Locating a new emission source in Io’s Bosphorus Regio

During late spring 2022, using JWST aperture masking interferometry (ERS program #1373) and ground-based adaptive optics at the Keck telescope, we detected a new emission feature in Io’s Bosphorus Regio.  To pinpoint the location more accurately we followed up with the Large Binocular Telescope (LBT).  An accurate location will help determine if this feature is part of the Emakong Patera, is part of the Seth Patera, or is an independent volcano emitting lava from its own magma source.  Here we report on the LBT observation and data analysis.

On UT November 8th, 2022, we observed Io with the Large Binocular Telescope Interferometer (LBTI).  We acquired over 30,000 14ms frames over a period of 4 hours and parallactic angle coverage of approximately 70 degrees.  Data were acquired at both M-band (4.8 microns) and a wide band-pass spanning 2.2 to 5.0 microns.  As in past LBTI observations of Io (Conrad et al., 2015), we employed lucky fringing and frame selection to assemble a data set in which all frames are co-phased.  From these data (taken with a 23-meter baseline), we expect to determine the location of the feature to a degree of accuracy approximately three times greater than is possible with adaptive optics on 8-10 meter ground-based telescopes.

Image reconstruction is the preferred method for combining interferometer data for most science programs.  However, for science programs that a) require only accurate astrometry of point sources (all volcanoes in our data are unresolved at the observed wavelengths) and b) utilize data taken with a Fizeau interferometer like LBTI, we have developed a simpler method.  This method has two advantages.  First, the method preserves the spatial information available in the raw data.  Image reconstruction can sometimes shift the location of a measured source.  Second, with our method data taken at different wavelengths can still be combined to yield a single measurement.  Image reconstruction methods can only combine images which were all taken with the same filter.

The method is quite simple.  Because a Fizeau interferometer like LBTI provides complete images (i.e., the image is not reconstructed from visibilities and closure phases), we can take a one-dimensional cut through each fringe pattern as it appears in the raw data.  From each cut we compute a one-dimensional centroid to get a sub-pixel location along that baseline.  These results, taken at different baseline angles (the LBTI baseline rotates with parallactic angle) are statistically combined to produce a single location measurement.  This location is then mapped from detector space to a latitude and longitude on the sphere of Io.  The uncertainty in the measurement is reflected as two orthogonal error bars, one for latitude and one for longitude, computed by statistically combining the individual uncertainties of each cut.

This same method can be used to locate other volcanoes visible in our data set, which will be the subject of a future work.