A&A 623, A61 (2019) Astronomy https://doi.org/10.1051/0004-6361/201834458 & c ESO 2019 Astrophysics Search for AGN counterparts of unidentified Fermi-LAT sources with optical polarimetry Demonstration of the technique N. Mandarakas1, D. Blinov1,2,3 , I. Liodakis4, K. Kouroumpatzakis1,2, A. Zezas1,2, G. V. Panopoulou5, I. Myserlis6, E. Angelakis6, T. Hovatta7, S. Kiehlmann5, K. Kokolakis1,8, E. Paleologou1,2, A. Pouliasi1, R. Skalidis1,2, and V. Pavlidou1,2 1 Department of Physics and Institute for Theoretical and Computational Physics (ITCP), University of Crete, 71003 Heraklion, Greece e-mail: [email protected] 2 Foundation for Research and Technology – Hellas, IESL, Voutes, 7110 Heraklion, Greece 3 Astronomical Institute, St. Petersburg State University, Universitetsky pr. 28, Petrodvoretz, 198504 St. Petersburg, Russia 4 KIPAC, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA 5 Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA 6 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany 7 Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Finland 8 Geodesy & Geomatics Engineering Lab, Technical University of Crete, 73100 Chania, Greece Received 18 October 2018 / Accepted 15 January 2019 ABSTRACT Context. The third Fermi-LAT catalog (3FGL) presented the data of the first four years of observations from the Fermi Gamma-ray Space Telescope mission. There are 3034 sources, 1010 of which still remain unidentified. Identifying and classifying γ-ray emitters is of high significance with regard to studying high-energy astrophysics. Aims. We demonstrate that optical polarimetry can be an advantageous and practical tool in the hunt for counterparts of the unidentified γ-ray sources (UGSs). Methods. Using data from the RoboPol project, we validated that a significant fraction of active galactic nuclei (AGN) associated with 3FGL sources can be identified due to their high optical polarization exceeding that of the field stars. We performed an optical polarimetric survey within 3σ uncertainties of four unidentified 3FGL sources. Results. We discovered a previously unknown extragalactic object within the positional uncertainty of 3FGL J0221.2+2518. We obtained its spectrum and measured a redshift of z = 0:0609 ± 0:0004. Using these measurements and archival data we demonstrate that this source is a candidate counterpart for 3FGL J0221.2+2518 and most probably is a composite object: a star-forming galaxy accompanied by AGN. Conclusions. We conclude that polarimetry can be a powerful asset in the search for AGN candidate counterparts for unidentified Fermi sources. Future extensive polarimetric surveys at high Galactic latitudes (e.g., PASIPHAE) will allow the association of a significant fraction of currently unidentified γ-ray sources. Key words. techniques: polarimetric – galaxies: active – gamma rays: galaxies 1. Introduction the unassociated sample, with a false-association rate of 11%. Since the launch of the Fermi spacecraft on 11 June 2008, a vast This technique provided a total of 231 new AGN candidates amount of data has been collected on γ-ray sources. The entire set among the 576 unassociated sources that were studied. of point sources detected during the first four years of observations Chiaro et al.(2016) and Salvetti et al.(2017) used the γ-ray is presented in the 3FGL catalog (Acero et al. 2015). Among the variability properties of unassociated sources and neural net- 3034 sources in 3FGL, about one-third (1010) are still unassoci- works in order to classify these sources. They demonstrated that ated with low-energy counterparts, while AGN account for ∼85% the percentage of sources of uncertain type in 3FGL can be of the associations and identifications (∼57% of the entire sample decreased from 52% to 10% with the use of their method. Simi- of 3034 sources). In order to classify and associate γ-ray sources, lar classification of UGSs can be useful for optimization of sur- various techniques have been used, as summarized below. veys dedicated for their identification. In Doert & Errando(2014), machine learning algorithms Kovalev(2009) proposed using very long baseline interfer- were used to find objects with AGN-like properties in the unas- ometry (VLBI) for identification of γ-ray sources. The author sociated sources of the 2FGL catalog. Machine training was con- cross-correlated positions of 205 γ-ray loud sources observed by ducted using 70% of the known AGN in the catalog, while the Fermi-LAT with VLBI coordinates of a large sample of extra- remaining 30% were used for testing. Results showed that the galactic sources. He was able to confirm the findings of LAT and algorithm is expected to recognize 80% of the AGN present in suggest six new identifications. Article published by EDP Sciences A61, page 1 of8 A&A 623, A61 (2019) Acero et al.(2013) studied Fermi-LAT sources that had also 2. Optical polarimetry as a tool for identification of been observed by the Swift satellite with its X-ray telescope UGSs (XRT). Swift XRT allowed precise localization at the level of a few arcseconds, with the detected sources being then observed in 2.1. Blazar detection efficiency the radio, IR, or optical. Seven high-latitude sources were inves- While blazars are typically moderately to highly polarized in the tigated, four of which were found to be AGN candidates and one optical, they are not the only type of source that can appear polar- a pulsar candidate. The authors speculated that the two remain- ized in the optical band. We must therefore take into account ing objects may belong to a new category subclass or point to a all processes that produce polarization in the optical, charac- new type of γ-ray emitter. terize their properties, and finally select the characteristics that In their search for pulsars in the 3FGL sample, Frail et al. isolate blazars from other types of polarized sources. In any (2016) examined radio spectra of unidentified sources within the given line of sight, light passing through the Galactic interstel- 95% confidence error ellipses, using existing catalogs. Compact lar medium (ISM) becomes linearly polarized due to dichroic objects that are bright in MHz frequencies but faint in GHz fre- extinction from dust grains that are aligned with the interstellar quencies were categorized as pulsar candidates. magnetic field (for a recent review see Andersson et al. 2015). Barr et al.(2013) conducted radio observations of 289 unas- The linear polarization fraction induced by the ISM is typically sociated sources from the 1FGL catalog using the Effelsberg at a level of a few percent. This can be enough to hinder the radio telescope in a search for pulsars. Objects studied were identification of a blazar within a typical field. Additionally, the located in the center of their 95% confidence ellipses. Using this intrinsic fractional polarization of blazars is known to be vari- method, one millisecond pulsar was discovered. able, which can also make them indistinguishable if observed Schinzel et al.(2017), using the Australia Telescope Com- only once at their low-polarization state. pact Array and Very Large Array in the range of 4.0–10.0 GHz, In order to evaluate the efficiency of our method, we devel- performed a survey of all unidentified Fermi sources in the 3FGL oped a Monte Carlo (MC) simulation that allowed us to inves- catalog, in their search for radio counterparts. They found 2097 tigate whether a blazar would be significantly more polarized candidates, with several fields containing multiple compact radio than foreground stars. To account for the fact that different parts sources, while others did not contain any above 2 mJy. For sev- of the sky exhibit different average interstellar polarization and eral of these targets they performed follow-up observations with the polarimetric properties in a single region vary between stars VLBI, which provided 142 new AGN associations, alternative in the same region, we relied on the detailed, high-accuracy associations for 7 objects, improved positions for 144 known optopolarimetric mapping of the well-known Polaris Flare cloud associations, as well as 36 extended radio sources. Among the using the RoboPol instrument (Panopoulou et al. 2015), and fields studied was 3FGL J0221.2+2518, which is the field of rescaled its polarization properties to different average polariza- interest of this paper. They propose two possible radio counter- tion values that may be applicable at different Galactic latitudes. parts lying within this field. We discuss the possibility of these First we estimated how much interstellar polarization varies associations with the Fermi source in Sect.6. from star to star in an area of the sky that is typical for UGSs Sowards-Emmerd et al.(2003) used a figure of merit (FoM) position uncertainty. To this end, we found that this area for approach to quantify the probability that an unassociated source sources in the 3FGL catalog is 0.0456 deg2, which corresponds is a blazar. To form this FoM, basic characteristics of blazars to a circle with a radius of 0.12 deg. We placed this circle in are taken into account: radio and X-ray properties as well as random positions in the Polaris Flare cloud region and mea- source position. Based on this approach, the authors evaluated sured the standard deviation of fractional polarization, σp, of associations of γ-ray and radio sources and presented ∼20 new stars within it, on the condition that there are five or more stars identifications. with measured polarization by Panopoulou et al.(2015) within These methods for the identification of Fermi sources make the selected area. We repeated the process until we obtained a set use of various characteristics of γ-ray emitters. Optical polariza- of 5000σp values from which we calculated the standard devia- tion is a frequent trait of γ-ray sources that has yet to be exploited tion σσp and the mean Mσp .