Swift follow-up observations of AT2021afpi / MASTER OT J030227.28+191754.5

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.

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A unified look at a famous black hole in M87

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):

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.

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MAGIC paper on GRB 160821B accepted in ApJ

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):

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.

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MAGIC detected the Geminga pulsar in VHE

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:

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

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Evidence for the expansion of accretion disc material beyond the Roche lobe of the accretor in HT Cas

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

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.

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A new paper on the nova-like cataclysmic variable RW Tri published in MNRAS

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.
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.

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Cosmic Cataclysm allows precise test of General Relativity

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.

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MAGIC observations of the diffuse gamma-ray emission in the vicinity of the Galactic Centre

June 2nd, 2020 / Comments Off on MAGIC observations of the diffuse gamma-ray emission in the vicinity of the Galactic Centre / by Vitaly

Another MAGIC paper entitled “MAGIC observations of the diffuse gamma-ray emission in the vicinity of the Galactic Centre” has been accepted for publication in A&A.

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The broadening of emission lines in TCP J21040470+4631129 occurs during a day at the end of rapid fading from the superoutburst

April 20th, 2020 / Comments Off on The broadening of emission lines in TCP J21040470+4631129 occurs during a day at the end of rapid fading from the superoutburst / by Vitaly

ATel #13646 (17 Apr 2020): V. Neustroev (U. Oulu), F. Sims (ARAS), K. L. Page (U. Leicester), T. Tordai (MCSE), A. V. Moiseev (SAO RAS), D. Boyd (ARAS), J. V. Echevarria (IA UNAM), B. T. Gaensicke (U. Warwick), C. Knigge (U. Southampton), T. R. Marsh (U. Warwick), R. Michel (IA UNAM), A. Oksanen (Hankasalmi Obs), J. P. Osborne (U. Leicester), S. Zharikov (IA UNAM)

We report the results of our optical and X-ray observations of a new superoutburst of the bright WZ Sge-type dwarf nova TCP J21040470+4631129 (hereafter TCP2104), discovered on July 12, 2019. At the maximum light, it was one of the brightest objects of this type ever observed (V~8.6 mag). However, the pre-outburst V magnitude of this object was ~18.1 (ATel #13122), thus the total amplitude of the outburst was ~9.5 mag. After the main superoutburst of a total duration of 22 days, the object has never returned to its original low state but instead experienced two smaller superoutbursts (V~10.8-11 mag at the maximums) and three rebrightenings (for previous reports, see ATel #12936, #12947, #13009, #13122, #13297).

On 2020 March 31, D. Denisenko reported that TCP2104 went into a new outburst (vsnet-alert 24120) which, after the detection of superhumps, appeared to be the third small superoutburst. Our first spectrum of TCP2104 was taken with the 6-m telescope of the SAO RAS within 14.5 hours after the observation by D. Denisenko. Using other smaller telescopes, we were also able to obtain spectra during all but one day of this superoutburst, and also during the rapid fading stage. Our photometric time-resolved observations were performed using 30-cm class telescopes.

The new superoutburst closely resembles the two previous small superoutbursts. At the maximum, the transient reached V~10.8 mag, it then slowly declined for ~9 days. Rapid fading started when the transient has reached V~11.85 mag – the exact same level as in all previous outbursts. As of 2020 April 16, TCP2104 is at V~15.3 mag.

We also obtained a series of Halpha images using a narrow-band filter, attempting to detect a nebulosity in emission (Hernandez Santisteban et al., 2019, 486, 2631). No sign of such a nebulosity was visible.

These new spectroscopic observations allowed us to trace a broadening of emission lines, which was first reported in ATel #13297. We found that the lines were narrow from the very beginning of the outburst to an initial part of the rapid fading stage (the FWHM of the Halpha line was ~450 km/s). However, during a single day at the end of rapid fading the lines became very broad (~2000 km/s, see Figure).

We also continued monitoring TCP2104 with Swift, getting a relatively dense coverage of this entire superoutburst. Before the first optical signature of an outburst, the XRT (0.3-10 keV) count rate of the transient was ~0.08+/-0.01 cts/s, consistent with (though slightly lower than) the inter-outburst observations obtained on 2019 December 24-30 after the previous superoutburst (0.10+/-0.01 cts/s). At the beginning of the optical outburst, during ~6.4 h the count-rate decreased sharply to 0.012+/-0.004 cts/s. 16 hours later the transient was found at 0.032+/-0.006 cts/s, staying at this level during the rest of the optical outburst. After the superoutburst ended, the count-rate returned to the pre-outburst level (see Figure).

We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for their ongoing support.

Light curves and spectra (Followup Reports page for TCP2104)



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Two Nature papers report the detection of the highest-energy photons from a GRB

November 24th, 2019 / Comments Off on Two Nature papers report the detection of the highest-energy photons from a GRB / by Vitaly

Gamma-ray bursts (GRBs) are the most violent explosions in the Universe suddenly appearing in the sky, about once per day. They are thought to result from the collapse of massive stars or the merging of neutron stars in distant galaxies.

The two papers published in Nature on 2019 November 21 report the first ever detection of very-high-energy photons from a GRB in the range of 0.2–1 teraelectronvolts, the highest energy photons measured from these objects. This ground-breaking achievement by the MAGIC telescopes provides critical new insight for understanding the physical processes at work in GRBs, which are still mysterious.

For more info about GRBs, afterglows, synchrotron emission, and the MAGIC telescopes, see the press release by MAGIC

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