August 30th, 2022 / Comments Off on EUROWD22: 22nd European Workshop on White Dwarfs / by Vitaly
I have just come back from a research visit to Tübingen, Germany, where I also attended the 22nd European Workshop on White Dwarf. I have given the talk “On the determination of fundamental parameters of accreting white dwarfs” which can be downloaded from here.
April 19th, 2022 / Comments Off on MAGIC paper in Nature Astronomy on the recurrent nova RS Oph explosion / by Vitaly
MAGIC collaboration published a paper in Nature Astronomy “Proton acceleration in thermonuclear nova explosions revealed by gamma rays”. For the first time, the gamma-ray emission from the recurrent nova RS Oph in the 60 GeV to 250 GeV energy range has been accurately characterized. Another interesting finding is that during nova explosions bubbles of enhanced cosmic-ray density can be created. Such protons will add to the Galactic cosmic ray budget, although primarily in the close neighbourhood of novae (of the order of 10 pc from the recurrent novae).
ArXiv: https://arxiv.org/abs/2202.07681
Link to the published paper: https://www.nature.com/articles/s41550-022-01640-z
March 1st, 2022 / Comments Off on Joint Letter Against the War in Ukraine of the Russian Scholars Working in Finland / by Vitaly
WE, Russian scientists and researchers, working in Finnish universities stand in solidarity with our Ukrainian colleagues, their families, Ukraine, and all the people who oppose the war. We declare a strong protest against the war with Ukraine. We call to end this war immediately!
We find no justification for the invasion and utterly condemn this unprovoked military assault. We reject Vladimir Putin’s distorted historical revisionism, myths and cynical lies that deny the right to self-determination of the Ukrainian nation and justify Russia’s imperialism. We condemn the Kremlin’s horrific misinformation campaign as well as Putin’s misrepresentation of Holocaust history to justify the war.
No to war in Ukraine! Putin’s war against Ukraine must be stopped!
МЫ, российские ученые и исследователи, работающие в финских университетах, солидарны с нашими украинскими коллегами, их семьями, Украиной и всеми людьми, которые выступают против войны. Мы заявляем решительный протест против войны с Украиной. Мы призываем немедленно прекратить эту войну!
Мы не находим оправдания вторжению и категорически осуждаем ничем не спровоцированную войну. Мы отвергаем искаженный исторический ревизионизм, мифы и циничную ложь Владимира Путина, отрицающие право на самоопределение украинской нации и оправдывающие российский империализм. Мы осуждаем ужасную кампанию Кремля по дезинформации, а также искажение Путиным истории Холокоста для оправдания войны.
Нет войне в Украине! Война Путина против Украины должна быть остановлена!
ME Suomessa toimivat ja asuvat venäläiset tutkijat olemme solidaarisia ukrainalaisille kollegoillemme, heidän perheilleen, Ukrainalle, sekä kaikille sotaa vastustaville ihmisille. Vastustamme kaikin keinoin sotaa Ukrainassa. Vaadimme, että sotatoimet on lopetettava välittömästi!
Me emme löydä minkäänlaista oikeutusta miehitykselle, ja tuomitsemme täysin provosoimattoman aseellisen hyökkäyksen. Hylkäämme Vladimir Putinin historian vääristelyn, myytit ja kyyniset valheet, jotka kieltävät Ukrainan kansalta itsemääräämisoikeuden ja pyrkivät oikeuttamaan Venäjän imperialismia. Tuomitsemme Kremlin valheisiin perustuvan disinformaatiokampanjan ja Putinin vääristyneen tulkinnan holokaustin historiasta sodan oikeutuksen keinona.
Ei sodalle Ukrainassa! Putinin sota Ukrainaa vastaan täytyy pysäyttää!
December 15th, 2021 / Comments Off on Swift follow-up observations of AT2021afpi / MASTER OT J030227.28+191754.5 / by Vitaly
ATel #15115 (15 Dec 2021): V. V. Neustroev (U. Oulu), J. P. Osborne (U. Leicester), K. L. Page (U. Leicester)
We report the results of follow-up observations of AT2021afpi = MASTER OT J030227.28+191754.5 carried out with the Neil Gehrels Swift Observatory. The object was discovered on 2021 November 27 as a bright optical transient by MASTER (ATel #15067) as a result of follow-up observations of the IceCube neutrino alert IceCube-211125A (GCN #31126). However, it has been shown (ATel #15081) that AT2021afpi started brightening at least 8.5 hours before the IceCube-211125A trigger (November 24, 21:53 UT). Based on the large amplitude of the optical outburst (~10 mag), the transient was initially classified as a classical nova (ATel #15069). Nevertheless, the following optical spectroscopic and time-resolved photometric observations have confirmed that AT2021afpi is a very high amplitude WZ Sge-type dwarf nova (ATel #15072, #15074).
Swift started monitoring AT2021afpi 3.9 days after the transient discovery. Although initially bright, with an XRT count rate ~1.1 c/s (ATel #15073, #15087), AT2021afpi has rapidly faded for ~10 days before stabilizing at ~0.02 c/s on December 4-5. On December 11, the X-ray flux dropped to the level of about ~0.005 c/s, but returned back to the level of ~0.02 c/s by the time of the next observation on December 13. However, the optical and UV light curves show only a smooth slow decline and no response to this X-ray drop. So far, the X-ray and UV flux evolution of AT2021afpi is similar to that of the superoutbursts of SSS J122221.7-311525 and GW Lib (Neustroev+18 A&A 611 13).
An X-ray spectrum consisting of the data with the lowest XRT count rates < 0.03 c/s is also consistent with the outburst spectra of SSS J122221.7-311525 and GW Lib. It can be satisfactorily fitted with one optically thin emission component. However, the two first Swift/XRT observations show the presence below 0.8 keV of an additional strong soft component. Thus, 2 optically-thin components are needed for a satisfactory fit of these "higher-state" data (see also ATel #15087). We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for their ongoing support.
April 16th, 2021 / Comments Off on A unified look at a famous black hole in M87 / by Vitaly
In April 2019, scientists released the first image of a black hole in the galaxy M87 using the Event Horizon Telescope (EHT). Now data from nineteen observatories, including MAGIC, are being released that promise to give unparalleled insight into this black hole and the system it powers, and to improve tests of Einstein’s General Theory of Relativity. See the full press release of the EHT here.
The paper entitled “Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign” is just accepted in ApJ Letters (arXiv:2104.06855):
Abstract:
In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass approximately 6.5 x 10^9 M_solar. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87’s spectrum. We can exclude that the simultaneous gamma-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the gamma-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
December 19th, 2020 / Comments Off on MAGIC paper on GRB 160821B accepted in ApJ / by Vitaly
The MAGIC telescopes followed up the afterglow emission of the short GRB 160821B starting from 24 seconds and lasting until ∼4 hours after the burst trigger. We obtained a TeV gamma-ray excess with a significance of 3.1 σ. Assuming that this excess is indeed a gamma-ray signal associated with GRB 160821B, in a new paper we discussed some models that may potentially account for such TeV emission from afterglows of short GRBs.
The GRB 160821B paper entitled “MAGIC observations of the nearby short gamma-ray burst GRB 160821B” is just accepted in ApJ (arXiv:2012.07193):
Abstract:
The coincident detection of GW170817 in gravitational waves and electromagnetic radiation spanning the radio to MeV gamma-ray bands provided the first direct evidence that short gamma-ray bursts (GRBs) can originate from binary neutron star (BNS) mergers. On the other hand, the properties of short GRBs in high-energy gamma rays are still poorly constrained, with only ∼20 events detected in the GeV band, and none in the TeV band. GRB~160821B is one of the nearest short GRBs known at z=0.162. Recent analyses of the multiwavelength observational data of its afterglow emission revealed an optical-infrared kilonova component, characteristic of heavy-element nucleosynthesis in a BNS merger. Aiming to better clarify the nature of short GRBs, this burst was automatically followed up with the MAGIC telescopes, starting from 24 seconds after the burst trigger. Evidence of a gamma-ray signal is found above ∼0.5 TeV at a significance of ∼3σ during observations that lasted until 4 hours after the burst. Assuming that the observed excess events correspond to gamma-ray emission from GRB 160821B, in conjunction with data at other wavelengths, we investigate its origin in the framework of GRB afterglow models. The simplest interpretation with one-zone models of synchrotron-self-Compton emission from the external forward shock has difficulty accounting for the putative TeV flux. Alternative scenarios are discussed where the TeV emission can be relatively enhanced. The role of future GeV-TeV observations of short GRBs in advancing our understanding of BNS mergers and related topics is briefly addressed.
November 21st, 2020 / Comments Off on MAGIC detected the Geminga pulsar in VHE / by Vitaly
Another important MAGIC paper entitled “Detection of the Geminga pulsar with MAGIC hints at a power-law tail emission beyond 15 GeV” has been published in A&A:
Abstract:
We report the detection of pulsed gamma-ray emission from the Geminga pulsar (PSR J0633+1746) between 15 GeV and 75 GeV. This is the first time a middle-aged pulsar has been detected up to these energies. Observations were carried out with the MAGIC telescopes between 2017 and 2019 using the low-energy threshold Sum-Trigger-II system. After quality selection cuts, ∼80 h of observational data were used for this analysis. To compare with the emission at lower energies below the sensitivity range of MAGIC, 11 years of Fermi-LAT data above 100 MeV were also analysed. From the two pulses per rotation seen by Fermi-LAT, only the second one, P2, is detected in the MAGIC energy range, with a significance of 6.3σ. The spectrum measured by MAGIC is well-represented by a simple power-law of spectral index Γ = 5.62 ± 0.54, which smoothly extends the Fermi-LAT spectrum. A joint fit to MAGIC and Fermi-LAT data rules out the existence of a sub-exponential cut-off in the combined energy range at the 3.6σ significance level. The power-law tail emission detected by MAGIC is interpreted as the transition from curvature radiation to Inverse Compton Scattering of particles accelerated in the northern outer gap.
And it is a HIGHLIGHT of the journal!
As a reminder, 4 years ago MAGIC has published another paper on Geminga. At that time, we did not find any significant detection, but we could do it now!
Also, 22 years ago I 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
October 14th, 2020 / Comments Off on Evidence for the expansion of accretion disc material beyond the Roche lobe of the accretor in HT Cas / by Vitaly
The paper entitled “Voracious vortices in cataclysmic variables. II. Evidence for the expansion of accretion disc material beyond the Roche lobe of the accretor in HT Cassiopeia during its 2017 superoutburst” has been finally published in A&A:
V.V. Neustroev & S.V. Zharikov, 2020, A&A, 642, A100 (Arxiv: arXiv:1908.10867)
It took a while to be accepted. Received: 28 August 2019; Accepted: 7 August 2020.
This is the second paper of a series of two. The first paper can be found here
Abstract:
Context. In Paper I we showed that the accretion disc radius of the dwarf nova HT Cas in its quiescent state has not changed significantly during many years of observations. It has remained consistently large, close to the tidal truncation radius. This result is inconsistent with the modern understanding of the evolution of the disc radius through an outburst cycle.
Aims. Spectroscopic observations of HT Cas during its superoutburst offered us an exceptional opportunity to compare the properties of the disc of this object in superoutburst and in quiescence.
Methods. We obtained a new set of time-resolved spectra of HT Cas in the middle of its 2017 superoutburst. We used Doppler tomography to map emission structures in the system, which we compared with those detected during the quiescent state. We used solutions of the restricted three-body problem to discuss again the location of emission structures and the disc size of HT Cas in quiescence.
Results. The superoutburst spectrum is similar in appearance to the quiescent spectra, although the strength of most of the emission lines decreased. However, the high-excitation lines significantly strengthened in comparison with the Balmer lines. Many lines show a mix of broad emission and narrow absorption components. Hα in superoutburst was much narrower than in quiescence. Other emission lines have also narrowed in outburst, but they did not become as narrow as Hα. Doppler maps of Hα in quiescence and of the Hβ and He I lines in outburst are dominated by a bright emission arc at the right side of the tomograms, which is located at and even beyond the theoretical truncation limit. However, the bulk of the Hα emission in outburst has significantly lower velocities.
Conclusions. We show that the accretion disc radius of HT Cas during its superoutburst has become hot but remained the same size as it was in quiescence. Instead, we detected cool gas beyond the Roche lobe of the white dwarf that may have been expelled from the hot disc during the superoutburst.
August 2nd, 2020 / Comments Off on A new paper on the nova-like cataclysmic variable RW Tri published in MNRAS / by Vitaly
The paper entitled “Structure of accretion flows in the nova-like cataclysmic variable RW Tri” is published in MNRAS (Subebekova G., et al., 2020, MNRAS, V. 497, pp.1475-1487)
Preprint: arXiv:2007.07478.
Subebekova G., Zharikov S., Tovmassian G., Neustroev V., et al.
Abstract:
We obtained photometric observations of the nova-like (NL) cataclysmic variable RW Tri and gathered all available AAVSO and other data from the literature. We determined the system parameters and found their uncertainties using the code developed by us to model the light curves of binary systems. New time-resolved optical spectroscopic observations of RW Tri were also obtained to study the properties of emission features produced by the system. The usual interpretation of the single-peaked emission lines in NL systems is related to the bi-conical wind from the accretion disc’s inner part. However, we found that the Hα emission profile is comprised of two components with different widths. We argue that the narrow component originates from the irradiated surface of the secondary, while the broader component’s source is an extended, low-velocity region in the outskirts of the accretion disc, located opposite to the collision point of the accretion stream and the disc. It appears to be a common feature for long-period NL systems – a point we discuss.
July 14th, 2020 / Comments Off on Cosmic Cataclysm allows precise test of General Relativity / by Vitaly
In 2019, the MAGIC telescopes detected the first Gamma Ray Burst at very high energies. This was the most intense gamma-radiation ever obtained from such a cosmic object. But the GRB data have more to offer: with further analyses, the MAGIC scientists could now confirm that the speed of light is constant in vacuum – and not dependent on energy. So, like many other tests, GRB data also corroborate Einstein’s theory of General Relativity. The study has now been published in Physical Review Letters.