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Chandra observation of SAX J1818.6-1703: confirmation of optical counterpart & suggestion of non-transient nature

ATel #915; Jean in 't Zand (SRON & Utrecht University), Peter Jonker (SRON, Utrecht University & CfA), Mariano Mendez (SRON & University of Amsterdam) & Craig Markwardt (University of Maryland & NASA-GSFC)
on 13 Oct 2006; 08:29 UT
Credential Certification: Jean in' t Zand (jeanz@sron.nl)

Subjects: X-ray, Binary, Transient

Referred to by ATel #: 1482, 1493, 2044, 5980

SAX J1818.6-1703, discovered in 1998 with the BeppoSAX Wide Field Cameras (in 't Zand et al. 1998, IAUC # 6840 ), was found with INTEGRAL and RXTE to exhibit few-hours long outbursts similar to the Supergiant Fast X-ray Transients (Grebenev & Sunyaev 2005, AstL 31, 672; Sguera et al. 2005, A&A 444, 221). Negueruela & Smith (2006; ATel #831), on the basis of this analogy, identified within the 1.5 arcmin radius error region determined by Grebenev & Sunyaev a likely optical/NIR counterpart whose I-band spectrum is typical for an OB supergiant.

On Sep. 19, 2006, we observed the error region of SAX J1818.6-1703 for 9.8 ksec with Chandra, using the ACIS-S CCD array in the focal plane. We find one bright and mildly variable point source which we identify as SAX J1818.6-1703. Its position is R.A. = 18h18m37s.89, Decl. = -17d02'47".9 (equinox 2000.0) with a standard 90%-confidence error of 0.6 arcsec. This position is 0.21" from the 2MASS position and 0.47" from the USNO-B1.0 position, thus confirming the counterpart proposed by Negueruela & Smith (2006).

The source exhibited a monotonic 50% increase in flux and a strongly absorbed power law spectrum with N_H = (6.0 +/- 0.7) x 10**+22 cm**-2, a photon index of 1.9 +/- 0.3 and an average 0.5-10 keV flux of (7.5 +/- 0.1) x 10**-12 erg s-1 cm-2 (errors are 1-sigma). The typical outburst peak flux of SAX J1818.6-1703 is a few times 10**-9 erg s**-1 cm**-2 (Sguera et al. 2005) suggesting a dynamic range of 10**3. This is somewhat different from another SFXT observed with Chandra, IGR J17544-2619, that showed a dynamic range of 6 x 10**4 (in 't Zand 2005; A&A 441, L1), although it should be mentioned that both were merely snapshot observation. Nevertheless, a difference between both sources is also suggested by the semi-weekly fluxes measured through the RXTE PCA bulge scan program (e.g., Markwardt 2006, AIP Conf. Proc. 840, 45) that show a significantly higher duty cycle for SAX J1818.6-1703 than for IGR J17544-2619. It may be that SAX J1818.6-1703 is not a true transient but a persistent HMXB with luminous flares such as the HMXB supergiant neutron star systems 4U 1907+097 (e.g., Marshall & Ricketts 1980, MNRAS 193, 7P) or Vela X-1 (e.g., Krivonos et al. 2003, ATel #211). The persistent/transient difference is probably related to the apastron distance and supergiant wind mass flux. Repeated sensitive-enough measurements are needed to confirm these results.