 2012-2013 (V-I=0.74, V=16.9, Neustroev et al. 2014, MNRAS, 445, 2424) and even during a temporary fading stage in June 2015 (V-I=0.73, V=17.19, Neustroev et al. 2015, ATel #7697). However, Russell et al. (ATel #9708) reported the reddening of light during this fading that is confirmed by our earlier observations taken on 2016 September 9 (V-I=0.87, V=17.29+-0.04).
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For more details about SWIFT J1753.5-0127 see here.
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Tags: ATel, News
October 24th, 2016 / Comments Off on Superoutburst of ASASSN-15po / by Vitaly
A paper entitled “Superoutburst of WZ Sge-type Dwarf Nova Below the Period Minimum: ASASSN-15po” has been accepted for publication in PASJ.
Preprint: ArXiv:1610.04941.
Namekata K., …, Neustroev V., et al.
Abstract:
We report on a superoutburst of a WZ Sge-type dwarf nova (DN), ASASSN-15po. The light curve showed the main superoutburst and multiple rebrightenings. In this outburst, we observed early superhumps and growing (stage A) superhumps with periods of 0.050454(2) and 0.051809(13) d, respectively. We estimated that the mass ratio of secondary to primary (q) is 0.0699(8) by using Porb and a superhump period Psh of stage A. ASASSN-15po [Porb∼ 72.6 min] is the first DN with the orbital period between 67-76 min. Although the theoretical predicted period minimum Pmin of hydrogen-rich cataclysmic variables (CVs) is about 65-70 min, the observational cut-off of the orbital period distribution at 80 min implies that the period minimum is about 82 min, and the value is widely accepted. We suggest the following four possibilities: the object is (1) a theoretical period minimum object (2) a binary with an evolved secondary (3) a binary with a metal-poor (Population II) secondary (4) a binary which was born with a brown-dwarf donor below the period minimum.
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