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IAUC 5613: GRO J0422+32

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                                                  Circular No. 5613
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM     EASYLINK 62794505

GRO J0422+32
     C. Telesco, Marshall Space Flight Center (MSFC), NASA; R. Pina,
University of California at San Diego; S. Fajardo, State University
of New York; G. Fishman and C. Kouveliotou, MSFC; and J. van
Paradijs, University of Amsterdam, report:  "We have detected the
gamma-ray transient GRO J0422+32 (cf. IAUC 5580) at 10.8 microns
using the MSFC 20-pixel bolometer array at the 3-m Infrared Telescope
Facility on Mauna Kea.  The 10.8-micron flux density was 51
+/- 9 mJy (detected S/N = 5.7; the uncertainty does not include +/-
7 percent in absolute calibration).  Only the pixel centered on the
optical counterpart (IAUC 5588) showed statistically significant
S/N.  Total on-source integration time was 1.17 hr, distributed
uniformly throughout the interval Sept. 9.52-9.58 UT."
     P. Garnavich, Dominion Astrophysical Observatory; and A.
Noriega-Crespo, University of Washington, communicate:  "We
obtained CCD photometry of GRO J0422+32 on five nights between Aug.
17 and Sept. 7, using the 0.8-m telescope at the Manastash Ridge
Observatory of the University of Washington.  On four of the
nights, images were taken using a B filter and 30- to 40-s
integrations. The power spectra of the light curves are flat from 1 to
about 15 mHz.  Irregular variations of 5-10 percent in brightness
are seen at frequencies lower than about 1 mHz.  On Sept. 2, a series
of unfiltered CCD images were obtained with 3.0-s integrations
and 8.4 s between mid-exposures.  The light curve spans 1.5 hr.
The power spectrum shows no evidence of a peak near the frequency
of the 35-mHz QPO seen at x-ray wavelengths (IAUC 5608).   We find,
however, that the variance in the brightness estimated from individual
exposures is 5 times larger than that expected from the combination
of CCD read-noise and photon shot-noise.  Photometry of other
stars on the CCD images confirms the surprisingly large variance.
We conclude that the power spectrum of the GRO J0422+32 light curve
is 'white' from 1 to 50 mHz and that the amplitude measured as a
standard deviation about a mean brightness is about 0.05 magnitude
in unfiltered light.  A closer examination of our low-time-resolution,
B-filter data also shows excess noise, but with a standard
deviation of < 0.03 magnitude."
     Further visual magnitude estimates by P. Schmeer, Bischmisheim,
Germany (cf. IAUC 5597):  Aug. 29.08 UT, 12.9; Sept. 7.13, 12.7.

1992 September 17              (5613)            Daniel W. E. Green

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