Mid-infrared Variability Properties of Gamma-ray-loud Narrow Line Seyfert 1 Galaxy TXS 1206+549

TXS 1206+549 (redshift z=1.344) is the most distant gamma-ray-loud narrow line Seyfert 1 galaxy discovered so far. Using long-term photometric data from the Wide-field Infrared Survey Explorer (WISE), we systematically investigate the mid-infrared variability properties of TXS 1206+549 from two aspects. First, the mid-infrared Intraday Variability (IDV) of TXS 1206+549 was investigated by combining three statistical methods. It is found that TXS 1206+549 reached the brightest level of all WISE observational epochs on April 29, 2022, exhibiting a very specific IDV:In the 3.4 μm and 4.6 μm bands, TXS 1206+549 continuously darkened by about 1.1 and 1.0 magnitudes, respectively, within about 1.24 days. The duty cycle (DC) of TXS 1206+549 in the 3.4 μm and 4.6 μm bands are 27.8% and 13.7%, respectively, which are significantly lower than the results reported by Rakshit et al. (2021). This ascribes to our more conservative IDV criterion (>3σ). The root-mean-square-flux (rms-flux) relationship was investigated based on the photometric data from all WISE observational epochs (21 in total), and a significant linear correlation was found. TXS 1206+549 did not exhibit significant color changes (i.e., achromatic) during the long-term timescale, but generally exhibited a trend of bluer-when-brighter (BWB) during day-long timescales. These results imply different mechanisms for the mid-infrared variability of TXS 1206+549 at different timescales:the long-term variability may be related to Doppler effect, while the IDVs may be related to shocks in the jet.