Astronomy Object of the Month: 2018, September
Detection of Periodic Radio Signal from the Blazar PKS 0219-164
Blazars are the most luminous sources in the Universe. They are believed to harbor monstrously giant black holes fed by the matter
falling from the parsec-scale accretion disks. These sources launch the most powerful relativistic jets beamed upon us and ejecting matter with high
speeds. In the radio images, the jets appear to extend up to a large distance from the central engine, and in some cases radio
knots show superluminal motion with apparent velocity up to about 37 times the speed of light. Blazars consist of two kinds of sources:
flat-spectrum radio quasars (FSRQ) and BL Lacertae objects. FSQRs are more powerful, show emission lines over the continuum and have the
synchrotron peak in the lower part of the spectrum; whereas BL Lac objects are less powerful, show weak or no emission lines, and have synchrotron
peak in the higher part of the spectrum. BL Lacs represent an extreme class of sources with maximum synchrotron and inverse-Compton energies
(hard X-rays to TeV emission); however, in comparison to the more luminous FSRQs, they accrete at relatively low rates and do not possess strong
circum-nuclear photon fields.
Blazar continuum emission is variable, both in flux and polarization, on diverse timescales ranging from a few minutes to decades. Most of the blazars are so far away us that the compact active region can not be resolved by any of the current instruments. In such context, the study of variability becomes one of the most important tools to investigate the processes occurring close to the central region. Previously, astronomers have detected quasi-periodic oscillations (QPO) in the multi-frequency light curves of a number of blazars. However, as their light curves are largely dominated by red-noise variability, establishing significance against spurious detection is a challenge.
PKS 0219-164 (z=0.7) is a BL Lac source that have been detected over a broad range of the electromagnetic spectrum including radio, infra-red optical, X-ray and γ-ray. The accurate position of the source at radio frequency (2700 MHz) and its optical counterpart was measured in 1977, and later in 1993 the source was classified as quasi-stellar object (QSO). The decade-long 15 GHz radio observations of the blazar PKS 0219-164 from the 40-m telescope at the Owens Valley Radio Observatory (OVRO) were analyzed employing Lomb-Scargle periodogram and weighted wavelet z-transform. The study revealed a strong repeating signal with a periodicity of ∼270 days.
For the observed period of 270 days, the corresponding period in the source rest frame at z=0.7 turns out to be about 160 days. For a blazar with a black hole of mass 109 solar masses, the radius of the Keplerian orbit is estimated as nearly 3 milli-parsecs. For the binary mass ratios in the range from 0.1 up to 0.01, the orbital decay timescale in the GW-driven regime is about 35 years only, which implies that we should expect that the system will gravitationally collapse with the release of gravitational waves within our life time. However, some astronomers believe that the probability of observing such a close binary SMBH system might be too small. The closest binary SMBH so far observed is separated by 0.35 pc and lies in a Seyfert type 2 (AGN Mrk 533).
But, on the other hand, if the observed periodicity is Doppler boosted by the relativistic motion of the emission regions, such as along the helical path of the magnetized jets, the periodic timescales can be longer by Doppler factor (typically 10 to 20) so that corresponding Keplerian orbit of the secondary black hole can lie farther away up to a distance of a few sub-parsecs. The binary SMBH systems spend relatively longer time at sub-parsecs separation during their evolution.
Alternatively, in General Relativistic treatment of a rotating massive objects, the nearby inertial frames are distorted by frame-dragging. This results in the nodal precession of the tilted orbits known as Lense-Thirring precession. For a fast spinning black hole with a mass of 109solar masses, the precessing orbit should lie around 12 gravitational radii. Similarly, a periodic swing of the blazar jet angle by a small angle also can explain the observed flux modulation.
In another probable scenario, a highly magnetized accretion disk can lead to the formation of so-called magnetically choked accretion flow. In that case, the Rayleigh-Taylor and Kelvin-Helmholtz instabilities can cause QPO oscillations at the interface of the disk and the magnetosphere. Such QPOs are observed to naturally develop in the magneto-hydrodynamical simulations of the large scale jets.
Original publication: Gopal Bhatta: Radio and gamma-Ray Variability in the BL Lac PKS 0219−164: Detection of Quasi-periodic Oscillations in the Radio Light Curve, Astrophysical Journal, 14 September 2017.
The research was conducted at the Department of High Energy Astrophysics of the Jagiellonian University’s Astronomical Observatory (OAUJ). The work was largely supported by the Polish National Science Centre through the grant DEC-2012/04/A/ST9/00083. More recently, Dr. Bhatta detected a transient QPO in the gamma-ray emission of the famous blazar Mrk 501. The discovery was also reported in Phys.org.
Gopal.Bhatta [at] uj.edu.pl