Mid-infrared Variability of gamma-ray emitting blazars

Abstract

Using data from the Wide-field Infrared Survey Explorer we studied the mid infrared 3.4 $\mu$m (W1-band) and 4.6 $\mu$m (W2-band) flux variability of $\gamma$-ray emitting blazars. Our sample consists of 460 flat spectrum radio quasars (FSRQs) and 575 BL Lacertae (BL Lac) objects. On intra-day timescales, the median amplitude of variability ($\sigma_{m}$) for FSRQs is 0.04$^{+0.03}_{-0.02}$ mag and 0.05$^{+0.03}_{-0.02}$ mag in W1 and W2 bands. For BL Lacs we found median $\sigma_{m}$ in W1(W2) bands of 0.04$^{+0.01}_{-0.02}$ (0.04$^{+0.02}_{-0.02}$) mag. On long timescales, for FSRQs we found a median $\sigma_{m}$ of 0.44$^{+0.28}_{-0.27}$ mag and 0.45$^{+0.27}_{-0.27}$ mag in W1 and W2 bands, while for BL Lacs the median values are 0.21$^{+0.18}_{-0.12}$ mag and 0.22$^{+0.18}_{-0.11}$ mag in W1 and W2 bands. From statistical tests, we found FSRQs to show larger $\sigma_m$ than BL Lacs on both intra-day and long timescales. Among blazars, low synchrotron peaked (LSP) sources showed larger $\sigma_m$ compared to intermediate synchrotron peaked (ISP) and high synchrotron peaked (HSP) sources.The larger $\sigma_{m}$ seen in FSRQs relative to BL Lacs on both intra-day and long timescales could be due to them having the most powerful relativistic jets and/or their mid infrared band coinciding with the peak of the electron energy distribution. BL Lacs have low power jets and the observational window too traces the emission from low energy electrons, thereby leading to low $\sigma_{m}$. In both FSRQs and BL Lacs predominantly a bluer when brighter behaviour was observed. No correlation is found between $\sigma_m$ and black hole mass.