The Infrared Counterpart of the X-Ray Nova XTE J1720—318

We report on the discovery of an infrared counterpart to the X-ray transient XTE J1720−318 on 2003 January 18, nine days after an X-ray outburst, and the infrared light curve during the first 130 days after the outburst. The infrared light curve shows ad ecline of∼ 1.2mag from the peak magnitude of Ks ∼ 15.3 over the observation period, and a secondary maximum, about 40 days after the outburst. Another small increase in the flux was also recorded about 20 days after the outburst. These increases were also detected in the X-ray light curve. The JHK s colors are consistent with an X-ray irradiated accretion disk suffering an extinction of AV ∼ 8, which is also inferred from its X-ray spectrum and the extinction map constructed from far-infrared dust emission of this line of sight. These J, H ,a ndKs observations demonstrate that useful data can be obtained even for such an object, which suffers heavy optical extinction, possibly located beyond the galactic center.