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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MAGIC paper on Geminga Pulsar

March 4th, 2016 / Comments Off on MAGIC paper on Geminga Pulsar / by Vitaly

Our MAGIC paper entitled “Search for VHE gamma-ray emission from Geminga pulsar and nebula with the MAGIC telescopes” has been accepted for publication in Astronomy and Astrophysics. Preprint: ArXiv:1603.00730.

Abstract:
The Geminga pulsar, one of the brighest gamma-ray sources, is a promising candidate for emission of very-high-energy (VHE > 100 GeV) pulsed gamma rays. Also, detection of a large nebula have been claimed by water Cherenkov instruments. We performed deep observations of Geminga with the MAGIC telescopes, yielding 63 hours of good-quality data, and searched for emission from the pulsar and pulsar wind nebula. We did not find any significant detection, and derived 95% confidence level upper limits. The resulting upper limits of 5.3 x 10^{-13} TeV cm^{-2} s^{-1} for the Geminga pulsar and 3.5 x 10^{-12} TeV cm^{-2} s^{-1} for the surrounding nebula at 50 GeV are the most constraining ones obtained so far at VHE. To complement the VHE observations, we also analyzed 5 years of Fermi-LAT data from Geminga, finding that the sub-exponential cut-off is preferred over the exponential cut-off that has been typically used in the literature. We also find that, above 10 GeV, the gamma-ray spectra from Geminga can be described with a power law with index softer than 5. The extrapolation of the power-law Fermi-LAT pulsed spectra to VHE goes well below the MAGIC upper limits, indicating that the detection of pulsed emission from Geminga with the current generation of Cherenkov telescopes is very difficult.

As a reminder, 18 years ago I already had a paper on the analysis of this pulsar, but in the optical wavelengths:
Shearer A., …, Neustroev V., et al.: “Possible pulsed optical emission from Geminga”, 1998, A&A., V.335, L21-L24

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Limits to dark matter annihilation cross-section from a combined analysis of MAGIC and Fermi-LAT observations of dwarf satellite galaxies

January 26th, 2016 / Comments Off on Limits to dark matter annihilation cross-section from a combined analysis of MAGIC and Fermi-LAT observations of dwarf satellite galaxies / by Vitaly

Our MAGIC paper “Limits to dark matter annihilation cross-section from a combined analysis of MAGIC and Fermi-LAT observations of dwarf satellite galaxies” has been accepted for publication in Journal of Cosmology and Astroparticle Physics. Preprint: arXiv:1601.06590.

Abstract:

We present the first joint analysis of gamma-ray data from the MAGIC Cherenkov telescopes and the Fermi Large Area Telescope (LAT) to search for gamma-ray signals from dark matter annihilation in dwarf satellite galaxies. We combine 158 hours of Segue 1 observations with MAGIC with 6-year observations of 15 dwarf satellite galaxies by the Fermi-LAT. We obtain limits on the annihilation cross-section for dark matter particle masses between 10 GeV and 100 TeV – the widest mass range ever explored by a single gamma-ray analysis. These limits improve on previously published Fermi-LAT and MAGIC results by up to a factor of two at certain masses. Our new inclusive analysis approach is completely generic and can be used to perform a global, sensitivity-optimized dark matter search by combining data from present and future gamma-ray and neutrino detectors.

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Multicolour Optical Photometry of V404 Cyg

January 11th, 2016 / Comments Off on Multicolour Optical Photometry of V404 Cyg / by Vitaly

ATel #8531: Vitaly Neustroev (University of Oulu) & George Sjoberg (AAVSO)

The black-hole X-ray binary V404 Cyg spent only few months in quiescence since the previous outburst in the summer of 2015 (ATel #8515). On 2015 December 23, the transient entered a new period of activity (GCN #18716; ATel #8453, #8454, #8455, #8457, #8458, #8462, #8501). In order to study the optical photometric behaviour of V404 Cyg during this outburst, we obtained five sets of multicolour photometric data. These observations were performed on 2015 December 29 and 31 and on 2016 January 4, 6 and 10 with the 0.35-m Celestron C14 robotic telescope, located at New Mexico Skies in Mayhill, New Mexico. We used an SBIG ST-10XME CCD camera with Johnson-Cousins BVRI Astrodon Photometric filters.

The images were taken for approximately 40-60 minutes after sunset, in a sequence B-I-R-V with exposure times of 120-240 s for each filter. In order to improve the confidence of our measurements, we aligned and summed all the nightly images for each filter and then measured the average magnitudes. Fluxes were calibrated using the secondary standards established by A. Henden (see the link to the AAVSO web-site below). The quoted uncertainties of these stars are less than 0.02 mag.

During our observations we were lucky to catch the system in both relatively high and low states. On 2015 December 29 (MJD=57385.104), the observations gave the following magnitudes:
B=17.73+-0.15, V=16.16+-0.05, R_c=14.65+-0.04, I_c=13.39+-0.03

On 2015 December 31 (MJD=57387.106), we found the following magnitudes:
B=18.77+-0.18, V=17.03+-0.07, R_c=15.64+-0.05, I_c=14.51+-0.04

The observations on the following nights gave the following magnitudes:
MJD=57391.091: V=18.04+-0.15, R_c=16.59+-0.06, I_c=15.36+-0.05
MJD=57393.064: V=17.68+-0.07, R_c=16.21+-0.05, I_c=15.15+-0.06
MJD=57397.078: V=18.12+-0.19, R_c=16.49+-0.04, I_c=15.36+-0.04

During the latter nights the source was too faint to be detected in the B band, although on 2016 January 10 (MJD=57397.078) there was a tentative detection with the B magnitude being 19.5+-0.3. V404 Cyg is still about a half magnitude brighter than during the previous quiescent period (ATel #8509).

We note, that despite a 2 mag variation of the source during our observations, we detected no significant changes in colours. The average colour indices are B-V=1.57+-0.17, V-R=1.49+-0.09, R-I=1.16+-0.08. However, these colours are bluer than they were in quiescence (Szkody & Margon, 1989, IAU Circ. No. 4794), and redder than they were during a brighter state of previous outbursts (Jones & Carter, 1989, IAU Circ. No. 4794; Marti et al., 2016, A&A, in press). We will continue our monitoring in the following nights, while the source is still visible.

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Voracious vortexes in cataclysmic variables: Multi-epoch tomographic study of HT Cassiopeia

December 9th, 2015 / Comments Off on Voracious vortexes in cataclysmic variables: Multi-epoch tomographic study of HT Cassiopeia / by Vitaly

The paper entitled “Voracious vortexes in cataclysmic variables: Multi-epoch tomographic study of HT Cassiopeia” has been accepted on November 4, 2015 for publication in A&A.
DOIPreprint

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Workshop on Relativistic Astrophysics

September 1st, 2015 / Comments Off on Workshop on Relativistic Astrophysics / by Vitaly

I participated in an international workshop on Relativistic Astrophysics in Turku (Finland) and have given the talk “Evolution of accretion discs of dwarf novae and low-mass X-ray binary transients through their whole outburst cycle”.

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