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
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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.
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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.
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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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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.
arXiv:2006.00623
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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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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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November 20th, 2019 / Comments Off on Dramatic broadening of emission lines in TCP J21040470+4631129 / by Vitaly
ATel #13297 (19 Nov 2019): V. Neustroev, A. E. Watkins, P. E. Kvist, E. P. Halsio, A. E. N. Ruokanen, M. M. Anetjarvi (U. Oulu), T. Tordai (MCSE), K. L. Page, J. P. Osborne (U. Leicester), G. Sjoberg (AAVSO), D. Boyd (ARAS), T. R. Marsh, B. T. Gaensicke (U. Warwick), C. Knigge (U. Southampton), S. Zharikov (UNAM), R. P. V. Rautio, T. A. Rikkola; L. Poranen; E. Sarkar (U. Oulu), N. P. M. Kuin (UCL-MSSL)
We report on new optical and X-ray observations of the WZ Sge-type dwarf nova TCP J21040470+4631129 (hereafter TCP2104) discovered on 2019 July 12. The object experienced two superoutbursts and three rebrightenings, and now is slowly declining (ATel #12936, #12947, #13009, #13122). Our new optical spectroscopic data were obtained on November 6, 50 days after the most recent rebrightening observed on September 15-17 (ATel #13122). We used the ALFOSC spectrograph at the Nordic Optical Telescope (NOT) on La Palma.
This observation shows a remarkable change in TCP2104’s emission line profiles. All the Balmer and He I lines are now much broader than they were during the superoutbursts and rebrightenings. For example, the FWHM of the Halpha line is now ~1550 km/s, 3-4 times larger than it was during the superoutburst (350-500 km/s). In the blue part of the spectrum, the Balmer emission lines are now superposed on the broad absorption lines of the white dwarf.
Time-resolved optical photometric observations show that after the last rebrightening TCP2104 has been declining at a rate of ~0.006 mag/day, and has been showing double-wave periodic modulations with an amplitude of ~0.08 mag. Their period of 77.09+/-0.01 min is consistent with the spectroscopic orbital period of 77.07+/-0.02 min (ATel #13009). TCP2104 is currently at the level of V~15.25 mag.
On November 3, we obtained another observation of TCP2104 with Swift. The observation showed no significant change in both the X-rays or UV compared to previous observations. The XRT count rate was 0.088 +/- 0.011 count/sec, while the UV magnitudes were: w2 = 12.64 +/-0.02, m2 = 12.75 +/- 0.03, and w1 = 12.97 +/- 0.02.
The data presented here were obtained in part with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOTSA. 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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October 12th, 2019 / Comments Off on The disappearance and reappearance of optical emission lines and the drop in a Swift/XRT count rate during the recent rebrightening of TCP J21040470+4631129 / by Vitaly
ATel #13122 (23 Sep 2019): V. Neustroev (U. Oulu), D. Boyd, F. Sims (ARAS), K. L. Page (U. Leicester), T. Tordai (MCSE), S. M. Brincat, C. Galdies, G. Sjoberg (AAVSO), S. Zharikov (UNAM), J. P. Osborne (U. Leicester), N. P. M. Kuin (UCL-MSSL), T. R. Marsh, B. T. Gaensicke (U. Warwick), C. Knigge (U. Southampton)
We report the results of our continuing optical and X-ray monitoring of the bright WZ Sge-type dwarf nova TCP J21040470+4631129 (hereafter TCP2104) discovered on July 12, 2019 (for previous reports, see ATel #12947, #13009). Our optical photometric observations are mostly performed using 30-cm class telescopes, while spectroscopic data are obtained with the 2.1-m telescope at the OAN-SPM, the 2.5-m Isaac Newton Telescope on La Palma, and other smaller telescopes.
The observations show that the object is exhibiting remarkable, very unusual behaviour. After the end of the plateau stage on August 3, TCP2104 experienced 2 short rebrightenings on August 8-10 and 14-16, after which the object underwent the second superoutburst with a relatively short plateau stage of about 9 days (from August 25 to September 3). After the second plateau ended, TCP2104 showed another short rebrightening on September 15-17. We note that the fading after this rebrightening has again decelerated suggesting that another rebrightening is possible. We also point out that superhump modulations which were observed from the very beginning of the superoutburst have never completely disappeared (see also ATel #12947 and #13009), and they are still present in a light curve after the end of the most recent rebrightening (their amplitude is about 0.1 mag). Using the observations obtained between September 18-21, we measured a period of modulations to be P=77.15+/-0.3 min, which is consistent with the orbital period Porb=77.07+/-0.02 min (ATel #13009).
TCP2104 is currently at the level of V~14.7 mag, which is still about 1.3 mag brighter than just before the discovery (according to ASAS-SN). However, the Pan-STARRS1 (PS1) catalog lists much lower fluxes; at a mean epoch of 2014 August 23 the mean V magnitude was measured to be ~18.1 mag (converted from PS1 magnitudes). We note, however, the PS1 detection tables indicate that between 2010 and 2014 TCP2104 has gradually brightened by ~0.5 mag.
Our extensive optical spectroscopic monitoring showed a notable difference between the spectra obtained during the first and the second plateau stages. During the second plateau, the flux in the emission lines was much reduced, and especially higher-order Balmer lines were significantly weakened. But the most dramatic spectral changes were observed during the recent rebrightening. The spectrum obtained on September 15.84, just before the rebrightening maximum, shows only Balmer absorption lines, while all the emission lines completely disappeared. However, another spectrum obtained 9 hours later (September 16.23) shows very strong emission lines again.
We also continued monitoring TCP2104 with Swift. The observations obtained between rebrightenings showed a very stable XRT count rate at the level of 0.11+/-0.02 cts/s. However, during rebrightenings it has dropped by about 10 times.
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
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August 9th, 2019 / Comments Off on The orbital period of the bright WZ Sge-type dwarf nova TCP J21040470+4631129 / by Vitaly
ATel #13009: V. Neustroev (U. Oulu), D. Boyd, P. Berardi (ARAS), S. Zharikov, A. L. Medina (IA UNAM), K. L. Page, J. P. Osborne (U. Leicester), N. P. M. Kuin (UCL-MSSL), C. Knigge (U. Southampton), T. R. Marsh, B. T. Gaensicke (U. Warwick), L. Franco, F. Teyssier (ARAS), G. Sjoberg (AAVSO)
We report optical and X-ray follow-up observations of the bright WZ Sge-type dwarf nova TCP J21040470+4631129 discovered on July 12, 2019 (ATel #12936, #12947). Our optical spectroscopic data were obtained with the 2.1-m telescope at the OAN-SPM, the 2.5-m Isaac Newton Telescope on La Palma, and other smaller telescopes, while photometric observations were mostly performed using 30-cm class telescopes.
Our observations show that during about three weeks after the discovery, the transient monotonically declined to the level of ~11.7 mag. On August 3, it started to fade more rapidly reaching ~13.5V on August 7. It was still more than 4 mag brighter than in quiescence (ATel #12936). However, on August 8.8 we found that TCP J21040470+4631129 rebrightened again to the level of ~11.9V confirming thus the observations of Y. Sato and H. Nishimura (vsnet-alert 23465). Such a behavior resembles that of another WZ Sge-type dwarf nova, the solid period-bounce candidate SSS J122221.7-311525 (Kato et al., 2013, PASJ, 65, L11; Neustroev et al., 2017, MNRAS, 467, 597). Time-resolved photometric observations obtained during the first days of the superoutburst revealed the presence of early superhumps with the period of 77.04+/-0.58 min (ATel #12947), suggesting a very short orbital period. Normal, full-grown superhumps appeared on July 23 (vsnet-alert 23422). Their period is not stable; using the observations obtained between July 23-29, we measured a period of normal superhumps to be Psh=78.9+/-0.4 min.
Between July 13 and 31, we obtained 18 nights of time-resolved spectroscopic observations. In general, the appearance of spectra was similar to that reported by F. Teyssier (ATel #12936). We note, however, that the He II 4686 line and the Bowen blend were quite strong during the first days of the superoutburst peaking at 30% and 15% above the continuum, respectively, but then almost disappeared on July 19-20. Although the emission lines of TCP J21040470+4631129 are relatively narrow, the spectral resolution of our time-resolved data (R=1000-6000) is enough to trace the variability of line profiles. For consistency, we extracted two sets of quantities from the H-alpha line – the radial velocities measured using the double-Gaussian method, and the line asymmetry measured as the ratio between the areas of the blue and red parts of the line – and calculated periodograms for them. Both sets of quantities revealed the presence of the orbital variability with the period of Porb=77.07+/-0.02 min, which is consistent with the period of early superhumps.
Such a short orbital period places TCP J21040470+4631129 at the very short-period end of the orbital period distribution of cataclysmic variables where the main-sequence donor stars transform into brown dwarf-like objects. However, it seems that TCP J21040470+4631129 is still at the start of this transition. Indeed, the fractional superhump period excess eps = (Psh-Porb)/Porb is ~2.3%. Employing the empirical relation between eps and the mass ratio q (Patterson et al. 2005, PASP, 117, 1204), we find q to be ~0.1. Such a large q suggests that the donor star is still on the main sequence.
We also continued monitoring TCP J21040470+4631129 with Swift. Between July 29 and August 5, Swift/XRT obtained four additional 1 ks observations during which an XRT count rate of the transient was at the level of 0.025+/-0.004 cts/s, that is about 3 times lower than during the previous observation on July 17 (ATel #12947).
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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