Letter to the Editor

Discovery and monitoring of the likely IR counterpart of SGR 1806–20 during the 2004 γ-ray burst-active state

¹ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy e-mail: gianluca@mporzio.astro.it
² INAF - Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (Lc), Italy
³ European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching, Germany
⁴ European Southern Observatory, Casilla 19001, Santiago, Chile
⁵ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica “G.Occhialini”, via Bassini 15, 20133 Milano, Italy
⁶ SRON - National Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
⁷ Department of Astrophysical and Planetary Sciences and JILA, University of Colorado, 440 UCB, Boulder, CO 80309, USA

Received: 7 April 2005
Accepted: 4 June 2005

Abstract

The sky region including the Chandra position of SGR 1806–20 was monitored in the IR band during 2004, following its increased high energy bursting activity. Observations were performed using NAOS-CONICA, the adaptive optics IR camera mounted on Yepun VLT, which provided images of unprecedented quality (FWHM better than 01). After the 2004 December 27th giant flare, the source position has been nailed by VLA observations of its radio counterpart, reducing the positional uncertainty to 004. Using IR data from our monitoring campaign, we discovered the likely IR counterpart to SGR 1806–20 based on positional coincidence with the Chandra and VLA uncertainty regions and flux variability of a factor of about 2 correlated with that at higher energies. We compare our findings with other isolated neutron star classes thought to be related, at some level, with SGRs.