A Glance in the I-band to XMM J174457-2850.3 and GRS 1741.9-2853.
ATel #522; S. Laycock (CfA), P. Zhao (CfA), M. A. P. Torres (CfA), R. Wijnands (Amsterdam), D. Steeghs (CfA), J. Grindlay (CfA), J. Hong (CfA), P. G. Jonker (SRON/CfA).
on 17 Jun 2005; 18:23 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Manuel Torres (email@example.com)
Subjects: Optical, X-ray, Binary, Black Hole, Neutron Star, Transient
Following the recent report of renewed activity in the Galactic Bulge X-ray transients
GRS 1741.9-2853 and XMM J174457-2850.3 (ATEL #512), we used the IMACS imaging spectrograph
mounted at the Magellan-Baade telescope on 2005 June 8th at 01:49 UT to acquire two
5-min I-band images of the field covering both targets. Seeing was 0.5" sampled at 0.2"
per pixel with the 8Kx8K CCD mosaic. The field was previously observed in 2003, June 3
with the CTIO 4m + Mosaic imager (I-band 15min, R-band 20min).
The Magellan and CTIO images were calibrated using observations of both the field and
Stetson standard-field PG1657 made with the CTIO 1.3 m telescope + ANDICAM imager.
XMM J174457-2850.3: The IMACS images showed a single optical source within the 4" XMM
error circle reported in Sakano et al. (2005,MNRAS,357,1211) and This star was also
detected in the CTIO image, located at R.A.(J2000)=17:44:57.29, DEC(J2000)=-28:50:18.4
in the USNO A2 reference frame (0.1" uncertainty). Absolute photometry gives I=21.96
(2003), I=22.04 (2005), with uncertainty 0.1 mag. Differential photometry with respect
to 5 nearby stars shows a change < 0.02 mag in its I-band brightness between June 2003
and June 2005. The R-I color is +1.95 (+/-0.2) mag, which translates to Av=7.3. Given
that the total Av to the Galactic Bulge is ~25mag, this object has approx 1/3 of the total Av
and from the Av vs, distance relation for this l,b (Drimmel et al 2003, A&A,409,205)
is therefore located between 2-4 kpc. The column density toward XMM J174457-2850.3
is around 5.9E22 cm^-2, using the Predehl & Schmitt 1995 relation between
Nh and Av this gives an Av for the source of ~33. This is ~4.5 times
more than the Av measured for the optical star in the XMM error circle.
In order to rigorously rule out candidacy as
optical counterpart of the X-ray source, the Chandra HRC observations (ATEL #512) were
re-investigated to derive an improved position for XMM J174457-2850.3 of
R.A(J2000)=17:44:57.44, DEC(J2000)=-28:50:20.3 (error radius 2"). This new X-ray
position is 2.74' away from the optical source and combined with the lack of variability
and low extinction leads us to conclude that this cannot be the counterpart to the XMM
source. Therefore any optical counterpart must be fainter than I=25.6 (the 3-sigma
limiting magnitude for the combined IMACS images), as can be expected for a system
located in the Galactic Bulge with Av=25.
GRS 1741.9-2853: The I-band images show a single optical source (I=21.23 (2003), I=21.21
(2005), error 0.1 mag) within 4" and 4 additional optical sources between 4" and 9".
None are consistent with the co-ordinates published for this source by Muno et al.
(2003,ApJ,589,225), R.A.(J2000)=17;45:02.33, DEC(J2000)=-28:54:49.7 (error radius 0.7").
Therefore, also in this case the optical counterpart must be fainter than I=25.6.
We conclude that no I-band counterparts could be detected for either of these sources.
To counter the expected reddening in Bulge fields, we encourage future efforts in the