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Register a new userBlazar OJ 287 After First VHE Activity: Tracking the Re-emergence of the HBL like Component in 2020
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We report the re-emergence of a new broadband emission through a detailed and systematic study of the multi-wavelength spectral and temporal behavior of OJ 287 after its first-ever reported VHE activity in 2017 to date, which includes the second-highest X-ray flux of the source. The source shows high optical to X-ray flux variations, accompanied mainly by strong spectral changes. The optical to X-ray flux variations are correlated and simultaneous except for two durations when they are anti-correlated. The flux variations, however, are anti-correlated with the X-ray spectral state while correlated with optical-UV (ultraviolet). Weekly binned {it Fermi}-LAT data around the duration of the highest X-ray activity show a few detections with a log-parabola model but none with a power-law; yet the extracted LAT spectral energy distribution (SED) of the high activity duration for both the models is similar and show a hardening above 1 GeV. Further, near-infrared (NIR) data indicate strong spectral change, resembling a thermal component. Overall, the combined optical to gamma-ray broadband spectrum establishes the observed variations to a new high-energy-peaked (HBL) broadband emission component, similar to the one seen during the highest reported X-ray flux state of the source in 2017. The observed activities indicate some peculiar features that seem to be characteristic of this emission component while its appearance, a few years around the claimed (sim 12)-year optical outbursts strongly indicate a connection between the two.
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We report three AstroSat observations of BL Lacertae object OJ 287. The three observations caught it in very different flux states that are connected to different broadband spectral states. These observations trace the source spectral evolution from the end-phase of activity driven by a new, additional HBL like emission component in 2017 to its complete disappearance in 2018 and re-emergence in 2020. The 2017 observation shows a comparatively flatter optical-UV and X-ray spectrum. Supplementing it with the simultaneous NuSTAR monitoring indicates a hardening at the high-energy-end. The 2018 observation shows a harder X-ray spectrum and a sharp decline or cutoff in the optical-UV spectrum, revealed thanks to the Far-UV data from AstroSat. The brightest of all, the 2020 observation shows a hardened optical-UV spectrum and an extremely soft X-ray spectrum, constraining the low-energy peak of spectral energy distribution at UV energies -- a characteristic of HBL blazars. The contemporaneous MeV-GeV spectra from LAT show the well-known OJ 287 spectrum during 2018 but a flatter spectrum during 2017 and a hardening above ~1 GeV during 2020. Modeling broadband SEDs show that 2018 emission can be reproduced with a one-zone leptonic model while 2017 and 2020 observations need a two-zone model, with the additional zone emitting an HBL radiation.
We present a comprehensive analysis of all XMM-Newton spectra of OJ 287 spanning 15 years of X-ray spectroscopy of this bright blazar. We also report the latest results from our dedicated Swift UVOT and XRT monitoring of OJ 287 which started in 2015, along with all earlier public Swift data since 2005. During this time interval, OJ 287 was caught in extreme minima and outburst states. Its X-ray spectrum is highly variable and encompasses all states seen in blazars from very flat to exceptionally steep. The spectrum can be decomposed into three spectral components: Inverse Compton (IC) emission dominant at low-states, super-soft synchrotron emission which becomes increasingly dominant as OJ 287 brightens, and an intermediately-soft (Gamma_x=2.2) additional component seen at outburst. This last component extends beyond 10 keV and plausibly represents either a second synchrotron/IC component and/or a temporary disk corona of the primary supermassive black hole (SMBH). Our 2018 XMM-Newton observation, quasi-simultaneous with the Event Horizon Telescope observation of OJ 287, is well described by a two-component model with a hard IC component of Gamma_x=1.5 and a soft synchrotron component. Low-state spectra limit any long-lived accretion disk/corona contribution in X-rays to a very low value of L_x/L_Edd < 5.6 times 10^(-4) (for M_(BH, primary) = 1.8 times 10^10 M_sun). Some implications for the binary SMBH model of OJ 287 are discussed.
Binary black hole (BH) central engine description for the unique blazar OJ 287 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 OJ 287 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 $mu$m 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 hours 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 OJ 287. These multi-epoch Spitzer observations provide a parametric constraint on the celebrated BH no-hair theorem.
We present a multi-wavelength spectral and temporal investigation of OJ 287 emission during its strong optical-to-X-ray activity between July 2016 - July 2017. The daily $gamma$-ray fluxes from emph{Fermi}-LAT are consistent with no variability. The strong optical-to-X-ray variability is accompanied by a change in power-law spectral index of the X-ray spectrum from $< 2$ to $>2$, with variations often associated with changes in optical polarization properties. Cross-correlations between optical-to-X-ray emission during four continuous segments show simultaneous optical-ultraviolet (UV) variations while the X-ray and UV/optical are simultaneous only during the middle two segments. In the first segment, the results suggest X-rays lag the optical/UV, while in the last segment X-rays lead by $sim$ 5-6 days. The last segment also shows a systematic trend with variations appearing first at higher energies followed by lower energy ones. The LAT spectrum before the VHE activity is similar to preceding quiescent state spectrum while it hardens during VHE activity period and is consistent with the extrapolated VHE spectrum during the latter. Overall, the broadband spectral energy distributions (SEDs) during high activity periods are a combination of a typical OJ 287 SED and an HBL SED, and can be explained in a two-zone leptonic model, with the second zone located at parsec scales, beyond the broad line region, being responsible for the HBL-like spectrum. The change of polarization properties from systematic to chaotic and back to systematic, before, during and after the VHE activity, suggest dynamic roles for magnetic fields and turbulence.
We present a multi-wavelength spectral and temporal analysis of the blazar OJ 287 during its recent activity between December 2015 -- May 2016, showing strong variability in the near-infrared (NIR) to X-ray energies with detection at $gamma$-ray energies as well. Most of the optical flux variations exhibit strong changes in polarization angle and degree. All the inter-band time lags are consistent with simultaneous emissions. Interestingly, on days with excellent data coverage in the NIR--UV bands, the spectral energy distributions (SEDs) show signatures of bumps in the visible--UV bands, never seen before in this source. The optical bump can be explained as accretion-disk emission associated with the primary black hole of mass $sim rm 1.8 times10^{10} M_{odot}$ while the little bump feature in the optical-UV appears consistent with line emission. Further, the broadband SEDs extracted during the first flare and during a quiescent period during this span show very different $gamma$-ray spectra compared to previously observed flare or quiescent spectra. The probable thermal bump in the visible seems to have been clearly present since May 2013, as found by examining all available NIR-optical observations, and favors the binary super-massive black hole model. The simultaneous multi-wavelength variability and relatively weak $gamma$-ray emission that shows a shift in the SED peak is consistent with $gamma$-ray emission originating from inverse Compton scattering of photons from the line emission that apparently contributes to the little blue bump.
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