Optical Reconnaissance of the nova V1405 Cas on Day +1546

ATel #17218; C. E. Woodward (U. Minnesota), S. Starrfield (Arizona State U.), K. L. Page (U. Leicester, UK)
on 4 Jun 2025; 13:08 UT
Credential Certification: C.E. Woodward (chickw024@gmail.com)

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V1405 Cas is a very slow nova that has remained bright across the electromagnetic spectra since its discovery on JD 2459277.268 (ATel#14471). The nova entered a recombination phase near day +71 (ATel#14622). Munari and Valisa (ATel#15796) identified it as a neon nova on day +618, based on the great strength of [Ne V] 342.7 nm, from optical spectra that also included detection of the [Fe VII] 608.629 nm line. Here we report recent, deep (1200 sec) optical spectra obtained on 2025 May 28.407 UT, day +1546, with the MMT Blue Channel spectrograph using a 1.0 arcsecond slit under photometric conditions. As the nova continues to evolve towards quiescence, the optical spectrum exhibits numerous emission lines from He I 402.398 nm, He II 419.99 nm, He II 454.20 nm, He II 468.60 nm, H I 397.00 nm, H I 410.10 nm, HI 486.10 nm, H I 656.28 nm as well as a variety of Fe I and [Fe II] lines. None of these lines show any structure across their peaks. The [Fe VII] 608.629 nm coronal line is still present (ATel#16089) although it is heavily blended with other lines near this wavelength. There is no evidence for the [Fe VI] 517.499 nm coronal line or strong lines of oxygen. The observed flux of H I 486.10 nm is approximately 1.10e-13 erg/s/cm2/A, with observed integrated intensity ratios of (HI 656.28/H I 486.10) = 2.86; (H I 434.00/H I 486.10) = 0.66; (H I 410.10/H I 486.10) = 0.43, while the (He II 468.60/H I 486.10]) is approximately 1.95 and the ([Fe VII] 608.629/H I 486.10) is approximately 0.30. The average FWHM velocity of the H I lines is approximately 1260 km/s. In a complementary effort, the Neil Gehrels Swift Observatory continues to monitor V1405 Cas with a weekly cadence. The X-ray emission remains soft (since at least day +1315; ATel#16876), with a count rate typically in the range 0.05-0.1 count/s. Over the same interval, the UV source varies between mag 13.5 and 12.5, with emission in the uvw1 filter (central wavelength 260 nm) being the brightest, and uvm2 (224.6 nm) being the faintest. Continued observations at all wavelengths are encouraged. Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution.