Central Bureau for Astronomical Telegrams
IAUC 4393: OPTICAL COUNTERPART OF 4U 1915-05
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Circular No. 4393
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
TWX 710-320-6842 ASTROGRAM CAM Telephone 617-495-7244/7440/7444
OPTICAL COUNTERPART OF 4U 1915-05
J. E. Grindlay, Center for Astrophysics, and H. Cohn, Indiana
University, write: "We have discovered the optical counterpart for
the x-ray burst source 4U 1915-05, which has an about 50-min orbital
period as revealed through x-ray dips (Walter et al. 1982, Ap.J. 253,
L67; White and Swank 1982, Ap.J. 253, L61); the optical counterpart
is the candidate noted by Grindlay (1986, in NATO ASI Series, 167,
25), initially identified as an ultraviolet-excess object from UBV
photometry obtained with the Cerro Tololo (CTIO) 4-m telescope on
1985 May 13. Analysis yields the following photometry: V = 20.99,
B-V = +0.41, U-B = -0.52 (typical errors +/- 0.05). The object is
2".5 northwest of star 3 and 7".9 southwest of star 2 on the finding
chart of Doxsey et al. (1977, Nature 269, 112); it is clearly visible
on the finding chart of Walter et al. and is about 3" south of the
edge of the Einstein HRI error circle shown (offset due to severe
distortion in the x-ray image). Extensive photometry of the field
with the CTIO 1.5-m telescope and a lower-noise CCD (+ Corning 9780
filter, which extends the Johnson B filter response by about 50 nm to
both the blue and red) on 1987 Apr. 27-29 revealed that the object
is variable; observations were obtained with 7-min integrations so
that typically 6 measurements per 50-min cycle were obtained. Dips
were recorded as follows: Apr. 28.385 UT, Delta-m = +0.4; 29.365, +0.5;
29.399, +0.4. These are consistent with a period of 50.34 +/- 0.1
min; this appears to be offset from the about 50.0-min x-ray period
determinations of Walter et al. and White and Swank from 1978-1980 x-
ray data. Follow-up observations by P. Schmidtke, Arizona State
University, on the CTIO 1.5-m (different CCD detector, same filter)
showed similar dips on May 3.381, 4.374, and 4.405. All six dips
are consistent with a 50.4-min period and rms phase jitter of 0.14
cycle in the times of optical dips; dips were observed at each
cycle for which there was coverage. The light curve appears
relatively flat-topped. The dips may be due to partial (and variable)
eclipses of a raised (> 0.1 Ro) bulge on the accretion disk rim
which occupies about 20 percent of the (about 3 Ro) disk circumference and
produces the x-ray absorption dips by partially (and variably) obscuring
the central (about 1.4 Mo) neutron star. The eclipsing companion
star must then have radius < 0.1 Ro (and thus mass < 0.1 Mo)
and may possibly have a He (degenerate) core and H-rich envelope as
suggested by Swank, Taam, and White (1984, Ap.J. 277, 274).
Additional coordinated optical/x-ray observations are needed."
1987 May 15 (4393) Daniel W. E. Green
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