Two new MAGIC papers accepted for publication in A&A

March 7th, 2017 / No Comments » / 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

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MAGIC paper on the low-luminosity blazar 1ES 1741+196

February 26th, 2017 / No Comments » / 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.

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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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