Photometry and spectroscopy of TCP J17440698-2125195 (AT 2024rxm)

ATel #16769; U. Munari (INAF Padova), S. Dallaporta, A. Vagnozzi, A. Maitan, M. Fiorucci (ANS Collaboration), and Irene Albanese (Univ. Padova)
on 13 Aug 2024; 13:42 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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TCP J17440698-2125195 was discovered by Tadashi Kojima on Aug 11.456 UT at unfiltered 13.7 mag, being missing from similar data taken a day earlier. Follow-up observations obtained on Aug 11.559 UT by K. Yoshimoto report B=14.86, V=13.76, Ic=11.80 and revise the position end figures to 07s.03 and 21".2; additional data by A. Pearce for Aug 11.903 UT provide B=15.45, V=13.61, R=12.54 and refine position end figures to 07s.05, 21".0.

Such coordinates correspond to better than 0.3 arcsec to the position of a field star of B=18.50, V=16.80, R=17.03 and J=13.887, H=13.121, K=12.769 according to DSS Nomad catalog and 2MASS survey. The red color is confirmed by Gaia DR3 that lists BP=17.835 and RP=15.577, with a not significant 0.07+/-0.07 mas parallax and just 5 mas/yr proper motion, supporting a large distance to the object. Adopting E(B-V)~0.7 as the asymptotic reddening in the direction of TCP J17440698-2125195, the available photometry for the progenitor fits an early K-giant at about 5 kpc distance.

We have obtained BVRI photometry of TCP J17440698-2125195 with ANS Collaboration telescopes ID 310, 1302, and 2202 at a mean epoch Aug 12.862 UT. The mean magnitudes and standard deviations among the three sets of independent measurements are: B=17.13 +/-0.08, V=14.89 +/-0.05, R=13.80 +/-0.04, I=12.93 +/-0.03. While not yet back to the brightness level shown on DSS, the object has markedly declined since Pearce's measurements obtained 24 hours earlier: 1.68 mag in B, 1.28 in V, and 1.26 in R.

A low resolution spectrum (4300-7900 Ang range, dispersion 2.3 Ang/pix) of TCP J17440698-2125195 has been obtained with the Asiago 1.22m + B&C telescope on Aug 12.863 UT. The S/N of the spectrum, that can be inspected here, has been affected by the low height on the local horizon (20deg) and not-optimal sky conditions. No emission line is readly visible. The spectral continuum rises steeply toward the red in fine agreement with results of above listed BVRI photometry. Fitting the spectrum simultaneously for reddening and spectral classification against the Asiago Spectral Atlas provides as the best match a spectral type K2 to K3, luminosity class III to II, and a reddening E(B-V)=0.80 +/-0.04. The Halpha line presents an absorption profile of normal intensity, excluding significant filling in, while the NaI doublet is made particoularly deep by the interstellar component associated to the high reddening.

The nature of TCP J17440698-2125195 and what caused the 3-mag amplitude outburst remain unclear. The object remained at maximum brightness for a few hours before embarking on a steep decline (at least 1.28 mag/day in V band), during which its spectrum resembled that normal K2/3 III-II giant.