Abstract
V1674 Her was not only the fastest nova on record, it was also a rare hybrid event, showing a transition between the Fe ii and He/N nova classes as early as 5.5 days after the outburst. Spectra now indicate the outburst is over; emission lines from the underlying binary system are now seen, although broad but weaker forbidden nebular emission lines are also present. Intriguing at this late evolutionary stage is a P Cygni-like line profile associated with Hα suggesting the presence of a wind emanating from the binary system. The profile exhibits intensity and velocity variations perhaps correlated with its 0.153 day orbital period.
Export citation and abstract BibTeX RIS
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
1. Introduction
Nova V1674 Her was discovered in outburst on 2021 June 12.5484 UT reaching a peak V magnitude ∼6.3 on the same day. It was detected from VHE γ rays to radio (Li 2021; Sokolovsky et al. 2021). Quimby et al. (2021) found a t2 ≲ 1 day and a t3 ≃ 2.2 days making it the fastest nova on record exceeding the decline of V838 Her (Vanlandingham et al. 1996). Its evolution was so rapid that by day 17 it had entered the nebular phase showing strong emission from [Ne v] 3425.50 Å and the [Ne iii] pair at 3869.07, 3967.69 Å (all cited wavelengths in air). Balmer emission line widths implied ejection velocities ≳5000 km s−1 (Albanese et al. 2021). The distance to this nova is ≃5 kpc, while day 0 is MJD 59,377.96 (Woodward et al. 2021) corresponding to peak optical brightness.
2. Results and Discussion
Our observations of V1674 Her (Figure 1) were obtained with the 2 × 8.4 m Large Binocular Telescope, using MODS (Pogge et al. 2006) at two epochs: 2021 September 12.228 UT (MJD 59,476.223, day 98.27, 3 × 600 s exposures) and 2022 Apr 30.382 UT (MJD 59,699.382, day 321.78, 26 × 120 s exposures, with Δt = 193 s [62% duty cycle]) in a dual-grating spectroscopic configuration using a 0
8 wide entrance slit yielding a final resolution of 3.4 Å at a dispersion of 0.5 Å per pixel. Data reduction used python scripts (Astropy Collaboration et al. 2018) and IRAF (Tody 1993). Day 321.78 data were obtained to assess spectral variations associated with the system orbital period.
Figure 1. V1674 Her spectra (dereddened, see Section 2). (top) Day 98.27 (blue) vs. day 312.78 (red, composite sum of 26 120 s exposures) illustrating the spectral evolution (offset = +16.0). (bottom) Day 312.78 time series spectra centered near Hα, highlighting changes in the emission line profile (spectra continuum normalized and smoothed with a 7th order Savitsky-Golay filter, with offsets for clarity). The P Cygni-like absorption component is indicated by the dotted red vertical line. Time increasing upwards (green arrow).
Download figure:
Standard image High-resolution image
As of day 321.78 (Figure 1-top, red spectrum), broad optical forbidden emission lines are still evident including those of [Ne v] 3345.40, 3425.50 Å, and [Ne iii] 3869.07, 3967.69 Å. A broad feature between 4910 and 5060 Å likely is residual [O iii] 4958.91 and 5006.84 Å emission. The latter feature exhibits many subcomponents. The FWHM of [Ne v] 3425.50 Å emission is narrower, ≃4900 km s−1, compared to the 5900 km s−1 observed on day 98.27 (blue spectrum). The integrated intensity (all cited intensities dereddend using E(B − V) = 0.55, Woodward et al. (2021) with R = 3.1 and a Cardelli et al. (1989) extinction function; units of ergs s−1 cm−2) of the [Ne v] 3425.50 Å emission, I321.78 = (8.49 ± 0.24) × 10−14, has declined by a factor of ≃ 124 compared to the earlier spectrum, I98.27 = (1.05 ± 0.03) × 10−11. The I98.27/I321.78 = (3.35 ± 0.25) × 10−12/(6.25 ± 0.98) × 10−15 for [Ne v] 3345.40 Å emission decline was even more precipitous ≃536. V1674 Her was first classified as a neon nova based on the exceptionally strong [Ne v] and [Ne iii] emission lines (Wagner et al. 2021), e.g., for [Ne iii] 3869.07, 3967.60 Å, I98.27 = (1.95 ± 0.06) × 10−12 and (6.20 ± 0.44) × 10−13 respectively. The continued presence of neon emission substantiates this classification based on observed spectral signatures.
At both epochs the spectra have a strong blue continuum. Other measured line intensities include: day 98.27 Hδ (3.70 ± 0.13) × 10−14, Hγ (1.09 ± 0.15) × 10−13 (highly asymmetric, broad red wing), Hβ (5.27 ± 0.38) × 10−14, and day 327.78 H β (8.75 ± 0.20) × 10−15. Within the uncertainties, the Hβ FWHM velocity remained unchanged between the two epochs, ≃944 ± 14 km s−1.
Mroz et al. (2021) reported that V1674 Her exhibited a 501.42 s periodic signal in Archival Zwicky Transient Facility pre-outburst data. As the nova was declining, Shugarov & Afonina (2021) and Patterson et al. (2021) reported the detection of a 3.67 hr (0.15290 day) orbital period. Mroz et al. (2021) interpreted the ∼501 s period as the spin period of a white dwarf in an Intermediate Polar cataclysmic binary system. A 2022 May 18 Geherls-Swift observation shows that the system is still bright in soft-X-rays and that an ∼500 s oscillation is still evident in the X-rays (Page, private communication). It is clear that the source has yet to exit the supersoft phase indicating that the great strength of He ii 4687.71 Å, I321.78 = (2.31 ± 0.05) × 10−14, is produced by a hot white dwarf ionizing helium rich material. By day 321.78 the He ii line profile is narrow with a FWHM ∼ 945 km s−1.
On day 321.78 (Figure 1-bottom) the deconvolved Hα emission line exhibits two relatively narrow components at − 490 and + 32 km s−1 with FWHM of 390 km s−1 and 630 km s−1, respectively. A much broader but considerably weaker emission component is also evident with a FWHM of ≃3430 km s−1. A P Cygni-like profile is clearly evident at some temporal phases, centered at − 1206 km s−1, and which has a terminal velocity of ≃ − 1930 km s−1.
The appearance of a P Cygni-like line profile of Hα in the spectrum of V1674 Her is reminiscent of similar line profiles observed in the UV resonance lines of some cataclysmic variables (CVs) implying mass loss in a stellar or an accretion driven disk wind (Mauche & Raymond 1987; Drew 1987). These line profiles are seen in some CVs with high accretion rates and relatively low inclination angles suggesting a bipolar line forming region. Orbital variations of the UV resonance lines that exhibit P Cygni-like line profiles are seen in some systems like YZ Cnc, IR Gem, and V3885 Sgr in which the maximum absorption occurs at ∼0.7 to 0.8 orbital phase after the transition from red to blue of the optical emission lines (Woods et al. 1992), and suggests an asymmetry of the inner disk region. Continued orbital phase resolved spectroscopy of V1674 Her may provide insights to the mass accretion rate and the geometry of the accretion disk and rotating magnetic field that could be the origin of the 501 s period.