November 14, 2022
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Dr. Hans Moritz Günther (MIT) requests AAVSO observers' assistance in monitoring the rotating variable ASAS J060415+1245.9 (= HD 251108, in Orion) in a follow-up to exceptional flaring activity.
Dr. Günther writes: "On November 7th LEIA (Lobster Eye Imager for Astronomy) discovered a new, bright X-ray transient source (https://www.astronomerstelegram.org/?read=15748). This was followed up by two other X-ray telescopes, NICER (https://www.astronomerstelegram.org/?read=15755) and Swift (https://www.astronomerstelegram.org/?read=15754) and likely originates from HD 251108.
"HD 251108 is a K2 giant star known to be rotationally variable and TESS data points to a period around 40 days. GAIA places the source at about 500 pc, but given its size it is still relatively bright and observable from the ground (m_V=9.9).
"The X-ray instruments observed what seems to be one of the largest flares ever studied in detail from a K-type giant. Yet, not much is known about this star and we thus request photometric and spectroscopic follow-up in any band. First spectroscopic follow-up shows emission lines (e.g. Halpha, Ca H & K) that seem to be decaying. Spectroscopic observations are particularly valuable is they can resolve those lines (R > 5000) so that we can determine if these lines decay after the flare or remain high for several rotational periods.
"We see an active chromosphere and rotational variability, but the presence of such a large and long-lasting flare may have some relation to features on the stellar surface. For example, there might be an unusual spot pattern that increases or decreases the magnitude of the optical variability and depending on the properties of those spots they may show up with an unusual color signature.
"We also do not know if this flare is a singular event that possibly decayed already since LEIA, Swift, and NICER observed it or if we will see enhanced activity for a period in the future. Thus, we suggest monitoring in any available photometric or spectroscopic band.
"Since the rotation period seems to be long and the flare also lasted for at least several days, this monitoring does not have to be very dense."
Observers are requested to begin coverage now and continue through 2022 December 31. Dr. Günther requests a cadence (per observer) of one image per filter per night through November 29, changing to every other night through December 31. As stated above, all filters are welcome, and spectroscopic resolution above 5000 will be very valuable. The range of ASAS J060415+1245.9 is 9.72 - 10.63 V; its period (based on ASAS-3 data analyzed by Sebastián Otero) is 21.18 days.
Coordinates (2000): RA 06 04 14.99 Dec. +12 45 51.1 (from VSX entry for ASAS J060415+1245.9)
Charts: Finder charts with comparison stars for ASAS J060415+1245.9 may be created using the AAVSO Variable Star Plotter (VSP).
Submit observations: Please submit observations to the AAVSO International Database or to the AAVSO Spectroscopy Database using the name ASAS J060415+1245.9.
This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material provided by Dr. Günther.
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