|185 Nov 20, 2015||Multiwavelength evidence for quasi-periodic modulation in the gamma-ray blazar PG 1553+113|
Italian scientists using about 7 years of data from NASA's Fermi Gamma-ray Large Area Telescope (LAT) have detected the first evidence of quasi-periodic changes in the gamma-ray flux emitted by the BL Lac object PG 1553+113. PG 1553+113 lies in the direction of the constellation Serpens Caput, with a redshift lying between 0.395 and 0.6. This optical/X-ray-selected blazar is intensively observed from Cherenkov telescopes at very-high energy gamma rays and may have associated PeV neutrino emission, potentially increasing the interest for this result.
If confirmed in the next years, the discovery would mark the first years-long cyclic gamma-ray and correlated multifrequency emission ever detected from any AGN, providing new insights into physical processes near the central supermassive black hole.
This research is led by S. Ciprini and S. Cutini two INFN researchers belonging the Fermi team at the ASI Science Data Center (ASDC) in strict cooperation with A. Stamerra, INAF Senior Scientist at ASDC. The work has seen also a synergetic international collaboration with scientists like S. Larsson (Royal Institute of Technology, Stockholm Sweden), R. Corbet (NASA Goddard Space Flight Center, USA), both periodical visitors and collaborators of the ASDC, D. Thompson (NASA Goddard Space Flight Center, USA and Deputy Project Scientist of the Fermi mission), W. Max-Moerbeck (National Radio Astronomy Observatory, Socorro, USA) and M. Perri (INAF Rome and ASDC).
Motivated by the possibility of regular gamma-ray changes, the researchers examined a decade of multiwavelength data. These included long-term optical observations from Tuorla Observatory program in Finland, Lick Observatory, KAIT observatory in USA, and the Catalina Sky Survey near Tucson, Arizona, as well as optical and X-ray data from Swift XRT and UVOT instruments and radio, 15 GHz, data from the Owens Valley Radio Observatory, USA. Ciprini, Cutini, Stamerra and their collaborators published the findings in the Nov. 10 edition of The Astrophysical Journal Letters.
The indication of a possible 2-year periodic modulation was possible thanks to the continuous all-sky survey of Fermi; the increased capability of the new Fermi LAT Pass 8 data; and the long-term radio/optical monitoring of LAT gamma-ray blazars. If the gamma-ray cycle of PG 1553+113 is in fact real, the blazar will peak again in 2017 and 2019, well within Fermi's expected operational lifetime.
The scientists identified several scenarios that could drive periodic emission, including different mechanisms that could produce a years-long wobble in the jet of high-energy particles emanating from the black hole. For example pulsational accretion flow instabilities, jet precession, rotation and/or helical structure, or mechanisms analogous to low-frequency QPO of high-mass binary stars. The most exciting scenario involves the presence of a second supermassive black hole closely orbiting at milliparsec scales the one producing the jet we observe. The gravitational pull of the neighboring black hole would periodically tilt the inner part of its companion's accretion disk, where gas falling toward the black hole accumulates and heats up. The result would be a slow oscillation of the jet much like that of a lawn sprinkler, which could produce the cyclic gamma-ray changes we observe.
In this rather less probable but exciting scenario very-low frequency gravitational wave emission would make PG 1553+113 an ideal multimessenger high-energy-photon/neutrino/gravitational-waves source, in this sense the blazars could represent the major "cost-free" accelerators in the nature and ideal multifrequency and multimessenger physics laboratories.
Fermi observations suggest possible years-long cyclic changes in gamma-ray emission from the blazar PG 1553+113.
The marginal significance of the 2.18+/-0.08 year period gamma-ray cycle is strengthened by correlated oscillations observed in radio and optical fluxes.
The first, top panel, shows Fermi Large Area Telescope data from August 2008 to July 2015 for gamma rays with energies above 100 million electron volts (MeV) with one possible explanation for the gamma-ray cycle, an oscillation of the jet produced by the gravitational pull of a second massive black hole, seen at top left in background in this artist's rendering [credits: NASA's Goddard Space Flight Center/CI Lab].
The panels following below the pictorial artist's rendering show all the data used in this research. From top to bottom the gamma-ray (E>100 MeV, and at E>1 GeV) flux light curves (the second, third and fourth panel), the X-ray (0.3-2.0 keV) integral flux by Swift XRT, the optical flux density (R filter) from Tuorla, Catalina CSS and KAIT monitoring programs and Swift UVOT, where the dotted line is the gamma-ray light curve scaled and superposed, and the 15 GHz flux density from the OVRO 40 m radio telescope and by VLBA (the fifth, sixth and seventh panel).