Publications

Continuous optical monitoring of the highly active blazar Mrk421

K. Gazeas, M. Petropoulou, A. Mastichiadis


ABSTRACT

We present the recent photometric monitoring of blazar Mrk421, obtained from the Gerostathopouleio Observatory at the University of Athens. Follow-up observations have been performed on this source after a highly energetic flare which occurred on 12 April, 2013. The flare was observed in X- rays by Nustar & Swift and in GeV-TeV gamma-rays by the Fermi satellite and MAGIC/VERITAS telescopes respectively. Continuous photometric monitoring in the optical BVRI bands during 3 months after the flaring activity reveals a quasi-periodic light variation. This is one of the few times that Mrk421 was observed for such a long period without large observational gaps in four different filters. We find a strong correlation between the different optical bands. Although we did not detect any signs of intra-day variability, the optical flux is variable in longer time scales (days/weeks) with the relative amplitude of variations being approximately the same in all four bands.


Reference: Gazeas K., Petropoulou M., Mastichiadis A., 2013, Hel.A.S. Conf. Series



Multi-wavelength monitoring of the highly active blazar Mrk421. Investigating high vs. low energy correlated variability

K. Gazeas, G. Bhatta, W. Max-Moerbeck, M. Petropoulou, T. Hovatta, G. Vasilopoulos, A. Mastichiadis


ABSTRACT

We present a long-term multi-wavelength monitoring of blazar Mrk 421, obtained simulta- neously with orbital and ground-based instruments. Daily optical observations from the University of Athens Observatory started as a follow-up monitoring of the major gamma-ray are of 12 April, 2013. This data set is compared with the gamma-ray, X-ray and UV observations obtained with Fermi/LAT and SWIFT satellite telescopes respectively, as well as with the 95 GHz and 15 GHz data from the CARMA and OVRO 40-m radio telescope.We investigate the existence of correlated variations among the different wavebands using the discrete correlation function (DCF) technique.


Reference: Gazeas et al., 2015, Hel.A.S. Conf. Series



Blazar Optical Sky Survey -BOSS project (2013-2017). The long-term blazar variability monitoring

K. Gazeas


ABSTRACT

Blazar Optical Sky Survey (BOSS Project) is a dedicated observational survey with the aim of monitoring known blazars in optical wavelengths. The project was initiated in March 2013 at the University of Athens Observatory (UOAO), performing ground-based optical photometric observations in parallel with orbital (SWIFT/XRT, FERMI/LAT) X-ray observatories. BOSS Project immediately met international attention, attracting the interest of several collaborators worldwide. It is currently running as an international collaboration of the National and Kapodistrian University of Athens, utilizing the robotic and remotely controlled telescope at the UOAO. Several targets of interest are monitored in the frame of BOSS Project, such as highly variable blazars and AGNs. The targets are continuously observed on a daily basis, with the aim to achieve dense temporal coverage in optical wavelengths. Furthermore, simultaneous observations in high and low energy bands are cross-correlated with BOSS database and crucial information are gathered, in order to understand the mechanisms that are taking place in these objects. In this work, some of the major achievements after the first 4 years of operation of the BOSS Project are given, while the advantage of small, robotic and remotely controlled telescopes is highlighted.


Reference: Gazeas K., 2017, Hel.A.S. Conf. Series



Multi-wavelength monitoring of the highly active blazar Mrk421. Investigating high vs. low energy correlated variability

K. Gazeas


ABSTRACT

Blazar Optical Sky Survey (BOSS Project) is a dedicated observational survey with the aim of monitoring known blazars in optical wavelengths. The project was initiated in March 2013 at the University of Athens Observatory (UOAO), performing ground-based optical photometric observations in parallel with orbital (SWIFT/XRT, FERMI/LAT) X-ray observatories. BOSS Project is currently running as an international collaboration of the National and Kapodistrian University of Athens, utilizing the robotic and remotely controlled telescope at the UOAO. The prototype blazar BL Lac is monitored in the frame of the BOSS Project, during the period of 2014-2018. BL Lac is continuously observed on a daily basis, in order to achieve dense temporal coverage in optical wavelengths and study the short time-scale flux variability. Several long-runs have been conducted, with the target monitored for several hours during the night, aiming towards the IDV detection and characterization. In this presentation, the preliminary results of the frequency analysis are given, summarizing the achievements after the 5-year long operation of the BOSS Project, while the advantage of small, robotic and remotely controlled telescopes is highlighted.


Reference: Gazeas K., 2018, HAP, Cochem



Long-Term Optical Monitoring of Blazars

K. Gazeas


ABSTRACT

Systematic monitoring of specific targets in the optical regime was historically applied on a very narrow sample of known variable stars. The discovery of blazars in the 20th century brought to the foreground the need for new global sky surveys, covering the entire sky and fainter sources. Full-sky surveys are conducted more easily from space observatories, while radio telescopes perform follow up observations from the ground. Blazars are detected in a wide range of energies, while they exhibit strong variability in various wavelengths from -rays and X-rays to the optical and radio domain. This results in a detailed classification, according to their emission properties in each region. The rapid variability in optical domain makes blazars interesting targets for optical sky surveys, o ering a new opportunity to study their variability in the time domain. Digital sky surveys in optical and near-IR found a fertile ground with the aid of sensitive sensors. Only a few dedicated programs are focusing on blazar variability, a trend which evolved rapidly in the last decade. Modern techniques, in combination with dedicated sky survey programs lead towards a new era of long-term monitoring of blazars, aiming towards the search or variability on various time scales. In this work, an overview of blazar optical surveys and monitoring projects is given, addressing the major points of each one, and highlighting the constraints that the long-term study of blazars will bring through future international campaigns.


Reference: Gazeas K., 2019, Galaxies, 7, 58



Publications in collaboration with other groups

The 2015 outburst of the OJ287 blazar

M. Valtonen, S. Zola, A. Gopakumar, K. Gazeas, W. Ogloza, M. Drozdz, M. Siwak, B. Debski, J. Dalessio, K. Sadakane, M. Kidger, K. Nilsson, A. Berdyugin, E. Lindfors, L. Takalo, K. Baliyan, M. Mugrauer, F. Alicavus, A. Erdem, J. Provencal, J. Webb, M. Zejmo, E. Sobas, H. Er, W. Keel, T. Schweyer


ABSTRACT

A model that contains a massive BH binary was proposed to explain the double peaked quasi-periodical (roughly about 12 years) outbursts of the blazar OJ287. A regular photometric monitoring of this target has been performed since the very beginning of this season with the aim of catching the next outburst, predicted by the model to occur this winter, between mid November and early January. Brightness of OJ287 was changing in the range between 14.4 and 14.8 mag in the R filter for most of this season but starting from Nov 18 we observed its gradual light increase, followed by a rapid brightness rise also announced by the ASAS project (ATEL #8372). On Dec 4, we recorded the highest brightness of about 12.9 mag (R) and OJ287 started to fade. We believe that the current outburst, consistent with the inspiralling spinning massive BH binary model for OJ287, could be the expected GR centenary flare.


Reference: Valtonen et al. 2015, ATel



Detection of possible quasi-periodic oscillations in the long-term optical light curve of the BL Lac object OJ 287

G. Bhatta, S. Zola, L. Stawarz, M. Ostrowski, M. Winiarski, W. Ogloza, M. Drozdz, M. Siwak, A. Liakos, D. Koziel-Wierzbowska, K. Gazeas, B. Debski, T. Kundera, G. Stachowski, and V. S. Paliya


ABSTRACT

The detection of periodicity in the broadband non-thermal emission of blazars has so far been proven to be elusive. However, there are a number of scenarios that could lead to quasi-periodic variations in blazar light curves. For example, an orbital or thermal/viscous period of accreting matter around central supermassive black holes could, in principle, be imprinted in the multi-wavelength emission of small-scale blazar jets, carrying such crucial information about plasma conditions within the jet launching regions. In this paper, we present the results of our time series analysis of the ~9.2 yr long, and exceptionally well-sampled, optical light curve of the BL Lac object OJ 287. The study primarily used the data from our own observations performed at the Mt. Suhora and Krakow Observatories in Poland, and at the Athens Observatory in Greece. Additionally, SMARTS observations were used to fill some of the gaps in the data. The Lomb-Scargle periodogram and the weighted wavelet Z-transform methods were employed to search for possible quasi-periodic oscillations in the resulting optical light curve of the source. Both methods consistently yielded a possible quasi-periodic signal around the periods of ~400 and ~800 days, the former with a significance (over the underlying colored noise) of >99%. A number of likely explanations for this are discussed, with preference given to a modulation of the jet production efficiency by highly magnetized accretion disks. This supports previous findings and the interpretation reported recently in the literature for OJ 287 and other blazar sources.


Reference: Bhatta et al. 2016, ApJ, 832, 47



Primary black hole spin in OJ 287 as determined by the general relativity centenary flare

M. Valtonen, S. Zola, S. Ciprini, A. Gopakumar, K. Matsumoto, K. Sadakane et al.


ABSTRACT

OJ287 is a quasi-periodic quasar with roughly 12 year optical cycles. It displays prominent outbursts that are predictable in a binary black hole model. The model predicted a major optical outburst in 2015 December. We found that the outburst did occur within the expected time range, peaking on 2015 December 5 at magnitude 12.9 in the optical R-band. Based on Swift/XRT satellite measurements and optical polarization data, we find that it included a major thermal component. Its timing provides an accurate estimate for the spin of the primary black hole, c = 0.313 +/- 0.01. The present outburst also confirms the established general relativistic properties of the system such as the loss of orbital energy to gravitational radiation at the 2% accuracy level, and it opens up the possibility of testing the black hole no-hair theorem with 10% accuracy during the present decade.


Reference: Valtonen et al. 2016, ApJL, 819, L37



Detection of possible quasi-periodic oscillations in the long-term optical light curve of the BL Lac object OJ 287

S. Zola, M. Valtonen S. Ciprini, D. E. Reichart, J. P. Moore, A. P. LaCluyze, K. M. Ivarsen, R. Groom, K. Stranger, K. Sadakane, K. Matsumoto, M. Kagitani, S. Okano, T. Sakanoi, J. Provencal, J. Dalessio, M. Drozdz, M. Siwak, K. Gazeas, V. Fallah Ramazani, M. Kidger, R. Naves, Montse Campas, H. Jermak, I. A. Steele, E. Sonbas, H. Er, M. Zejmo, D. B. Caton, G. Poyner


ABSTRACT

We have continued monitoring the blazar OJ287 in the optical at several sites after its November/December, 2015 outburst. After a further large outburst that started in the beginning of February, 2016 (Atels #8667, #8697, #8705) OJ287 remained brighter than 14th magnitude [in R] for the rest of the month. A peak was reached in mid February, followed by a gradual decline until almost the end of the month. On Feb 28th a new large flare occurred and the brightness of OJ287 increased from about 14.04 mag to 13.46 in the R filter within a single day. After a short standstill, the blazar continued to rise to 13.11 mag (R), close to the peak of the December outburst. The most recent measurement taken with the SKYNET R-COP telescope in Perth, Australia, indicates that the target is fading. At 13UT on March 3rd, its brightness has decreased to R=13.21. Further multiwavelength coverage is encouraged.


Reference: Zola et al. 2016, ATel



A Search for QPOs in the Blazar OJ287: Preliminary Results from the 2015/2016 Observing Campaign

Zola S., M. Valtonen, G. Bhatta, A. Goyal, B. Debski, A. Baran, J. Krzesinski et al.


ABSTRACT

We analyse the light curve in the R band of the blazar OJ287, gathered during the 2015/2016 observing season. We did a search for quasi-periodic oscillations (QPOs) using several methods over a wide range of timescales. No statistically significant periods were found in the high-frequency domain both in the ground-based data and in Kepler observations. In the longer-period domain, the Lomb-Scargle periodogram revealed several peaks above the 99% significance level. The longest one-about 95 days-corresponds to the innermost stable circular orbit (ISCO) period of the more massive black hole. The 43-day period could be an alias, or it can be attributed to accretion in the form of a two-armed spiral wave.


Reference: Zola et al. 2016, Galaxies, 4, 41



Radio and optical intra-day variability observations of five blazars

X. Liu, P. P. Yang, J. Liu, B. R. Liu, S. M. Hu, O. M. Kurtanidze, S. Zola, A. Kraus, T. P. Krichbaum, R. Z. Su, K. Gazeas, K. Sadakane, K. Nilson, D. E. Reichart, M. Kidger, K. Matsumoto, S. Okano, M. Siwak, J. R. Webb, T. Pursimo, F. Garcia, R. Naves Nogues, A. Erdem, F. Alicavus, T. Balonek and S. G. Jorstad


ABSTRACT

We carried out a pilot campaign of radio and optical band intra-day variability (IDV) observations of five blazars (3C66A, S5 0716+714, OJ287, B0925+504 and BL Lacertae) on 2015 December 18-21 by using the radio telescope in Effelsberg (Germany) and several optical telescopes in Asia, Europe and America. After calibration, the light curves from both 5 GHz radio band and the optical R band were obtained, although the data were not smoothly sampled over the sampling period of about four days. We tentatively analyse the amplitudes and time-scales of the variabilities, and any possible periodicity. The blazars vary significantly in the radio (except 3C66A and BL Lacertae with only marginal variations) and optical bands on intra- and inter-day time-scales, and the source B0925+504 exhibits a strong quasi-periodic radio variability. No significant correlation between the radio- and optical-band variability appears in the five sources, which we attribute to the radio IDV being dominated by interstellar scintillation whereas the optical variability comes from the source itself. However, the radioand optical-band variations appear to be weakly correlated in some sources and should be investigated based on well-sampled data from future observations.


Reference: Liu et al. 2017, MNRAS, 469, 2457



Polarization and Spectral Energy Distribution in OJ 287 during the 2016/17 Outbursts

M. Valtonen, Zola S., H. Jermak, S. Ciprini, R. Hudec, L. Dey, A. Gopakumar, D.L. Reichart, D.B. Caton, K. Gazeas et al.


ABSTRACT

We report optical photometric and polarimetric observations of the blazar OJ 287 gathered during 2016/17. The high level of activity, noticed after the General Relativity Centenary flare, is argued to be part of the follow-up flares that exhibited high levels of polarization and originated in the primary black hole jet. We propose that the follow-up flares were induced as a result of accretion disk perturbations, travelling from the site of impact towards the primary SMBH. The timings inferred from our observations allowed us to estimate the propagation speed of these perturbations. Additionally, we make predictions for the future brightness of OJ 287.


Reference: Valtonen et al. 2017, Galaxies, 5, 83



High accuracy measurement of gravitational wave back-reaction in the OJ287 black hole binary

M.J. Valtonen, L. Dey, R. Hudec, S. Zola, A. Gopakumar, S. Mikkola, S. Ciprini, K. Matsumoto, K. Sadakane, M. Kidger, K. Gazeas, K. Nilsson, A. Berdyugin, V. Piirola, H. Jermak, K.S. Baliyan, D. E. Reichart, D. B. Caton, S. Haque and the OJ287-15/16 Collaboration


ABSTRACT

Blazar OJ287 exhibits large thermal flares at least twice every 12 years. The times of these flares have been predicted successfully using the model of a quasi-Keplerian eccentric black hole binary where the secondary impacts the accretion disk of the primary, creating the thermal flares. New measurements of the historical light curve have been combined with the observations of the 2015 November/December flare to identify the impact record since year 1886, and to constrain the orbit of the binary. The orbital solution shows that the binary period, now 12.062 years, is decreasing at the rate of 36 days per century. This corresponds to an energy loss to gravitational waves that is 6.5 +/- 4 % less than the rate predicted by the standard quadrupolar gravitational wave (GW) emission. We show that the difference is due to higher order gravitational radiation reaction terms that include the dominant order tail contributions.


Reference: Valtonen et al. 2017, IAU



Authenticating the Presence of a Relativistic Massive Black Hole Binary in OJ 287 Using Its General Relativity Centenary Flare: Improved Orbital Parameters

L.Dey, M. J. Valtonen, A. Gopakumar, S. Zola, R. Hudec, P. Pihajoki, S. Ciprini, K. Matsumoto, K. Sadakane et al.


ABSTRACT

Results from regular monitoring of relativistic compact binaries like PSR 1913+16 are consistent with the dominant (quadrupole) order emission of gravitational waves (GWs). We show that observations associated with the binary black hole (BBH) central engine of blazar OJ287 demand the inclusion of gravitational radiation reaction effects beyond the quadrupolar order. It turns out that even the effects of certain hereditary contributions to GW emission are required to predict impact flare timings of OJ287. We develop an approach that incorporates this effect into the BBH model for OJ287. This allows us to demonstrate an excellent agreement between the observed impact flare timings and those predicted from ten orbital cycles of the BBH central engine model. The deduced rate of orbital period decay is nine orders of magnitude higher than the observed rate in PSR 1913+16, demonstrating again the relativistic nature of OJ287's central engine. Finally, we argue that precise timing of the predicted 2019 impact flare should allow a test of the celebrated black hole "no-hair theorem" at the 10% level.


Reference: Dey et al. 2018, ApJ, 866, 11



Stochastic Modeling of Multiwavelength Variability of the Classical BL Lac Object OJ287 on Timescales Ranging from Decades to Hours

A. Goyal, L. Stawarz, S. Zola, V. Marchenko, M. Soida, K. Nilsson, S. Ciprini, A. Baran, M. Ostrowski, P. J. Wiita, Gopal-Krishna, A. Siemiginowska, M. Sobolewska, S. Jorstad, A. Marscher, M. F. Aller, H. D. Aller, T. Hovatta, D. B. Caton, et al.


ABSTRACT

We present the results of our power spectral density analysis for the BL Lac object OJ 287, utilizing the Fermi-LAT survey at high-energy γ-rays, Swift-XRT in X-rays, several ground-based telescopes and the Kepler satellite in the optical, and radio telescopes at GHz frequencies. The light curves are modeled in terms of continuous-time autoregressive moving average (CARMA) processes. Owing to the inclusion of the Kepler data, we were able to construct for the first time the optical variability power spectrum of a blazar without any gaps across ~6 dex in temporal frequencies. Our analysis reveals that the radio power spectra are of a colored-noise type on timescales ranging from tens of years down to months, with no evidence for breaks or other spectral features. The overall optical power spectrum is also consistent with a colored noise on the variability timescales ranging from 117 years down to hours, with no hints of any quasi-periodic oscillations. The X-ray power spectrum resembles the radio and optical power spectra on the analogous timescales ranging from tens of years down to months. Finally, the γ-ray power spectrum is noticeably different from the radio, optical, and X-ray power spectra of the source: we have detected a characteristic relaxation timescale in the Fermi-LAT data, corresponding to ~150 days, such that on timescales longer than this, the power spectrum is consistent with uncorrelated (white) noise, while on shorter variability timescales there is correlated (colored) noise.


Reference: Goyal et al. 2018, ApJ, 863, 175



High accuracy measurement of gravitational wave back-reaction in the OJ287 black hole binary

M.J. Valtonen, L. Dey, R. Hudec, S. Zola, A. Gopakumar, S. Mikkola, S. Ciprini, K. Matsumoto, K. Sadakane, M. Kidger, K. Gazeas, K. Nilsson, A. Berdyugin, V. Piirola, H. Jermak, K.S. Baliyan, D. E. Reichart, S. Haque and the OJ287-15/16 Collaboration


ABSTRACT

The binary black hole (BBH) central engine of OJ287 exhibits large thermal flares at least twice every 12 years. The times of these flares have been predicted successfully using the simple rule that they are generated at a constant phase angle of a quasi-Keplerian eccentric orbit. In this model a secondary black hole goes around a primary black hole, impacting the accretion disk of the latter twice per orbital period, creating the thermal flares. New measurements of the historical light curve have been combined with the observations of the 2015/2017 season. The 2015 November/December flare went into the phase of rapid flux rise on the centenary of Einstein's General Relativity, namely on November 25, and peaked on December 5. At that time OJ287 was at its brightest level in over 30 years in optical wavelengths. Using the light curve of this flare and subsequent synchrotron flares, and comparing it with the points in the historical light curve, we are able to identify the impact record since year 1886, altogether 25 impacts. Out of these, 14 are timed accurately enough to constrain the orbit of the black hole binary. The set of flare timings determines uniquely the 8 parameters of our BBH central engine model: the two masses, the primary spin, the major axis, eccentricity and the phase of the orbit, plus the two parameters of the standard accretion disk. Since the orbit solution is strongly over-determined, its parameters are known very accurately, at better than one percent level for the masses and the primary BH spin. The orbit solution shows that the period of the orbit, now 12.062 year, has decreased at the rate of 36 days per century. This corresponds to an energy loss to gravitational waves that is 6.5 +/- 4 % less than the rate predicted by the standard quadrupolar GW emission. We show that the difference is due to higher order gravitational radiation reaction contributions to BBH dynamics that includes the dominant order tail contributions. The orbital shrinkage rate agrees within error limits with the rate calculated by Damour, Gopakumar and Iyer (2004). The full list of participants in the OJ287-15/16 Collaboration is found in ApJL 819, L37, 2016.


Reference: Valtonen et al. 2018, IAU



Spitzer Observations of the Predicted Eddington Flare from Blazar OJ 287

Seppo Laine, Lankeswar Dey, Mauri Valtonen, A. Gopakumar, Stanislaw Zola, S. Komossa, Mark Kidger, Pauli Pihajok, Jose L. Gomez, Daniel Caton, Stefano Ciprini, Marek Drozdz, Kosmas Gazeas, Vira Godunova, Shirin Haque, Felix Hildebrandt, Rene Hudec, Helen Jermak, Albert K. H. Kong, Harry Lehto, Alexios Liakos, Katsura Matsumoto, Markus Mugrauer, Tapio Pursimo, Daniel E. Reichart, Andrii Simon, Michal Siwak, and Eda Sonbas


ABSTRACT

Binary black hole (BH) central engine description for the unique blazar OJ287 predicted that the next secondary BH impact-induced bremsstrahlung flare should peak on 2019 July 31. This prediction was based on detailed general relativistic modeling of the secondary BH trajectory around the primary BH and its accretion disk. The expected flare was termed the Eddington flare to commemorate the centennial celebrations of now-famous solar eclipse observations to test general relativity by Sir Arthur Eddington. We analyze the multi-epoch Spitzer observations of the expected flare between 2019 July 31 and 2019 September 6, as well as baseline observations during 2019 February-March. Observed Spitzer flux density variations during the predicted outburst time display a strong similarity with the observed optical pericenter flare from OJ287 during 2007 September. The predicted flare appears comparable to the 2007 flare after subtracting the expected higher base-level Spitzer flux densities at 3.55 and 4.49 μm compared to the optical R-band. Comparing the 2019 and 2007 outburst lightcurves and the previously calculated predictions, we find that the Eddington flare arrived within 4 hr of the predicted time. Our Spitzer observations are well consistent with the presence of a nano-Hertz gravitational-wave emitting spinning massive binary BH that inspirals along a general relativistic eccentric orbit in OJ287. These multi-epoch Spitzer observations provide a parametric constraint on the celebrated BH no-hair theorem.


Reference: Laine et al. 2020, ApJL, 894, L1



Created by K.Gazeas, based on a template from Quackit.com
Image sources include: NASA/JPL, IPAC-Caltech