Correlated X-ray and IR decaying flux from the Anomalous X-ray Pulsar XTE J1810-197
ATel #284; N. Rea(Univ. Rome2/INAF-OAR), G. L.Israel(INAF-OAR), V. Testa(INAF-OAR), L. Stella(INAF-OAR), S. Mereghetti(IASF-MI), A. Tiengo(IASF-MI), T. Oosterbroek(ESA-ESTEC), V. Mangano(IASF-PA), S. Campana(INAF-OAB), S. Covino(INAF-OAB), G. Lo Curto(ESO), R. Perna(Princeton)
on 21 May 2004; 11:30 UT
Credential Certification: Nanda Rea (rea@mporzio.astro.it)
Subjects: Infra-Red, X-ray, Neutron Star, Transient, Pulsar
The field of XTE J1810-197 was imaged by VLT-UT4 Yepun with the Nasmith Adaptive Optics System and the High Resolution Near IR Camera (NAOS-CONICA) on 2004 March 12, 13, and 14, as part of a monitoring program for this source. Observations were obtained with sub-arcsecond seeing and good sky conditions, resulting in a point-like source PSF between 0.09" and 0.12". J, H and Ks band images were obtained for a total exposure time of 48, 36 and 52 minutes, respectively. We found the following IR magnitudes for the proposed IR counterpart (Israel et al. 2004, 603, L97): Ks = 21.4 +\- 0.1, H= 22.7 +\- 0.2 and J > 23.0 (3 sigma upper limit). Within a radius of about 7" from the Chandra uncertainty region only the proposed IR counterpart to XTE J1810-197 showed a significant variation with respect to our previous VLT-NACO observation carried out on October 2003 (Israel et al. 2004, 603, L97), with DeltaH = 0.7 +\- 0.2 and DeltaKs = 0.5 +\- 0.1. This decrease by a factor of about two in the IR flux unambiguously confirms the link between the X-ray source and the IR object.
We also analysed an unsolicited 17ks Target of Opportunity observation of XTE J1810-197 carried out by XMM-Newton one day before the VLT run (2004 March 11). The source spin period was 5.53974 +/- 0.00005s (90% uncertainty). The pulsed fraction, pulse profile and spectral parameters did not change significantly compared to the previous XMM-Newton observation performed about six months before (Tiengo & Mereghetti 2003, ATel #193; Gotthelf et al. 2004, ApJ, 605, 368), except for the normalisation. The 0.5-10 keV flux on March 11th was 2x10^-11 erg/s/cm^-2 (absorbed flux; Nh=0.97 +/- 0.05 x10^22 atoms*cm-2), a factor of two lower with respect to the previous XMM-Newton observation.
These results together with a comparison with the emission models for anomalous X-ray pulsars are reported in full by Rea et al. (2004).
We thank the VLT team for their help in carrying out the IR NACO-images and the XMM-Newton team for prompt XTE J1810-197 data release.
