Alert Notice 397: Request for optical photometry of the bright X-ray binary V884 Sco (4U 1700-377)

June 30, 2009

Dr. Jerome Orosz (SDSU) has requested the assistance of southern bright star observers in observing the high-mass X-ray binary star V884 Sco (== 4U 1700-377). Orosz and collaborators are attempting to measure the mass of the compact object to clarify whether the object is a black hole or a neutron star. Optical photometry of the ellipsoidal variations of V884 Sco will be used in conjunction with optical spectroscopy to be performed at Las Campanas in mid-July and early August 2009 to measure the dynamical properties of the system. Orosz requests time-series B and V observations of this bright (V = 6.54, B-V=0.264) system beginning now (2009 June 30) and continuing through early August 2009.

V884 Sco is a binary system composed of a massive, main sequence O-type star in orbit with either a black hole or a neutron star. Dr. Orosz provides the following justification for the project:

"...[A] note about the importance of 4U 1700-377.  If it has a neutron star, it has by far the most massive neutron star known. Furthermore, the high mass would rule out all sorts of equations of state for neutron star matter. On the other hand, if the compact object is a black hole, it would be by far the least massive black hole known, and its mass would raise all sorts of interesting questions about the formation of black holes, and what kinds of stars produce black holes."

V884 Sco is a low-amplitude (amplitude < 0.04 mag) variable star which shows ellipsoidal variations rather than binary eclipses. The period of the star is approximately 3.45 days. Photometry is requested to obtain a current ephemeris; examination of data from the 1970s makes clear the period of the system has changed. ASAS-3 data do exist for this star and are being used, but the star is near the bright limit for the ASAS system. Observers capable of performing precision bright star photometry are encouraged to participate, particularly those in the southern hemisphere.

V884 Sco is located at the following (J2000) coordinates:

RA: 17 03 56.80 , Dec: -37 50 39.0

For this project, we recommend the use of HD 153767 (SAO 208340; RA: 17 03 00.34 , Dec: -37 43 10.8 , V=7.43, B-V=0.02) as the comparison star. This will require either a CCD camera with a field of view larger than 15 arcminutes, or a photoelectric photometer. If you are using a CCD camera, use short exposures to avoid saturating the chip, or stop down your telescope. If you choose to stop down your system, new flat fields should be created. For photoelectric observers, we recommend using HD 154310 (SAO 208406; RA: 17 06 20.19 , Dec: -37 13 39.3 , V=5.988, B-V=0.076) as the check star. This is a known double, but the B component is four magnitudes fainter than A, and should not interfere with the photometry. These comparison and check stars have been added to the PEPObs online processing program. Johnson B and V filters are the preferred filters if they are available, but data taken in any filter will be useful and all observers are encouraged to participate.

Charts for the field may be plotted with AAVSO's Variable Star Plotter:

Please submit data to the AAVSO with the name "V884 SCO".

This AAVSO Alert Notice was prepared by M. Templeton.



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