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I read with interest your recent paper on a variable BAL trough in a
BL Lac object (arXiv:1106.1587, Discovery of A Variable Broad Absorption Line
in the BL Lac object PKS 0138−097). The object is definitely worth following up
when it's in a high state to see if the trough shows up. However, I'm not quite
convinced the trough is real.
The 4200 Ang feature seen by Stickel et al. could be explained as just
narrow MgII absorption observed at low resolution. Whether or not there
is additional (broad) absorption in the S93 spectrum seems to me
critically dependent on where the continuum is drawn.
However, the main reason I'm skeptical is that the strongest trough is seen
in a SDSS observation with fiber 73 (MJD 53729, plate 1914). There is a
problem in the SDSS #1 blue CCD which may explain this feature as spurious.
Here's my footnote on that problem, from an unrelated paper in preparation:
The SDSS spectrum of the BL Lac object OM~280
(SDSS J115019.21+241753.8; 2510 53877 086)
appears to show a trough at $\sim$4150\,\AA. This trough is spurious,
likely arising from a region of non-optimal flatfielding (e.g., due to a
localized variable feature in the flatfield) in the CCD in the blue arm of
SDSS spectrograph \#1. This region is visible as a faint circular depression on
the reduced 2-D spectrograms of that CCD in that wavelength range, centered
near fiber 81. The affected wavelength region is interpolated over for
fibers 78-84, but its effects can sometimes be seen in other nearby fibers
such as \#86 (used for the spectrum of OM 280) or \#76 (producing a similar
spurious trough in SDSS J115013.88+234602.1).
If the broad feature in the fiber-73 (MJD 53729) spectrum of your object turns
out to be spurious, then the reality of the feature rests on the MJD 52174
spectrum and the Stickel et al. spectrum. The features in those spectra might
not be real broad troughs, which is why I'm not convinced. But I agree they
could be real, which is why I think followup observations are merited for
this object, and I hope you pursue them.
PS Note that a more accurate name for the object is PKS B0138-097;
the IAU asks that the B or J always be included to indicate the epoch,
and PKS 0138-097 recovers the object in SIMBAD but not in NED.
Also, I find it helpful to include not just the MJD of the SDSS spectra
in a publication, but the plate and fiber numbers as well.
PPS The raw SDSS 2-D spectrograms can be found at
sorted by MJD, with names corresponding to those found in the
EXPID01, EXPID02 etc. header keywords in the spSpec files.