Discovery of [Fe X] and super-soft X-ray emission from the gamma-ray nova ASASSN-17mt (Nova Vel 2017)
ATel #11649; K. L. Page (U. Leicester), F. M. Walter (Stony Brook), N. P.M. Kuin (MSSL/UCL), J. P. Osborne (U. Leicester and M. J. Darnley (LJMU)
on 17 May 2018; 07:55 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
Subjects: Optical, Ultra-Violet, X-ray, Nova
ASASSN-17mt (Nova Vel 2017) was discovered on 2017 September 24.39 (ATEL #10772). A weak γ-ray source was subsequently detected by Fermi-LAT starting on 2017 October 24 (ATEL #10977). X-ray data collected by the Neil Gehrels Swift Observatory between 2017 November and 2018 January showed a faint source (mean count rate = 0.05 count s-1), heavily absorbed [NH = (2.0 +0.3/-0.4)×1022 cm-2 if fitted with an optically-thin APEC of kT = 1.6 +0.3/-0.2 keV].
Photometry from the dual-channel photometer on the 1.3m telescope of SMARTS shows no obvious sign
of dust formation between days 70 and 240 after discovery.
Spectra obtained with the Chiron echelle spectrograph (R=27,800) on the SMARTS 1.5m telescope
show the evolution from an optically-thick Fe II spectrum with multiple
absorption systems in December into a nebular spectrum in April/May.
The emission lines of H and He have narrow central
cores superposed on the normal double-horned profiles.
Prominent forbidden lines include [N II], [Fe VII], [Fe X], and [Ca V].
[O III] 4959/5007 is very weak.
Following the detection of [Fe X] emission, a Swift observation was requested in order to determine whether a super-soft source had become detectable, as the presence of [Fe X] can suggest. 1 ks of data was obtained on 2018 May 16 (234 days after discovery), showing enhanced emission below ~0.8 keV. While there are only 28 source counts in the spectrum, this suggests the emergence of a new, soft component, which can be approximately parameterised with a blackbody of temperature kT = 28 +57/-19 eV, together with APEC kT > 2.4 keV and a decreased, although poorly constrained, column density of NH = (3.5 +5.9/-3.4)×1021 cm-2. The 0.3-10 keV observed (unabsorbed) flux from this fit is 1.7×10-12 (6.2×10-11) erg cm-2 s-1; the BB component contributes 2.1×10-13 erg cm-2 s-1 of the observed flux.
The UVOT magnitudes obtained on May 16 are u = 14.65 ± 0.04, uvw1 = 15.64 ± 0.06, uvm2 = 18.09 ± 0.16 and uvw2 = 16.64 ± 0.08. OIR magnitudes from May 15 are B = 14.56, V = 13.80, R = 11.91, I = 12.10, J = 9.91, H = 9.46 and K = 8.45, with
uncertainties of 0.02 mag at J, and < 0.01 mag elsewhere.
The Swift UVOT photometry shows a pronounced minimum in uvm2 (2246 Å) which may be due to the bump in the Galactic reddening law. The photometry in the three UV and u bands forms a straight slope when adopting a reddening correction using the Cardelli law with E(B-V) = 1.0 ± 0.1 (Rv = 3.1). This reddening is consistent with the column density measured from the X-ray data.
Further Swift observations are planned to monitor the super-soft
We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for their ongoing support.
