Multiwavelength observations of the symbiotic star V694 Mon during the current stable hydrogen-burning phase
ATel #16956; N. Masetti (INAF-OAS, Bologna) & U. Munari (INAF-OA Padova)
on 17 Dec 2024; 02:33 UT
Credential Certification: Nicola Masetti (nicola.masetti@inaf.it)
Subjects: Optical, Ultra-Violet, X-ray, Binary, Cataclysmic Variable, Transient, Variables
V694 Mon is a symbiotic star in which an M4 III red giant is orbited by
a white dwarf (WD). Since discovery (Merrill & Burwell 1943, ApJ, 98, 153)
it has presented highly variable absorption lines with velocities up to 6500
km/s and large-amplitude photometric flickering. Tomov et al. (1990, Nature,
346, 637) attributed the variable absorptions to discrete jet-like ejections
with a substantial degree of collimation and directed near the line of sight,
with the binary system being seen close to face-on. The observational features
of V694 Mon radically changed in 2018, with a steep rise in brightness accompanied
by the disappearance of both flickering and high-velocity absorptions, which never
reappeared since (ATels #12227, #12670, #13236, #14239, #14988, #15061, #15066, #15906).
Munari (2024, RNAAS 8, 306) has suggested that the changes experienced in
2018 are due to V694 Mon transitioning from the accreting-only state (dominating
since the object discovery) to non-explosive, thermal-equilibrium hydrogen
burning on the WD surface. The bolometric luminosity of such stable
burning is about 3500 Lsun, pointing to a WD mass of 0.6 Msun.
The maximum optical brightness and the weakest emission line state may have been
reached by V694 Mon in early 2024, when its spectral appearance resembled that
of an A7 II/I supergiant, with only a weak Halpha emission. The median optical
photometric values over the first 4 months of 2024 are U=8.589, B=8.622,
V=8.235, R=7.895 and I=7.304 from the ANS Collaboration daily monitoring.
We thus initiated an X-ray and ultraviolet (UV) monthly monitoring of V694 Mon
with the Neil Gehrels Swift satellite, starting at 06:50 UT on 2024 Dec. 10.
The source was observed in UV with UVOT with filters UVW1, UVW2 and UVM2 for 617 s,
637 s and 637 s, respectively; preliminary analysis shows that the source was well
detected, with Vega system magnitudes UVM2=11.60+-0.04 and UVW2=11.40+-0.04, while
an upper limit <11.01 mag (i.e. a lower limit in brightness) was obtained in the
UVW1 band due to coincidence loss in the detector produced by the source intensity;
a more detailed analysis using the method of Page et al. (2013, MNRAS, 436, 1684)
is underway. This corresponds to UVW1<12.52, UVM2=13.29+-0.04 and UVW2=13.13+-0.04
in the AB system. The source was instead not detected in X-rays: a simultaneous XRT
pointing of about 2000 s did not reveal any emission in the 0.3-10 keV band down to
a 3-sigma limit of 3.4e-3 counts/s.
Near-simultaneous optical photometry was obtained on 2024 Dec. 11.17 UT with
the Asiago 67/92cm Schmidt telescope, yielding U=8.729, B=8.723, and V=8.383
mag (with millimag errors).
Assuming a collisionally ionized plasma X-ray spectrum with kT=0.77 keV (i.e.
the soft component in Table 3 of Lucy et al. 2020, MNRAS 492, 3107), adopting
the Gaia DR3 distance of 2.35(+-0.20) kpc and considering a hydrogen column
density of 8.3e20 atoms/cm2 derived from the interstellar reddening value
E(B-V)=0.15 (Lucy et al. 2020) using the prescription of Predehl & Schmitt (1995,
A&A, 293, 889), the XRT data imply an unabsorbed flux upper limit <1e-13 erg/cm2/s
and a luminosity limit <7e31 erg/s.
The absence of observable X-rays and the UV/optical photometry are in good agreement
with the hypothesis of emission from a burning shell around the WD mimicking a normal
supergiant star. Correcting for the above E(B-V) value, the source absolute magnitude
is M(V)=-4.0 and the simultaneous optical and UV photometry well match a T=7200 K,
log(g)=2.0, [Fe/H]=0 model atmosphere from Castelli & Kurucz (2003, IAUS 210, A20),
corresponding to a F1 II/Ib spectral type, close to the A7 II/I classification inferred
from the early 2024 data.
We thank the Swift Team for the quick approval, scheduling and acquisition
of the observation presented in this communication.
