VLA Observations of XTE J0929-314

Michael Rupen, Amy Mioduszewski, & Vivek Dhawan

Last update: 20 March 2003

Please do not use these results without first contacting M. Rupen. These results are preliminary, and provided here primarily to aid other observers.

All error bars are 1sigma unless otherwise noted.

IAU Circulars

IAUC 7888: 30apr02 (duplicates ATel #92)
R. A. Remillard, Massachusetts Institute of Technology (MIT), for the RXTE ASM Team at MIT and Goddard Space Flight Center, reports the discovery of a faint x-ray transient at R.A. = 9h29m22s, Decl. = -31o22'.8 (equinox J2000.0; estimated uncertainty 3', 90- percent confidence). The source is visible in sky maps computed from 6-day intervals of ASM data. Average fluxes (2-12 keV): Apr. 13-18, 15 +/- 2 mCrab; Apr. 19-24, 20 +/- 2; Apr. 25-30, 26 +/- 1. The error circle contains no noteworthy sources in the Simbad catalogues. The ASM hardness ratios suggest an x-ray spectrum similar to transients associated with weakly magnetic neutron stars or some blackhole systems. Optical and radio observations are encouraged.

IAUC 7889: 1may02
J. G. Greenhill, A. B. Giles, and K. M. Hill, University of Tasmania, report a possible optical counterpart for XTE J0929-314 (cf. IAUC 7888). Observations obtained around May 1.42-1.58 UT at the 1-m Mt. Canopus telescope show a blue object with V about 18.8 at R.A. = 9h29m20s.16, Decl. = -31o23'02".7 (equinox J2000.0; uncertainty +/- 0".5). The object was also detected in B, R, and I, but not on a red plate from the Digitized Sky Survey. The source faded significantly during the observations.

IAUC 7893: 7may02
R. A. Remillard, Massachusetts Institute of Technology; and J. Swank and T. Strohmayer, Goddard Space Flight Center, NASA, report the discovery of 185-Hz pulsations in XTE J0929-314 (IAUC 7889). This source was observed in a brief pointed observation with RXTE on May 2, while the average flux was 28 mCrab (2-30 keV). A power spectrum was computed for 800 s of PCA data, and a highly significant pulsation is seen at 185.09 Hz, with a strong harmonic at 370.18 Hz. This is the third known pulsar in which pulsations faster than 10 ms can be seen in the persistent x-ray emission. Rasters across the source give an improved position R.A. = 9h29m18s, Decl. = -31o23'.1 (equinox J2000.0; systematic uncertainty 1'), consistent with the optical candidate (IAUC 7889).
M. P. Rupen, V. Dhawan, and A. J. Mioduszewski, National Radio Astronomy Observatory, report the detection of a radio counterpart to the x-ray transient XTE J0929-314 (IAUC 7888). Observations with the Very Large Array (VLA) at 4.86 GHz show a source with flux density 0.31 +/- 0.07 mJy on May 3, and 0.36 +/- 0.05 mJy on May 7, at R.A. = 9h29m20s.194, Decl. = -31o23'03".41 (equinox J2000.0; uncertainty +/- 0".3). This is 0".8 from the optical position reported by Greenhill et al. (IAUC 7889) and provides strong evidence that this optical identification is correct. Further optical and x-ray observations are strongly encouraged.
P. Cacella, Brasilia, Brazil, reports that an unfiltered CCD image taken with a 0.25-m reflector shows a variable (mag 18.3) that is possibly the optical counterpart to XTE J0929-314 at position end figures 20s.22, 03".6.

IAUC 7895: 9may02
A. J. Castro-Tirado, Instituto de Astrofisica de Andalucia, Consejo Superior de Investigaciones Cientificas (IAA-CSIC), Granada; A. Caccianiga, Osservatorio Astronomico di Brera, Milano; J. Gorosabel, IAA-CSIC; P. Kilmartin, University of Canterbury; P. Tristram and P. Yock, University of Auckland; C. Sanchez-Fernandez, Laboratorio de Astrofisica Espacial y Fisica Fundamental, Instituto Nacional de Tecnica Aeroespacial, Madrid; and M. E. Alcoholado- Feltstrom, Sociedad Malaguena de Astronomia, Malaga, communicate: "We have observed the optical counterpart of the x-ray transient XTE J0929-314 (IAUC 7888, 7889, 7893) with the European Southern Observatory 3.6-m telescope at La Silla. Two 1200-s spectra (350- 800 nm) were obtained on May 6.96 and 7.96 UT. The combined spectrum shows emission lines from the C III-N III (464.0-465.0 nm) blend (EW = 0.14 nm for N III) and H-alpha (656.3 nm, EW = 0.14 nm). These lines are superposed on a blue continuum and are typical of soft-x-ray transients in outburst. Monitoring with the 0.6-m telescope (+ MOA camera) at Mt. John Observatory reveals that the counterpart has not changed in brightness by more than 0.1 mag since May 1.6. Additional spectroscopy and photometry during the outburst will be highly valuable."

IAUC 7897: 9may02
D. K. Galloway, E. H. Morgan, R. A. Remillard, and D. Chakrabarty, Massachusetts Institute of Technology, write: "Observations of the recently discovered 185-Hz accreting millisecond pulsar XTE J0929-314 (cf. IAUC 7888, 7889, 7893, 7895) with RXTE/PCA on May 9.5 UT indicate that the pulsar is still active at a flux level of about 20 mCrab (2-10 keV), down from 30 mCrab on May 2. Orbital Doppler shifts of the pulse frequency were clearly detected. Further RXTE observations are scheduled over the next few days. Multiwavelength follow-up observations are strongly encouraged."

IAUC 7900: 16may02
D. K. Galloway, E. H. Morgan, R. A. Remillard and D. Chakrabarty, Massachusetts Institute of Technology, report: "We have computed provisional orbital elements for XTE J0929-314 (cf. IAUC 7897), using pulse frequency measurements from RXTE/PCA observations between May 2.54 and 13.83 UT. Assuming a constant pulsar spin frequency and a circular orbit, our preliminary orbital solution has binary period 2614.75(15) s, projected semimajor axis 6.1(3) light-ms, and orbital epoch (time of 90 deg mean longitude) 2002 May 11.4941(2) UT at the solar-system barycenter. The derived barycentric pulsar spin frequency is 185.1052(1) Hz. The inferred mass function of 2.7 x 10**-7 solar mass is the smallest measured for any stellar binary. For a 1.4-solar-mass neutron star, the minimum companion mass is 0.008 solar mass (or 8.5 Jupiter masses). The x-ray source was active at a flux of 13 mCrab (2-10 keV) on May 15.69. RXTE observations are continuing."

IAUC 7905: 20may02
Corrigendum. On IAUC 7900, line 8, *for* UT *read* TDB

X-ray/Radio Light Curves

The plot below shows the RXTE daily averages as of 11may02 (MJD 52405); the arrows show the dates of the VLA measurements so far.

X-ray light curve (11may02)

The plot below shows the radio light curve as of 8may02; the solid points with 1-sigma error bars represent the flux densities at 4.86 GHz, while the upper limit is a 3-sigma non-detection at 8.46 GHz. The 4.86 GHz data are consistent with a point source with a constant flux density of 0.320+/-0.027 mJy.

Radio light curve ( 8may02)

First detections

Observations at 4.89 GHz on 3, 7, and 8 May 2002 gave clear detections near the optical position. Data taken at 8.46 GHz at nearly the same time on 3may02 with an rms of 0.082 mJy/beam showed no source; the peak within 0.5arcsec was 0.29 mJy/beam, so this is not a very stringent limit.

4.89 GHz image ( 3may02) 4.89 GHz image ( 7may02)

In these contour plots the circle shows the optical position (+/-0.5arcsec) from IAUC 7889; the large cross to the SE shows the optical position from VSNET/P. Cacella (see below); and the two smaller crosses show the VLA positions from Gaussian fits to these data (3 and 7 May 2002). My interpretation is that the positions agree reasonably well.


The best VLA position, based on comparing the 3, 7, and 8 May 2002 4.86 GHz images:
    09 29 20.194+/-0.003   -31 23 03.46+/-0.1
The error bars are based on a generous estimate of the scatter between those three images. This position is measured relative to the IRCF source J0921-2618, a VLBI and VLA calibrator which is 5.4 degrees away. The reported optical positions are
    09 29 20.16     -31 23 02.7     +/-0.5arcsec; Greenhill et al., IAUC 7889
    09 29 20.222   -31 23 03.58     VSNet/P. Cacella.


Please send any questions, comments, or suggestions to Michael Rupen at the e-mail address given below.

Last modified 20 March 2003