[vsnet-campaign-v4641sgr 86] (fwd) V4641 Sgr ephemeris

[Ryoko Ishioka <ishioka@kusastro.kyoto-u.ac.jp>]

Date: Wed, 5 Jun 2002 12:53:48 +0200 (MET DST)
From: "Jerome A. Orosz" <J.A.Orosz@astro.uu.nl>
Subject: [vsnet-campaign-v4641sgr 0] V4641 Sgr ephemeris

Dear Colleagues,

I have been working on combining extensive CCD photometry (collected by
several collaborators) and radial velocities of V4641 Sgr in quiescence to
get a good ephemeris.  Here are the results, using light curves in the
UBVRIJK filters and the radial velocities. There are 1831 photometry
points distributed like this:

    296  points in U, time interval = 372.2 days,
    333  points in B, time interval = 372.2 days,
    406  points in V, time interval = 421.1 days,
    323  points in R, time interval = 372.2 days,
    382  points in I, time interval = 418.1 days,
     45  points in J, time interval = 113.6 days,
     46  points in K, time interval = 113.7 days,

and 68 radial velocity points, with a time interval of 642.2 days. The
total time baseline of the combined data set is HJD 2,451,440.6136 to
2,452,194.5808 or 753.97 days.

I am using my light curve synthesis code and its genetic fitting routine
to get the period and phase zeropoint (see Orosz et al. 2002, ApJ, 568,
845 and cited references for a discussion of the light curve synthesis and
genetic fitting code).

Here is the ephemeris:

T0     = HJD 2,452,002.385 +/- 0.0050 (time of B-star inferior conjunction)
period = 2.81737 +/- 0.00005 days

The distributions of the residuals of the fits are not quite Gaussian (the
tails do not fall off as rapidly), so the usual thresholds in the chi^2
statistic that one uses to get confidence limits may not strictly apply.
Therefore I have been a bit conservative in assigning errors to the period
and phase zeropoint.

Using the above ephemeris, the phase of the spectroscopic T_0 (epoch of
maximum B-star radial velocity) given in Orosz al. (2001, ApJ, 555, 498)
is -553.659 +/- 0.010, significantly different than the expected value of
-553.75.  Thus the T_0 given in Orosz et al. (ibid) is about 6 hours *too
late*.  It appears there are a few deviant radial velocities take near the
B-star superior conjunction that affected the determination of the
spectroscopic phase zero-point.  The T_0 given above is more reliable
since it makes use the photometry.

The above period is consistent with the period of 2.81730 +/- 0.00001 days
derived by Orosz et al. (ibid) from Goranskij's photographic measurements.


To summarize:

T0     =  HJD 2,452,002.385 +/- 0.0050
period =  2.81737 +/- 0.00005 days

At phase 0.00:      X-ray eclipses (if any) and partial eclipses of the
                    accretion disk will occur.  You will see the shallower
                    minimum of the quiescent ellipsoidal light curve.

At phase 0.50:      The B-star companion is behind the black hole and
                    disk.  You will see the deeper minimum in the
                    quiescent ellipsoidal light curve.

At phase 0.25:      You see the maximum B-star radial velocity, and the
                    maximum of the quiescent ellipsoidal modulations

At phase 0.75:      You see the minimum B-star radial velocity, and the
                    maximum of the quiescent ellipsoidal modulations.

Regards,

Jerry Orosz

____________________________________________________________________________
			   Jerome A. Orosz
	     Sterrenkundig Instituut, Universiteit Utrecht
	   Postbus 80.000, 3508 TA Utrecht, The Netherlands
phone: +31-30-253-5204;  dept. phone: +31-30-253-5200;  fax: +31-30-253-5201
  E-mail: J.A.Orosz@astro.uu.nl  WWW: http://vsnet.astro.uu.nl/~orosz

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