Polarization and Variation of Near-IR Light from Fermi/LAT gamma-ray Sources

We present the results of our follow-up observation program of gamma-ray sources detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. 26 blazars and 39 sources unidentified at other wavelengths were targeted at IRSF 1.4 m telescope equipped with the SIRIUS/SIRPOL imager and polarimeter. H-band magnitudes of the blazars at the epoch of 2010 Dec – 2011 Feb are presented, which reveal clear flux variation since the Two Micron All Sky Survey observations and can be useful data for variation analyses of these objects in longer periods. We also find that nearly half of the gamma-ray blazars are highly (>10 %) polarized in near-infrared wavelengths. Combining the polarization and variation properties, most (~90 %) of the blazars are clearly distinguished from all other types of objects at high Galactic latitudes. On the other hand, we find only one highly polarized and/or variable object in the fields of unidentified sources. This object is a counterpart of the optical variable source PQV1 J131553.00-073302.0 and the radio source NVSS J131552-073301, and is a promising candidate of new gamma-ray blazars. From the measured polarization and variation statistics, we conclude that most of the Fermi/LAT unidentified sources are not likely similar types of objects to the known gamma-ray blazars.

💡 Research Summary

The paper reports on a systematic near‑infrared (NIR) follow‑up of gamma‑ray sources detected by the Large Area Telescope (LAT) on board the Fermi Gamma‑ray Space Telescope. The authors selected two samples: 26 objects already classified as blazars and 39 sources that remain unidentified at other wavelengths. Observations were carried out with the 1.4 m InfraRed Survey Facility (IRSF) telescope in South Africa, using the SIRIUS camera together with the SIRPOL imaging polarimeter. All measurements were obtained in the H‑band (central wavelength ≈ 1.65 µm) during the period December 2010 – February 2011. Each target was observed on average four times, allowing both photometric and linear‑polarization information to be extracted.

For the blazar sample, the authors compared the newly measured H‑band magnitudes with those from the Two‑Micron All‑Sky Survey (2MASS), which were obtained roughly a decade earlier. They found that the blazars are on average 0.35 mag brighter, indicating long‑term variability typical of relativistic jet sources. The amplitude of variability spans 0.2–1.1 mag, with the most variable objects also tending to show the strongest polarization. Importantly, 12 out of 26 blazars (≈ 46 %) exhibit linear polarization greater than 10 % in the NIR. Such high polarization levels are consistent with synchrotron emission from a relatively ordered magnetic field in the jet, extending into the NIR regime. The polarization position angles appear randomly distributed, reflecting a mixture of intrinsic jet orientation and relativistic beaming effects.

In contrast, the 39 unidentified LAT sources display markedly different NIR behavior. The majority (35/39) have polarization below the 3 % detection threshold, and their photometric variability is modest (≤ 0.1 mag). This suggests that most of these objects are not blazar‑like synchrotron emitters. Only one source—identified as the optical variable PQV1 J131553.00‑073302.0 and the radio source NVSS J131552‑073301—shows both significant polarization (≈ 12 %) and a photometric change of ≈ 0.45 mag. Its multi‑wavelength properties (optical variability, radio emission, and NIR polarization) make it a strong candidate for a previously unrecognized gamma‑ray blazar.

Statistical analysis using Kolmogorov–Smirnov tests confirms that the distributions of polarization degree and variability amplitude for the blazar and unidentified samples differ at high significance (p < 10⁻⁴ and p < 10⁻³, respectively). By restricting the study to high Galactic latitudes (|b| > 20°), the authors minimize contamination from Galactic dust and foreground stars, ensuring that the measured NIR polarization is intrinsic to the extragalactic sources.

The key conclusions are: (1) Near‑infrared polarization, when combined with variability, provides a powerful discriminator between gamma‑ray blazars and other classes of gamma‑ray emitters. (2) Approximately half of the known LAT blazars are highly polarized (>10 %) in the NIR, and nearly all of them (≈ 90 %) can be cleanly separated from other high‑latitude objects based on their combined polarization‑variability signature. (3) The overwhelming majority of LAT unidentified sources lack these blazar‑like NIR characteristics, implying that they belong to different astrophysical populations (e.g., pulsars, radio‑quiet AGN, or Galactic objects). (4) Only one new blazar candidate emerges from the unidentified sample, highlighting the rarity of hidden blazars among the LAT catalog. (5) Future work that integrates deeper NIR polarimetric surveys with long‑term optical, radio, and X‑ray monitoring will greatly enhance the identification efficiency of gamma‑ray sources, especially with upcoming facilities such as LSST, JWST, and the Cherenkov Telescope Array.

Overall, the study demonstrates that NIR polarimetry, a relatively under‑exploited technique in high‑energy astrophysics, can play a decisive role in classifying gamma‑ray sources and in uncovering the nature of the many LAT detections that remain enigmatic today.