Swift and VLT monitoring of AXP 1E1048.1-5937

ATel #1077; G. L. Israel (OA Roma), S. Campana (OA Brera), V. Testa (OA Roma), S. Mereghetti (IASF Milan), S. Zane (MSSL), N. Rea (SRON), G. Lo Curto (ESO), L. Stella (OA Roma) on behalf of a larger Team
on 18 May 2007; 21:05 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

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X-ray and near-IR monitoring observations have been carried out during the latest weeks by means of the Swift XRT (0.3-10keV) and VLT-UT4 (equipped with the NAOS-CONICA camera). The observations cover the time interval 4th April - 17th May (for Swift) and 6th April - 10th May (for the VLT). The 1-10keV source flux remained at a relatively high level of 3.5x10^-11 ergs/cm^2/s (absorbed) until approximately the end of April 2007. The May 2007 pointings (8th and 17th) showed that the source was at a flux level of about 20% fainter. A similar behavior was also observed in the IR flux of the source between the first (Ks~19.6; 6th April 2007; 2400s exposure time) and latest (Ks~20.2; 10th May 2007; 2300s exposure time) pointings. The corresponding nearly-simultaneous Fx/FIR is of the order of 10^3 similar to those obtained for other AXPs.

The first Swift observation (4th April 2007; Campana & Israel, ATEL #1043) showed that the X-ray pulse shape and pulsed fraction have largely changed after the glitch reported by Dib et al. (2007, ATEL #1041), and recently confirmed by Chandra data (Gavriil et al. 2007, ATEL #1076). Moreover, the subsequent Swift monitoring observations found that the pulsed fraction (defined as semi-amplitude of the modulation divided by the mean source count rate) increased from about 22%+/-2% up to almost 38%+/-3% (the quiescent level being around 90%), while the pulse shape varied from being multi-peaked to almost sinusoidal. We also confirmed the anti-correlation between the count rate and the pulsed fraction first reported by Mereghetti et al. (2005)

We thank the Swift and Paranal teams for their support and effort in optimizing observations.