Observations and Characterization of the New Active Centaur 2023 RS₆₁

Abstract We report the cometary activity of the Centaur 2023 RS 61 (RS61) evident in our UT 2025 January 2 observations with the Lowell Discovery Telescope, showing a coma and a ∼5 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> </mml:math> 9-long tail. The archival observations from the Pan-STARRS1 survey reveal two previously unnoticed activity episodes in 2017 and 2023, suggesting SR61 could be a rare outbursting Centaur similar to 174P/Echeclus. The outburst on UT 2017 October 10 at r H = 13.5 au makes RS61 the second-most distant active Centaur after 95P/Chiron. We estimate a nucleus size between 4.7 km ≤ r n ≤ 7.0 km, assuming typical Centaur geometric albedos, an upper-limit Afρ ≤ 245 cm and dust production rate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>˙</mml:mo> </mml:mrow> </mml:mover> <mml:mspace width="0.25em"/> <mml:mo>≤</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>131</mml:mn> </mml:math> kg s −1 . As RS61 approaches the perihelion in 2028 its activity will likely increase, making this object an exciting observing target.