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Gaia16aqg/AT2016ecm was a Supernova Type Ia near the nucleus of the galaxy

ATel #10293; K. Sokolovsky (IAASARS NOA/ASC Lebedev/SAI MSU), M. Gromadzki, L. Wyrzykowski, A. Hamanowicz (Warsaw Observatory)
on 20 Apr 2017; 16:06 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

Subjects: Optical, Ultra-Violet, X-ray, Supernovae

The optical transient Gaia16aqg/AT2016ecm was discovered on 2016-05-23 coinciding with the center of the galaxy LEDA 581422.

SALT/RSS spectroscopy on 2016-07-08 revealed broad H_alpha and HeI emission at z=0.060 superimposed on the continuum dominated by the galaxy light (ATel #9497). The spectrum of host galaxy was obtained with VLT/FORS2 on 2017-03-20. It shows similar emission lines but the continuum is more smooth/flat compared to the SALT spectrum obtained while the transient was bright. The difference may be attributed to broad metal lines typical for SN Ia, which are not as strong as usual due to a significant contribution of the host galaxy light. The spectroscopic classification was done using SNID code (Blondin & Tonry 2007, ApJ, 666, 1024) on residual spectrum after host subtraction and is consistent with SN Type Ia 37 days after maximum.

Swift observed Gaia16aqg for 2.1ks on 2017-03-25. Swift/XRT detected no X-ray source at the position of the transient with an upper limit of 0.0014 +/-0.0008 cts/s. Assuming power law emission with the photon index of 2 and HI column density of 1.43x10^20 cm^-2 this translates to the unabsorbed 0.3-10 keV flux limit of 5.1x10^-14 ergs/cm^2/s.

An ultraviolet source is detected at the position of the transient with the following Swift/UVOT magnitudes:

 Band AB_mag. Err. 
  W2  19.30  0.06 
  M2  19.26  0.07 
  W1  19.15  0.06 
Taking into account the Galactic reddening of E(B-V)=0.011 (Schlafly et al. 2011 ApJ, 737, 103) these magnitudes suggest a black-body emission with the temperature ~18000 K.

An ultraviolet source associated with this galaxy was previously observed by GALEX having AB magnitudes FUV=19.05 +/-0.14, NUV=18.79 +/-0.08 (Bianchi et al. 2011 Ap&SS, 335, 161). GALEX NUV band approximately corresponds to the wavelength range covered by three UVOT filters (W2, M2, W1). The high color temperature observed before and after the transient appearance is consistent with the galaxy LEDA 581422 having an increased star formation. Lack of clear ultraviolet excess is consistent with the spectral classification as SN Type Ia occurring near the center of the galaxy.

We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (ATel #6593). We thank Brad Cenko and the Swift team for making this observation possible.

Gaia Photometric Science Alerts