SAX J1810.8-2609

The wide-field camera onboard BeppoSAX has discovered a new X-ray transient (IAU Circ. 6838) at R.A. = 18h10m46s, Decl.= -26o09'.1 (equinox 2000.0; preliminary error radius 3'). The source has been detected since Mar. 10.18 UT at a steady level of 15 mCrab (2-9 keV), and an x-ray burst of duration 45 s was detected from this source on Mar. 11.06634 with a peak intensity of about 1.6 Crab.

Previous IAU Circulars

• IAUC 6838 Discovery with SAX WFC
• IAUC 6843 Results of SAX NFI pointing
• IAUC 6985 ROSAT HRI observation

• A ROSAT TOO has revealed a bright X-ray source within the WFC error box. The image below shows the full field of view of the ROSAT HRI with the 0.5 cts/s source RX J1810.7-2609 (white bright spot near the center of the FOV) which we identify with SAX J1810.8-2609 (Details are reported in IAUC 6985). Overlayed are the SAX WFC error circle (yellow; IAUC 6838) and the revised SAX NFI error circle (green; prov. comm. by P. Ubertini). Note that the error circle of the NFI position is shifted and larger with respect to what has been reported in IAUC 6843 (see below).
  

  The images below show the ROSAT HRI error circle of this source (10 arcsec radius) overlayed on the digitized Sky-Survey (DSS1; left image) and on a Z band image taken on March 13, 1998 with the Danish 1.5 m telescope on La Silla (right image; note the different scale!).

  DSS1 image = UK Schmidt plate V12143 (blue band) taken on Sep. 15, 1987.

  Z band image taken on March 13, 1998 at the ESO Danish 1.5m telescope.

• Detailed checks of the BSAX attitude during the NFI pointing has revealed that the aspect solution for that NFI TOO was particularly unfavourable due to the lack of bright stars in the pointing direction (rather rare aspect control configuration). Conversely, the stability of the whole observation was very good. The revised coordinate is shown above as the small green circle (error radius of 1.5 arcmin).
  This solution reconciles consistency of the 3 error boxes (WFC, NFI and HRI) confirming that the same object was observed with SAX and ROSAT.
  (Information courtesy of P. Ubertini, A. Antonelli and L. Piro)
• Fig. 4
  

  Mar. 13, 1998; Gunn z band	Aug. 28, 1998; Gunn z band
  Mar. 13, 1998 - R band