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Galaxies 2016, 4(4), 41; https://doi.org/10.3390/galaxies4040041

Article
A Search for QPOs in the Blazar OJ287: Preliminary Results from the 2015/2016 Observing Campaign
by S. Zola 1,2,*, M. Valtonen 3,4, G. Bhatta 1, A. Goyal 1, B. Debski 1, A. Baran 2, J. Krzesinski 2, M. Siwak 2, S. Ciprini 5,6, A. Gopakumar 7, H. Jermak 8, K. Nilsson 4, D. Reichart 9, K. Matsumoto 10, K. Sadakane 10, K. Gazeas 11, M. Kidger 12, V. Piirola 3,4, F. Alicavus 13,14, K. S. Baliyan 15, A. Berdyugin 4, D. Boyd 16, M. Campas Torrent 17, F. Campos 18, J. Carrillo Gómez 19, D. B. Caton 20, V. Chavushyan 21, J. Dalessio 22, D. Dimitrov 23, M. Drozdz 2, H. Er 24, A. Erdem 13,14, A. Escartin Pérez 25, V. Fallah Ramazani 4, A. V. Filippenko 26, F. Garcia 27, F. Gómez Pinilla 28, M. Gopinathan 29, J. B. Haislip 30, J. Harmanen 4, R. Hudec 31,32, G. Hurst 33, K. M. Ivarsen 30, M. Jelinek 31, A. Joshi 34, M. Kagitani 35, N. Kaur 15, W. C. Keel 36, A. P. LaCluyze 30, B. C. Lee 37, E. Lindfors 4, J. Lozano de Haro 38, J. P. Moore 30, M. Mugrauer 39, R. Naves Nogues 17, A. W. Neely 40, R. H. Nelson 41, W. Ogloza 2, S. Okano 35, J. C. Pandey 34, M. Perri 5,42, P. Pihajoki 43, G. Poyner 44, J. Provencal 22, T. Pursimo 45, A. Raj 46, R. Reinthal 4, S. Sadegi 4, T. Sakanoi 35, Sameer 15, J.-L. Salto González 47, T. Schweyer 48,49, F. C. Soldán Alfaro 50, N. Karaman 51, E. Sonbas 51, I. Steele 8, J. T. Stocke 52, J. Strobl 31, L. O. Takalo 4, T. Tomov 53, L. Tremosa Espasa 54, J. R. Valdes 21, J. Valero Pérez 55, F. Verrecchia 5,42, J. R. Webb 56, M. Yoneda 57, M. Zejmo 58, W. Zheng 26, J. Telting 45, J. Saario 45, T. Reynolds 45, A. Kvammen 45, E. Gafton 45, R. Karjalainen 59 and P. Blay 60
1
Astronomical Observatory, Jagiellonian University, ul. Orla 171, Krakow 30-244, Poland
2
Mt. Suhora Observatory, Pedagogical University, ul. Podchorazych 2, Krakow 30-084, Poland
3
Finnish Centre for Astronomy with ESO, University of Turku, Turku F-21500, Finland
4
Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Turku F-21500, Finland
5
Agenzia Spaziale Italiana (ASI) Science Data Center, Roma I-00133, Italy
6
Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, Perugia I-06123, Italy
7
Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Mumbai 400005, India
8
Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, Brownlow Hill L3 5RF, UK
9
Departmentof Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599, USA
10
Astronomical Institute, Osaka Kyoiku University, 4-698 Asahigaoka, Kashiwara, Osaka 582-8582, Japan
11
Department of Astrophysics, Astronomy and Mechanics, National & Kapodistrian University of Athens, Zografos GR-15784, Athens, Greece
12
Herschel Science Centre, ESAC, European Space Agency, C/Bajo el Castillo, s/n, Villanueva de la Cañada, Madrid E-28692, Spain
13
Department of Physics, Faculty of Arts and Sciences, Canakkale Onsekiz Mart University, Canakkale TR-17100, Turkey
14
Astrophysics Research Center and Ulupinar Observatory, Canakkale Onsekiz Mart University, Canakkale TR-17100, Turkey
15
Physical Research Laboratory, Ahmedabad 380009, India
16
5, Silver Lane, West Challow, Wantage, Oxon OX12 9TX, UK
17
C/Jaume Balmes No 24, Cabrils, Barcelona E-08348, Spain
18
C/.Riera, 1, 1o 3a B, Vallirana, Barcelona E-08759, Spain
19
Carretera de Martos 28 primero Fuensanta, Jaen E-23001, Spain
20
Dark Sky Observatory, Department of Physics and Astronomy, Appalachian State University, Boone, NC 28608, USA
21
Instituto Nacional de Astrofisica, Óptica y Electrónica, Apartado Postal 51-216, Puebla 72000, Mexico
22
Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA
23
Institute of Astronomy and NAO, Bulgarian Academy of Science, 72 Tsarigradsko Chaussee Blvd., Sofia 1784, Bulgaria
24
Department of Physics, Faculty of Science, Atatürk University, Erzurum 25240, Turkey
25
Aritz Bidea No 8 4 B, Mungia, Bizkaia E-48100, Spain
26
Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
27
Muñas de Arriba La Vara, Valdés E-33780, Spain
28
C/Concejo de Teverga 9, 1 C, Madrid E-28053, Spain
29
Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital 263002, India
30
Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599, USA
31
Astronomical Institute, The Czech Academy of Sciences, Ondřejov 25165, Czech Republic
32
Faculty of Electrical Engineering, Czech Technical University, Prague 74864, Czech Republic
33
16 Westminster Close, Basingstoke, Hampshire RG22 4PP, UK
34
Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital 263002, India
35
Planetary Plasma and Atmospheric Research Center, Tohoku University, Sendai 980-8578, Japan
36
Department of Physics and Astronomy and SARA Observatory, University of Alabama, Box 870324, Tuscaloosa, AL 35487, USA
37
Korea Astronomy and Space Science Institute, 776, Daedeokdae-Ro, Youseong-Gu, Daejeon 305-348, Korea
38
Partida de Maitino, Pol. 2 Num. 163, Elche E-03206, Spain
39
Astrophysikalisches Institut und Universitäts-Sternwarte, Schillergäßchen 2-3, Jena D-07745, Germany
40
NF/Observatory, Silver City, NM 88041, USA
41
1393 Garvin Street, Prince George, BC V2M 3Z1, Canada
42
INAF—Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio Catone I-00040, Italy
43
Department of Physics, University of Helsinki, P.O. Box 64, Helsinki FI-00014, Finland
44
BAA Variable Star Section, 67 Ellerton Road, Kingstanding, Birmingham B44 0QE, UK
45
Nordic Optical Telescope, Apartado 474, Santa Cruz de La Palma E-38700, Spain
46
Indian Institute of Astrophysics, II Block Koramangala, Bangalore 560 034, India
47
Observatori Cal Maciarol Módul 8. Masia Cal Maciarol, Camí de l’Observatori s/n, Ager, Lerida E-25691, Spain
48
Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, Garching D-85748, Germany
49
Physik Department, Technische Universität München, James-Franck-Str., Garching D-85748, Germany
50
C/Petrarca 6 1a Sevilla E-41006, Spain
51
Department of Physics, University of Adiyaman, Adiyaman 02040, Turkey
52
Department of Astrophysical and Planetary Sciences, Center for Astrophysics and Space Astronomy, University of Colorado, Box 389, Boulder, CO 80309, USA
53
Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, ul. Grudziadzka 5, Torun 87-100, Poland
54
C/Cardenal Vidal i Barraquee No 3, Cambrils, Tarragona E-43850, Spain
55
C/Matarrasa 16, Ponferrada, León E-24411, Spain
56
Florida International University and SARA Observatory, University Park Campus, Miami, FL 33199, USA
57
Kiepenheuer-Institut fur Sonnenphysik, Freiburg D-79104, Germany
58
Janusz Gil Institute of Astronomy, University of Zielona Góra, Szafrana 2, Zielona Góra PL-65-516, Poland
59
Isaac Newton Group of Telescopes, Apartado 321, Santa Cruz de La Palma E-38700, Spain
60
IAC-NOT, C/Via Lactea, S/N, La Laguna E-38205, Spain
*
Author to whom correspondence should be addressed.
Academic Editors: Jose L. Gómez, Alan P. Marscher and Svetlana G. Jorstad
Received: 16 July 2016 / Accepted: 9 September 2016 / Published: 9 October 2016

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.
Keywords:
galaxies: active; BL Lacertae objects: individual (OJ287); supermassive black holes

1. Introduction

OJ287 is the only blazar known to exhibit certain quasi-periodic variability in its light curve, with a rough period of 12 years. A model that successfully explains this observational feature requires the blazar central engine to contain a binary consisting of two supermassive black holes (SMBHs; Valtonen et al. 2008 [1], and references therein). The two SMBHs orbit their common center of mass, and the less-massive one (150 million solar mass) pierces the accretion disk surrounding the more-massive one (18 billion solar mass) twice per orbit. The general relativistic orbital precession naturally explains the quasi-periodic light-curve variability of OJ287.
Since 2006, OJ287 has been regularly monitored at optical wavelengths at the Mt. Suhora Observatory, with supporting observations at Krakow and Athens. In the 2015/2016 season, we started observations in September, soon after the blazar became visible after the summer conjunction with the Sun. In anticipation of the outburst predicted for this season by the binary model, a multi-site campaign was organized. Polarimetric observations were also scheduled to help reveal the nature of the expected brightening. The predicted outburst started at the end of November 2015, with an initial slow rise in brightness followed by a very rapid brightening. After our alert, almost two dozen telescopes on four continents contributed photometric observations, providing very good coverage of the event as shown in the upper panel of Figure 1. Polarimetric observations were taken at Hawaii, the Canary Islands, Mt. Suhora, and in India. The full-season light curve of OJ287 taken until mid-May 2016 is presented in the bottom panel of Figure 1; symbols in green denote dates when low polarization ( p < 11 %) was measured. Ultraviolet (UV) and X-ray data were also obtained with the Swift satellite. Timing of this and previous outbursts allowed revision of the masses of the SMBHs, and the measured spin of the more-massive black hole (BH) is 0.31 ± 0.01 (Valtonen et al. 2016 [2]).

2. Search for QPOs

2.1. Ground-Based Data

Variability at all wavelengths is commonly observed in blazars. Amplitudes of flux changes in the optical band can reach a few magnitudes. These variations can be fast; often, intraday variability is seen. There are physical processes in blazars that could lead to periodic or quasi-periodic behaviour (e.g., those arising at the innermost stable circular orbit). Detection of such quasi-periodic oscillations (QPOs) could give a better understanding of the underlying physical processes in blazars. There were numerous periodicity analyses and discussions of the physical significance of the various frequencies in OJ287. Results covering the previous outburst in 2005 were published by Valtonen et al. (2012) [3] and by Pihajoki et al. (2013) [4].
The intensive multisite monitoring of OJ287 in the 2015/2016 season resulted in the best coverage ever obtained from the ground: between mid-November 2015 and mid-May 2016, OJ287 was observed a few times per day. Our first goal was to search for any periodic signal present in the data around the December flare. We analysed the residuals left after the trend plotted as the model line (Figure 1, top panel) was subtracted. Three methods were applied: regular Fourier transform (FT), wavelet, and running Fourier transform (rFT). We found no significant (above the 4 σ level) peaks with FT. A period of about 3 hr can be recognized, but only at the 2 σ level. Both the wavelet and rFT techniques revealed the presence of a statistically significant, short-lived period of about 3 days at the outburst maximum. The period of its visibility was centered at the maximum of brightness (Figure 2) — it showed up near JD 2,457,360 and disappeared after 4 days.
We also performed a thorough search using the entire season dataset covering the period from mid-September 2015 to mid-May 2016. Several statistical tools have been used, and we show the Lomb–Scargle periodogram (Lomb 1976 [5], Scargle 1982 [6]) in the left panel of Figure 3. The red-noise ( β = 1.5 ) light curves were simulated by the randomization of both phase and amplitude, as described by Timmer & Koenig 1995 [7]. The light curves were then resampled according to the sampling of the real light curve, and their Lomb–Scargle periodogram (LSP) was computed. The mean LSP of 1000 simulated light curves is shown in black in the left panel of Figure 3. No significant peaks corresponding to short periods were found. In the longer-period domain, there seem to be statistically significant peaks in the range between 0.01 and 0.1 c/d. However, the weighted wavelet Z-transform analysis (WWZ; Foster 1996 [8]) indicates that they are not stable. As seen in Figure 3 (right panel), the length of the longest period (about 95 days) has been increasing since it started to be visible at about JD 2457330.

2.2. K2 Observations

OJ287 was observed by the Kepler spacecraft during K2 Campaign 5. This run resulted in almost continuous coverage over 75 days ( 27 April 2015 to 10 July) with about 1-min cadence. We used both short- and long-cadence target pixel files. We employed our custom IRAF tasks to pull out fluxes, applying three-pixel circular apertures. We computed power spectral density (PSD) functions for the resulting light curve and also the 2015/2016 ground-based data. Neither show any statistically significant periodicities that could be attributed to QPOs.

3. Conclusions

We found no stable periods in the OJ287 photometric data over the entire 2015/2016 season. However, the 95-day peak in the power spectrum is close to the period for the more-massive BH ISCO, while the 43-day peak is half of this value. Accretion in the form of a one-armed stationary spiral density wave should show up as the full ISCO period, while a two-armed stationary wave will feed the central BH at one-half of the ISCO period. Both types of density waves are observed, such as in galactic disks under perturbation. These phenomena are not expected to produce stable periodicities, since interactions between the exact ISCO period and wave frequencies may occur. The 95-day period started to be visible somewhat before the December outburst, and its best visibility continued after the outburst. The period increased with time, and simultaneously, high optical variability of OJ287 was observed.
We found no firm evidence of any short-period variability that could be attributed to the secondary black hole (at the ISCO or the event horizon). The peaks that different techniques revealed are either transient—like the 3-day period found in the maximum of the December 2015 flare—or the periods and the variability amplitudes in the higher-frequency domain change with time. Such flux changes at shorter timescales most likely originate in the jet. The 3-day quasiperiodicity is the expected jet counterpart of the half-ISCO, with a Lorentz compression factor of 14.
The PSD analysis of both ground-based and Kepler data shows no statistically significant peaks. However, if they do exist, they could be hidden by the high-amplitude variability of the flaring component present after the unprecedented December 2015 outburst.

Acknowledgments

This work was partially supported by the NCN grant No. 2013/09/B/ST9/00599. The Czech team acknowledges GACR grant No. 13-33324S. The Berkeley team is grateful for NSF grant AST-1211916, NASA grant NNX12AF12G, the TABASGO Foundation, and the Christopher R. Redlich Fund. Research at Lick Observatory (KAIT) is partially supported by a generous gift from Google.

Author Contributions

S. Zola coordinated the observing campaign and wrote the paper; M. Valtonen and A. Gopakumar did GR computations; G. Bhatta, A. Goyal, and J. Krzesinski performed the statistical analysis; A. Baran reduced the Kepler observations; S. Ciprini gathered and reduced UV and X-ray data; everyone else contributed photometric and/or polarimetric data, and A.V. Filippenko also edited the paper.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
BHBlack Hole
SMBHSupermassive Black Hole
ISCOInnermost Stable Circular Orbit
PSDPower Spectral Density
LSPLomb–Scargle Periodogram
QPOQuasi-Periodic Oscillation
WWZWeighted Wavelet Z-transform

References

  1. Valtonen, M.; Lehto, H.J.; Nilsson, K.; Heidt, J.; Takalo, L.O.; Sillanpää, A.; Villforth, C.; Kidger, M.; Poyner, G.; Pursimo, T.; et al. Massive binary black-hole system in OJ287 and a test of general relativity. Nature 2008, 452, 851–853. [Google Scholar] [CrossRef] [PubMed]
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Figure 1. R-band light curve of OJ287 gathered during the 2015/2016 season. The December 2015 outburst is shown in the top panel, while the full-season light curve is in the bottom panel. The December 2015 high-amplitude flare turned out to be unpolarized.
Figure 1. R-band light curve of OJ287 gathered during the 2015/2016 season. The December 2015 outburst is shown in the top panel, while the full-season light curve is in the bottom panel. The December 2015 high-amplitude flare turned out to be unpolarized.
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Figure 2. Running Fourier transform Running Fourier transform (rFT) of the OJ287 data gathered during the 2015/2016 outburst.
Figure 2. Running Fourier transform Running Fourier transform (rFT) of the OJ287 data gathered during the 2015/2016 outburst.
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Figure 3. (a) Left panel: Lomb-Scargle periodogram (LSP) of the 2015/2016 data (blue line). The 99% confidence level is shown as the red contour; (b) Right panel: the resulting graph from the wavelet Z-transform analysis.
Figure 3. (a) Left panel: Lomb-Scargle periodogram (LSP) of the 2015/2016 data (blue line). The 99% confidence level is shown as the red contour; (b) Right panel: the resulting graph from the wavelet Z-transform analysis.
Galaxies 04 00041 g003
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