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XMM-Newton & SWIFT observations of the Supergiant Fast X-ray transient SAXJ1818.6-1703

ATel #1493; Bozzo E. (Inaf-OA Roma), Campana S. (INAF-OA Brera), Stella L. (Inaf-OA Roma), Falanga M. (CEA Saclay), Israel G. (INAF-OAR), Rampy R. (University of California, Santa Cruz), Smith D. (University of California, Santa Cruz), Negueruela I. (University of Alicante)
on 29 Apr 2008; 08:40 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

Subjects: X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 2044

SAXJ1818.6-1703 was discovered with BeppoSAX in 1998 (in't Zand et al. 1998, IAUC # 6840 ) and observed in outburst more than once (flux of few 10**-9 erg s**-1 cm**-2, Grebenev & Sunyaev 2005, AstL 31, 672; Sguera et al. 2005, A&A 444, 221). A Chandra observation in 2006 caught this source at a flux level of of 7.5x10**-12 erg s-1 cm-2 (in't Zand et al. 2006; ATel #915), thus suggesting a ratio of maximum to minimum X-ray flux of ~10**3. This is well below the values measured in some other SFXTs (up to ~10**4-10**5), and the association of SAXJ1818.6-1703 with sources of this class was called into question.

Here we report on XMM-Newton and SWIFT observations of SAXJ1818.6-1703 carried out on 8 October 2006 and 18 April 2008, respectively.

The total good exposure time in the XMM observation was of ~13 ks. The source was not detected, and, by adopting the spectral model used in the previous Chandra observation, we derived a 3 sigma upper limit on the X-ray flux (unabsorbed, 0.5-10 keV) of 1.1x10**-13 erg s**-1 cm**-2 (we used only Epic-PN data).

A 2 ks ToO SWIFT observation of SAX J1818.6-1703 was obtained following the discovery of a renewed activity of this source (Grebenev & Sunyaev 2008; Atel #1482). About 20 source photons were detected by SWIFT/XRT, but such a low statistics did not allow for a detailed characterization of the spectrum. By assuming the same spectral model as above, we derived a 2.0-10 keV unabsorbed flux of 4.2*10**-12 erg cm**-2 s**-1 and an upper limit on the absorption column N_H<9.4*10**23 (90% confidence level).

The flux obtained with SWIFT is compatible with that measured previously by Chandra (in't Zand et al. 2006), whereas the XMM-Newton upper limit is a factor of ~80 below.

We conclude that SAX J1818.6-1703 displayed an X-ray flux dynamic range that is similar to that observed in some other SFXTs.

Further observations of SAXJ1818.6-1703 are encouraged.