Near Infrared Intraday Variability of Mrk 421

We report results from our monitoring of the BL Lac object Mrk 421 in the near-IR $J$ band. The observations, aimed at studying the intraday variability (IDV) of the object, were carried out systematically over an extended (and near-continuous) period of eight nights from the 1.2m Mount Abu Infrared Telescope, India. There are limited studies for Mrk 421 in the $J$ band for such an extended period. The observation epoch for this study (25 February - 5 March 2003) was chosen to significantly overlap other concurrent studies of Mrk 421 in the X-ray/$\gamma$-ray regions being conducted using the Rossi X-ray timing explorer (RXTE) and the solar tower atmospheric Cherenkov effect experiment (STACEE). Hence these results could be useful for a multi-wavelength analysis of the variability behavior of Mrk 421. We find that Mrk 421 was quite active during the observed period and showed significant IDV and short term variability. A maximum variation of 0.89 magnitudes is seen over the entirety of the observed period. Flaring activity, with typical brightness variations of $\sim 0.4$, are also seen on several occasions. The extent of the variability observed by us is compared with the results of other similar studies of Mrk 421 in the $J$ band.