Infrared coronal line emission detected in the 2024 eruption of Recurrent Nova LMC 1968
ATel #16771; D. P.K Banerjee (Physical Research Lab., India), A. Polin (Purdue Univ.), E. Hsiao (Florida State Univ.), B. Mockler (Carnegie Observatories), A. Evans (Keele Univ.), C. E. Woodward (U. Minnesota), S. Starrfield (Arizona State Univ.), K. L. Page (U. Leicester) and T. R. Geballe (NOIRlab/Gemini Observatory)
on 13 Aug 2024; 18:32 UT
Credential Certification: Dipankar P.K. Banerjee (dpkb12345@gmail.com)
Subjects: Infra-Red, Nova
We report a near-infrared spectrum (0.8-1.8 micron) of the recurrent
nova "Nova LMC 1968" whose latest outburst occurred at some time between
1.1 August and 1.83 August 2024 UTC (ATel #16752). The long slit (0.6 arcsec width) spectrum was
taken on 2024-08-11.406 UT with the FIRE spectrograph on the 6.5m Baade telescope
of the Las Campanas Observatory at low resolution (~300-500) and at an airmass
of 1.5. The total exposure time was 507s.
The Large Magellanic Cloud Recurrent Nova (RN) LMC 1968 (LMC V1341)
has previously been seen in eruption in 1968, 1990, 2002, 2010, 2016
and 2020 (see Kuin et al.2020, MNRAS, 491, 655 for a review of most
of these eruptions). Following the 2020 eruption, a recurrence period
of four years was proposed (ATel #13731). It is believed to have the
the third shortest recurrence period amongst known RNe (galactic
or extra-galactic) and is the first known extra-galactic RN
(Shore et al. 1991, ApJ, 370,193). No infrared spectrum has
been taken of it.
The spectrum has low S/N and the weaker features are thus
suppressed, but we detect [S IX] 0.939 (weakly), HeII 1.012, HeI 1.083,
HeII 1.164, HI 1.282 (Paschen beta) and [Si X] 1.430 micron lines. Of these,
the [Si X] 1.430 micron line is by far the strongest (line flux = 1.2e-12 erg/s/cm^2)
and its presence indicates that the nova is in the coronal phase. The presence of
coronal line emission is consistent with the super-soft X-ray phase through which
the nova is currently passing (as evidenced by Swift observations).
This is one of the earliest onsets of the coronal line emission
phase in a nova, similar to the timescales of 9.41d and 11.5d
in U Sco and V1974 Her (Evans et al. 2023, MNRAS 522, 4841,
Woodward et al. 2021, ApJ, 922, L10). The FWZIs of the HeI 1.083 micron
(the second strongest line in the spectrum, observed line flux = 2.5e-13 erg/s/cm^2)
and the [Si X] 1.430 microns lines are in the range 12000-13000 km/s. Both lines
show a double-peaked structure; the inter-peak separation is ~4000 km/s for the
[Si X] 1.430 micron line. The de-reddened continuum (using E(B-V) = 0.07)
is reasonably well matched by free-free emission from a hot, ionized gas.
Further observations are deeply encouraged.
