SSS J122221.7-311525: the highly-evolved post period-minimum dwarf nova

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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Detection of a very red source at the position of SWIFT J1753.5-0127

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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Superoutburst of ASASSN-15po

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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A paper on RZ LMi

October 4th, 2016 / Comments Off on A paper on RZ LMi / by Vitaly

A new paper entitled “RZ Leonis Minoris Bridging between ER Ursae Majoris-Type Dwarf Nova and Novalike System” has been accepted for publication in PASJ.
Preprint: ArXiv:1609.08791.

Kato T, …, Neustroev V., et al.
Abstract:
We observed RZ LMi, which is renowned for the extremely (~19d) short supercycle and is a member of a small, unusual class of cataclysmic variables called ER UMa-type dwarf novae, in 2013 and 2016. In 2016, the supercycles of this object substantially lengthened in comparison to the previous measurements to 35, 32, 60d for three consecutive superoutbursts. We consider that the object virtually experienced a transition to the novalike state (permanent superhumper). This observed behavior extremely well reproduced the prediction of the thermal-tidal instability model. We detected a precursor in the 2016 superoutburst and detected growing (stage A) superhumps with a mean period of 0.0602(1)d in 2016 and in 2013. Combined with the period of superhumps immediately after the superoutburst, the mass ratio is not as small as in WZ Sge-type dwarf novae, having orbital periods similar to RZ LMi. By using least absolute shrinkage and selection operator (Lasso) two-dimensional power spectra, we detected possible negative superhumps with a period of 0.05710(1)d. We estimated the orbital period of 0.05792d, which suggests a mass ratio of 0.105(5). This relatively large mass ratio is even above ordinary SU UMa-type dwarf novae, and it is also possible that the exceptionally high mass-transfer rate in RZ LMi may be a result of a stripped core evolved secondary which are evolving toward an AM CVn-type object.

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MAGIC paper on the gravitationally-lensed blazar QSO B0218+357

September 21st, 2016 / Comments Off on MAGIC paper on the gravitationally-lensed blazar QSO B0218+357 / by Vitaly

Our new MAGIC paper entitled “Detection of very high energy gamma-ray emission from the gravitationally-lensed blazar QSO B0218+357 with the MAGIC telescopes” has been accepted for publication in Astronomy and Astrophysics.
Preprint: ArXiv:1609.01095.

Abstract:
Context. QSO B0218+357 is a gravitationally lensed blazar located at a redshift of 0.944. The gravitational lensing splits the emitted radiation into two components, spatially indistinguishable by gamma-ray instruments, but separated by a 10-12 day delay. In July 2014, QSO B0218+357 experienced a violent flare observed by the Fermi-LAT and followed by the MAGIC telescopes.
Aims. The spectral energy distribution of QSO B0218+357 can give information on the energetics of z ~ 1 very high energy gamma- ray sources. Moreover the gamma-ray emission can also be used as a probe of the extragalactic background light at z ~ 1.
Methods. MAGIC performed observations of QSO B0218+357 during the expected arrival time of the delayed component of the emission. The MAGIC and Fermi-LAT observations were accompanied by quasi-simultaneous optical data from the KVA telescope and X-ray observations by Swift-XRT. We construct a multiwavelength spectral energy distribution of QSO B0218+357 and use it to model the source. The GeV and sub-TeV data, obtained by Fermi-LAT and MAGIC, are used to set constraints on the extragalactic background light.
Results. Very high energy gamma-ray emission was detected from the direction of QSO B0218+357 by the MAGIC telescopes during the expected time of arrival of the trailing component of the flare, making it the farthest very high energy gamma-ray sources detected to date. The observed emission spans the energy range from 65 to 175 GeV. The combined MAGIC and Fermi-LAT spectral energy distribution of QSO B0218+357 is consistent with current extragalactic background light models. The broad band emission can be modeled in the framework of a two zone external Compton scenario, where the GeV emission comes from an emission region in the jet, located outside the broad line region.

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My 3rd MAGIC shift is over

July 15th, 2016 / Comments Off on My 3rd MAGIC shift is over / by Vitaly

I’ve spent another 4 weeks in La Palma at the Roque de los Muchachos Observatory, making observations with the MAGIC telescopes. Weather conditions were perfect all nights, we lost only a few hours because of high humidity. Although I was quite busy during these weeks, I was able to make some photos of sunrises, sunsets and telescopes.

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MAGIC paper on Long-term multi-wavelength variability and correlation study of Markarian 421

June 8th, 2016 / Comments Off on MAGIC paper on Long-term multi-wavelength variability and correlation study of Markarian 421 / by Vitaly

Our new MAGIC paper entitled “Long-term multi-wavelength variability and correlation study of Markarian 421 from 2007 to 2009” has been accepted for publication in Astronomy and Astrophysics.
Preprint: ArXiv:1605.09017.

Abstract:
We study the multi-band variability and correlations of the TeV blazar Mrk 421 on year time scales, which can bring additional insight on the processes responsible for its broadband emission. We observed Mrk 421 in the very high energy (VHE) gamma-ray range with the Cherenkov telescope MAGIC-I from March 2007 to June 2009 for a total of 96 hours of effective time after quality cuts. The VHE flux variability is quantified with several methods, including the Bayesian Block algorithm, which is applied to data from Cherenkov telescopes for the first time. The 2.3 year long MAGIC light curve is complemented with data from the Swift/BAT and RXTE/ASM satellites and the KVA, GASP-WEBT, OVRO, and Mets\”ahovi telescopes from February 2007 to July 2009, allowing for an excellent characterisation of the multi-band variability and correlations over year time scales. Mrk 421 was found in different gamma-ray emission states during the 2.3 year long observation period. Flares and different levels of variability in the gamma-ray light curve could be identified with the Bayesian Block algorithm. The same behaviour of a quiet and active emission was found in the X-ray light curves measured by Swift/BAT and the RXTE/ASM, with a direct correlation in time. The behaviour of the optical light curve of GASP-WEBT and the radio light curves by OVRO and Mets\”ahovi are different as they show no coincident features with the higher energetic light curves and a less variable emission. The fractional variability is overall increasing with energy. The comparable variability in the X-ray and VHE bands and their direct correlation during both high- and low-activity periods spanning many months show that the electron populations radiating the X-ray and gamma-ray photons are either the same, as expected in the Synchrotron-Self-Compton mechanism, or at least strongly correlated, as expected in electromagnetic cascades.

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On the seed photon source for Comptonisation in the black hole binary SWIFT J1753.5-0127

March 31st, 2016 / Comments Off on On the seed photon source for Comptonisation in the black hole binary SWIFT J1753.5-0127 / by Vitaly

Our paper, entitled “On the seed photon source for Comptonisation in the black hole binary SWIFT J1753.5–0127” accepted for publication in A&A. Preprint: arXiv:1603.08796

Kajava J. J. E., Veledina A., Tsygankov S., Neustroev V.:
Abstract:
Aims. The black hole binary SWIFT J1753.5-0127 is providing a unique data-set to study accretion flows. Various investigations of this system and of other black holes have not, however, led to an agreement on the accretion flow geometry nor on the seed photon source for Comptonisation during different stages of X-ray outbursts. We aim to place constraints on these accretion flow properties by studying long term spectral variations of this source. Methods. We performed phenomenological and self-consistent broad band spectral modeling of SWIFT J1753.5-0127 using quasi-simultaneous archived data from INTEGRAL/ISGRI, Swift/UVOT/XRT/BAT, RXTE/PCA/HEXTE and Maxi/GSC instruments. Results. 1. We identify a critical flux limit, F \sim 1.5 \times 10^{-8} erg/cm^2/s, and show that the spectral properties of SWIFT J1753.5-0127 are markedly different above and below that. Above the limit, during the outburst peak, the hot medium seems to intercept roughly 50 per cent of the disc emission. Below it, in the outburst tail, the contribution of the disc photons reduces significantly and the entire optical-to-X-ray spectrum can be produced by a synchrotron-self-Compton mechanism. 2. The long term variations in the hard X-ray spectra are caused by erratic changes of the electron temperatures in the hot medium. 3. Thermal Comptonization models indicate unreasonably low hot medium optical depths during the short incursions into the soft state after 2010, suggesting that non-thermal electrons produce the Comptonized tail in this state. 4. The soft X-ray excess, likely produced by the accretion disc, shows peculiarly stable temperatures for over an order of magnitude changes in flux. (abridged)

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Three new MAGIC papers

March 25th, 2016 / Comments Off on Three new MAGIC papers / by Vitaly

“Multi-Wavelength Observations of the Blazar 1ES 1011+496 in Spring 2008” accepted for publication in MNRAS. Preprint: arXiv:1603.07308
Abstract:
The BL Lac object 1ES 1011+496 was discovered at Very High Energy gamma-rays by MAGIC in spring 2007. Before that the source was little studied in different wavelengths. Therefore a multi-wavelength (MWL) campaign was organized in spring 2008. Along MAGIC, the MWL campaign included the Metsahovi radio observatory, Bell and KVA optical telescopes and the Swift and AGILE satellites. MAGIC observations span from March to May, 2008 for a total of 27.9 hours, of which 19.4 hours remained after quality cuts. The light curve showed no significant variability. The differential VHE spectrum could be described with a power-law function. Both results were similar to those obtained during the discovery. Swift XRT observations revealed an X-ray flare, characterized by a harder when brighter trend, as is typical for high synchrotron peak BL Lac objects (HBL). Strong optical variability was found during the campaign, but no conclusion on the connection between the optical and VHE gamma-ray bands could be drawn. The contemporaneous SED shows a synchrotron dominated source, unlike concluded in previous work based on nonsimultaneous data, and is well described by a standard one zone synchrotron self Compton model. We also performed a study on the source classification. While the optical and X-ray data taken during our campaign show typical characteristics of an HBL, we suggest, based on archival data, that 1ES 1011+496 is actually a borderline case between intermediate and high synchrotron peak frequency BL Lac objects.

“Insights into the emission of the blazar 1ES 1011+496 through unprecedented broadband observations during 2011 and 2012” accepted to A&A. Preprint: arXiv:1603.06776
Abstract:
1ES 1011+496 (z=0.212) was discovered in very high energy (VHE, E >100 GeV) γ-rays with MAGIC in 2007. The absence of simultaneous data at lower energies led to a rather incomplete characterization of the broadband spectral energy distribution (SED). We study the source properties and the emission mechanisms, probing whether a simple one-zone synchrotron-self-Compton (SSC) scenario is able to explain the observed broadband spectrum. We analyzed VHE to radio data from 2011 and 2012 collected by MAGIC, Fermi-LAT, Swift, KVA, OVRO, and Metsahovi in addition to optical polarimetry data and radio maps from the Liverpool Telescope and MOJAVE. The VHE spectrum was fit with a simple power law with a photon index of 3.69±0.22 and a flux above 150 GeV of (1.46±0.16)×10^{−11} ph /cm^2/s. 1ES 1011+496 was found to be in a generally quiescent state at all observed wavelengths, showing only moderate variability from radio to X-rays. A low degree of polarization of less than 10% was measured in optical, while some bright features polarized up to 60% were observed in the radio jet. A similar trend in the rotation of the electric vector position angle was found in optical and radio. The radio maps indicated a superluminal motion of 1.8±0.4c, which is the highest speed statistically significantly measured so far in a high-frequency-peaked BL Lac. For the first time, the high-energy bump in the broadband SED of 1ES 1011+496 could be fully characterized from 0.1 GeV to 1 TeV which permitted a more reliable interpretation within the one-zone SSC scenario. The polarimetry data suggest that at least part of the optical emission has its origin in some of the bright radio features, while the low polarization in optical might be due to the contribution of parts of the radio jet with different orientations of the magnetic field to the optical emission.

“Super-orbital variability of LS I +61°303 at TeV energies” has been accepted for publication in A&A. Preprint: arXiv:1603.06973
Abstract:
The gamma-ray binary LS I +61∘303 is a well established source from centimeter radio up to very high energy (VHE; E>100 GeV). Its broadband emission shows a periodicity of ∼26.5 days, coincident with the orbital period. A longer (super-orbital) period of 1667 ± 8 days was discovered in radio and confirmed in optical and high energy (HE; E>100 MeV) gamma-ray observations. We present a four-year campaign performed by MAGIC together with archival data concentrating on a search for a long timescale signature in the VHE emission. We focus on the search for super-orbital modulation of the VHE peak and on the search for correlations between TeV emission and optical determination of the extension of the circumstellar disk. A four-year campaign has been carried out by MAGIC. The source was observed during the orbital phases when the periodic VHE outbursts have occurred (ϕ=0.55-0.75). Additionally, we included archival MAGIC observations and data published by the VERITAS collaboration in these studies. For the correlation studies, LS I +61∘303 has also been observed during the orbital phases where sporadic VHE emission had been detected in the past (ϕ=0.75-1.0). These MAGIC observations were simultaneous with optical spectroscopy from the LIVERPOOL telescope. The TeV flux of the periodical outburst in orbital phases ϕ=0.5–0.75 was found to show yearly variability consistent with the ∼4.5 years long-term modulation found in the radio band. This modulation of the TeV flux can be well described by a sine function with the best fit period of 1610±58 days. The complete dataset span two super-orbital periods. There is no evidence for a correlation between the TeV emission and the mass-loss rate of the Be star but this may be affected by the strong, short timescale (as short as intra-day) variation displayed by the Hα fluxes.

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New MAGIC paper on the new VHE gamma-ray source H1722+119

March 22nd, 2016 / Comments Off on New MAGIC paper on the new VHE gamma-ray source H1722+119 / by Vitaly

A new MAGIC paper entitled “Investigating the peculiar emission from the new VHE gamma-ray source H1722+119” has been accepted for publication in MNRAS. The official date of acceptance is 2016 March 21. Preprint: ArXiv:1603.06523.

Abstract:
The MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes observed the BL Lac object H1722+119 (redshift unknown) for six consecutive nights between 2013 May 17 and 22, for a total of 12.5 h. The observations were triggered by high activity in the optical band measured by the KVA (Kungliga Vetenskapsakademien) telescope. The source was for the first time detected in the very high energy (VHE, E>100 GeV) γ-ray band with a statistical significance of 5.9 σ. The integral flux above 150 GeV is estimated to be (2.0±0.5) per cent of the Crab Nebula flux. We used contemporaneous high energy (HE, 100 MeV < E < 100 GeV) γ-ray observations from Fermi-LAT (Large Area Telescope) to estimate the redshift of the source. Within the framework of the current extragalactic background light models, we estimate the redshift to be z=0.34±0.15. Additionally, we used contemporaneous X-ray to radio data collected by the instruments on board the Swift satellite, the KVA, and the OVRO (Owens Valley Radio Observatory) telescope to study multifrequency characteristics of the source. We found no significant temporal variability of the flux in the HE and VHE bands. The flux in the optical and radio wavebands, on the other hand, did vary with different patterns. The spectral energy distribution (SED) of H1722+119 shows surprising behaviour in the ∼3×1014−1018 Hz frequency range. It can be modelled using an inhomogeneous helical jet synchrotron self-Compton model.

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