The burning symbiotic star QW Sge (=AS 360) is in outburst, the fifth of its recorded history
ATel #16739; Ulisse Munari (INAF Padova), Gian Luigi Righetti (ANS Collaboration), Andrea Farina (Univ. Padova)
on 27 Jul 2024; 17:02 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
The symbiotic star QW Sge (=AS 360) belongs to the burning type,
with the WD burning in stable conditions at its surface the material
accreted from the M4III cool giant companion. Strong [NeV] 3427, HeII 4686,
and Raman-scattered OVI 6825 emission lines have always been present, both
in old spectra (eg. Allen 1984, PASAu 5, 369; Munari & Buson 1991, A&A
249, 141; Munari & Zwitter 2002, A&A 383, 188), as well as in the more recent
ones that we are recording regularly since 2011 with the Asiago 1.22m and
1.82m telescopes.
QW Sge has an optical companion of similar brightness just 4.9 arcsec to
the north. Most of the photometry available from repositories like AAVSO or
ASASSN pertains to the unresolved pair (combined). ANS Collaboration is
regularly monitoring QW Sge in BVRI bands since 2005, primarily with the
long-focus ID 1507 telescope, and all data is reduced via PSF-fitting to
measure QW Sge separately from the optical companion. The median
values since 2005 for QW Sge and its companion are, respectively, B=15.231
and 13.561, V=13.958 and 13.018, R=12.432 and 12.760, I=10.764 and 12.653,
with no evidence of variability larger than 0.025 mag for the optical companion.
Previous known outburst of QW Sge occurred in 1963, 1983, 1997, and 2022.
Our recent observations with ANS Collaboration ID 1507 telescope
indicates that a new outburst of QW Sge is currently in progress (this therefore
being the fifth one known): our latest collected data are summarized in the table
below. Compared to quiescence, QW Sge is currently brighter by 2.1 mag in B
(13.093 vs. 15.231) and bluer in the (B-I) color by 1.7 mag (2.78 vs.
4.47).
| UT | U | B | V | R | I |
|---|
| 2024 Jul 23.916 | 12.718 | 13.093 | 12.127 | 11.085 | 10.317 |
| 2024 Jul 18.864 | | 13.017 | 11.947 | 11.134 | |
| 2024 Jul 13.868 | | 13.268 | 12.065 | 11.146 | |
A low-resolution spectrum of QW Sge (3200-7900 Ang, 2.3 Ang/pix) has
been obtained with the Asiago 1.22m telescope on July 23.854 UT. In
this figure we compare the outburst spectrum
with a quiescence one obtained with the same instrumentation almost exactly
one 360d-period earlier (Munari & Jurdana-Sepic 2002, A&A 386, 237), on 2023 Jul 8. The observed absolute flux (erg/cm2/s/Ang) of some representative emission lines measured on these two spectra is listed in the table below.
| date | Bowen | HeII | Hbeta | [OIII] | HeI | [FeVII] | Halpha | Raman OVI |
|---|
| 4640 | 4686 | 4861 | 5007 | 5876 | 6087 | 6563 | 6825 |
|---|
| 2024-Jul-23 | 6.98E-13 | 1.45E-12 | 1.42E-12 | 1.73E-13 | 5.43E-13 | 2.25E-14 | 1.37E-11 | 6.24E-14 |
| 2023-Jul-08 | | 3.39E-13 | 8.92E-13 | | 2.44E-13 | 8.25E-14 | 1.09E-11 | 4.45E-13 |
The reduction in the integrated flux of the highest ionization emission
features ([FeVII], [CaVII], [NeV], Raman scattered OVI) and the corresponding
surge in flux of those of intermediate ionization (HeII, [OIII], [NeIII],
NIII Bowen blend at 4640) suggests that the rise in optical brightness has
probably been driven by a shift to milder temperatures (and therefore the
emission to longer wavelengths) of the outer layers of the WD burning shell.
