[vsnet-alert 9901] Re: bright variable quasar QSO B0133+47

da55 at Safe-mail.net da55 at Safe-mail.net
Sat Feb 9 21:08:46 JST 2008

Seiichi noted :-

"that the CMC14 and UCAC2 catalogs suggest this quasar could be about 14-15 mag at their observations, in 2000 or later"

Although not an Alert matter, but relating to variable sources possibly noted in these surveys, it should be noted that the epoch of both CMC14 and UCAC2 mags can be found, except that they can't.  But you can get a rough idea of when they happened.

For CMC14 use VizieR and you will see that the measures for this object's photometry are 2, as are those of astrometry, and as is the total number of observatiosn, with a mean epoch of 

"MJD-51263	1298 	d 	Mean epoch of astrometry, as days since 26-3-1999 (=MJD-51263, or JD-2451263.5)"

so 1298 days after 26th March 1999, 13th Oct 2002 ish


however, there's no indication to the range this mean exists within.  So you can only assume an approximate time point, late 2002, say.

This is a fairly straightforward one, as other observations may have as many as 9 to 16 used in the total observations column, taken over half a decade or so, half a dozen or so of which were used in the astrometric solution, and maybes a similar number in the photometric solution, with no evidence that these latter are the same half dozen, so the context of the quoted mean epoch for such is more problematic and only a rough idea of  date can be found.

But late 2002 seems safe enough here for this object being near a bright state.

UCAC2 is even more problematic.  The magnitudes, though useful at times as indicative quantities, are not photometric, or meant to be taken serious, only being meant to be used as double check "place holders" to quote the ucac literature.

If access to the full ucac2 catalogue is available a rough idea of date can be found.  A mean epoch of the astrometry exists.  However, as the mean epoch for the RA is very often totally different from the mean epoch of the declination (a good trick if you can manage it) and there is no way to figure out how this relates to any epoch for the photometry, the situation might not be all that clear.

Fortunately, I've just found out, through severe good luck, that the mean epoch of both RA and Dec for this object's ucac2 entry is 2002.531, or roughly 194 days into 2002, or about mid July, however this is a mean epoch from 3 observations, with again no indication of what date range they span over.

Generally these data _suggest_ the CMC14 data is on the way to a return to a faint state after a recent very bright state, however the variation rate may be higher than that, according to the misao images, and much could have happened inbetween.  Certainly the thing shows some activity.

r'_CMT (from CMC14) and UCAC2 instrumental red magnitudes are not directly comparable, but as with most magnitudes that hark back to a Vega or even AB system, experience shows that white objects in NSVS, CMC14, UCAC2 "red" magnitudes, as well as Rc, and even amateur unfiltered stuff on some CCD cameras, all usually have red mags within a few tenths of each other.

Finally, whilst I'm at it, and as an aside in this instance, but not generally unrelated, people should try never to use NOMAD for this sort of thing, but instead use the primary sources it is based upon, which are just as readily accessible, if not moreso in some cases.  Some fascination seems to exist because an ostensible B-V can be obtained from the mostly undocumented YB6 (strange what faith people will put into mystical undocumented material whilst simultaneously ignoring well documented and readily accessible robust online resources).  YB6 is pretty much, as best as I can tell, a photmoetric re-re-reduction of of the Yale proper motion surveys' plate digitisation (which latter the USNO PMM people did for the Yale people), so the source data is only plate magnitudes, for a start, Y being "yellow" plates, B being blue plates, so there's no Johnson B and V about it, and further, they were proper motion surveys (yes, plural, catalogued as an NPM1 re-reduction, the NPM2 [both Northern, mostly] and SPM3.3 [Southern]), ie these plates were exposed in periods up to decades apart, which although possibly acceptable in terms of constant stars, given the photographic plate source caveat, is not acceptable for variables that may have phase dependent colours, or worse are of high amplitude in either yellow or blue, and thus needing to have been in the coincidence of being in exactly the same phase when both plates were imaged if results are not going to be disparate.

Basically, just don't bother using it for variable star work, unless it's strictly (ie irreplaceably) needed for an astrometric solution.

Or read the revised NPM1, the NPM2 and the SPM papers, which are knocking around.  The revised NPM1, the separate NPM2 and the SPM3.3 are all re-reductions based, if I remember rightly, on Tycho2 photometric calibrations.  I seem to remember that the 6th version of the USNO NOFS YB catalogue (YB6) is yet another re-reduction of this data, using Tycho2 possibly, but I'm not certain whether they bothered to redigitise the plates, or just re-reduced the data.  Basically, when documentation is sparse, people shouldn't use data from a source as if it was definitive, because there's no way to gauge said data.


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