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 (email@example.com)
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.