COMET 313P/GIBBS
H. H. Hsieh, Planetary Science Institute and Academia Sinica Institute of
Astronomy and Astrophysics, reports that wide-bandpass RI-band observations
obtained by J. Pittichova on Aug. 3 UT using the Palomar 5-m Hale Telescope,
r'-band observations obtained by queue observers P. Prado, M. Gomez, and A.
Shugart on Aug. 5 and 31 using the 8.1-m Gemini South telescope (program
GS-2019B-LP-104), and r'-band observations obtained by M. M. Knight on Aug.
26 using the 4.3-m Discovery Channel Telescope show that comet 313P/Gibbs
(cf. CBETs 3991, 4003, 4307, 4343), which has been previously identified as
a "main-belt comet" (MBC, or a comet with orbital elements like those of
main-belt minor planets), is currently active. Despite passing through dense
star fields during this period, the object is clearly active, exhibiting a
clear coma (full-width-at-half-maximum 0".62 in the Aug. 5 Gemini data, where
nearby stars have FWHM = 0".55) and a tapered tail extending at least 6" from
its photocenter in p.a. about 75 degrees (the tail roughly coinciding with the
projected direction on the sky of the anti-solar vector). During the period
covered by these observations, comet 313P ranged in heliocentric distance
from 2.75 to 2.69 AU, and in true anomaly from 292 to 298 degrees.
Including a previously observed active apparition in 2014 (cf. CBETs 3991,
4003) and archival Sloan Digital Sky Survey observations showing the object
to be active in 2003 (e.g., Hsieh et al. 2015, Ap.J. Let. 800, L16), these
observations mark the third time that 313P has been seen to be active.
Comet 313P's multiple observed periods of activity, interspersed with periods
of quiescence, is a strong indicator that its activity is driven by the
sublimation of volatile material. The current observing window for 313P
extends until approximately November 2019, by which time it will have reached
a true anomaly of about 320 degrees. The comet's next available observing
window extends from approximately July 2020 to February 2021, over which time
it will cover a true anomaly range of about 20 to 80 degrees. Observational
monitoring during both of these observing windows is highly encouraged to
characterize the evolution of 313P's current active episode for comparison to
its previous active episodes and to other MBCs.
The astrometry below were measured by M. Micheli and H. H. Hsieh.

2019 UT R.A. (2000) Decl. Mag. Observer
Aug. 3.16900 18 54 25.55 -24 43 38.7 19.3 Pittichova
3.20588 18 54 24.07 -24 43 48.5 19.1 "
3.20973 18 54 23.90 -24 43 49.7 19.0 "
5.04263 18 53 15.05 -24 51 41.3 19.5 Gomez
5.04651 18 53 14.90 -24 51 42.3 19.5 "
5.05041 18 53 14.76 -24 51 43.3 19.6 "
26.24625 18 45 55.79 -26 06 07.9 20.3 Knight
26.24892 18 45 55.78 -26 06 08.3 20.3 "
26.25226 18 45 55.75 -26 06 08.8 20.5 "
31.05497 18 45 58.84 -26 18 29.5 19.9 Gomez
31.05887 18 45 58.86 -26 18 30.1 19.9 "
31.06278 18 45 58.87 -26 18 30.6 20.0 "

Calculations by the undersigned suggest that the astrometry reported
to the Minor Planet Center in June and July of last year (the first known
observations since 2014 Nov. 28) indicated a notable outburst in mid-2018.
Both of the prior sets of observations of comet 313P (2003-2004 and 2014)
were made post-perihelion and at heliocentric distances (r) less than
about 3.2 AU; the power-law photometric parameters at those two perihelia
can be represented fairly well by H = 13.5, 2.5n = 10 and by H = 14.5,
2.5n = 10 (for r = 2.4-3.2 AU). Such parameters represent the above
(Aug. 2019) magnitudes reasonably, but the reported 2018 magnitudes are 3-4
magnitudes brighter than would be expected (when the comet was at r = 3.6 AU).
This comet reaches perihelion at q = 2.42 AU on 2020 Apr. 14 (Q = 3.9 AU,
P = 5.63 yr).