Swift and VLT observations of the supernova OGLE18act = AT2018act

ATel #11435; K. Sokolovsky (IAASARS NOA/ASC Lebedev/SAI MSU), M. Gromadzki, L. Wyrzykowski (Warsaw Observatory), E. Kankare (QUB), A. Pastorello (INAF-Padova), M. Fraser (UCD), A. Udalski, I. Soszynski, K. Ulaczyk, J. Skowron (Warsaw Observatory, on behalf of the OGLE team)
on 16 Mar 2018; 18:01 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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The optical transient OGLE18act = AT2018act was detected on 2018-03-08 coinciding with an uncataloged galaxy visible at the stacked OGLE-IV image. The previous OGLE-IV observations show the transient slowly rising to I=19.2 since the first detection on 2018-01-21.

We obtained the spectrum of OGLE18act on 2018-03-10 using the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) on the Very Large Telescope (VLT). The spectrum reveals blue continuum with superimposed emission lines of the Balmer series, [O II], [O III] and faint [S II] at redshift z=0.078. Balmer emission lines have a very complex shape. An unresolved narrow emission peak (FWHM ~<500 km/s) is superimposed on a broad component with an extended red wing (total width ~10000 km/s). There is a narrow absorption feature blue shifted by 700-800 km/s with respect to the narrow emission component.

Swift observed OGLE18act for 4.1ks on 2018-03-14. The XRT detected no X-ray source at the position of the transient with an upper limit of 0.004 cts/s. Assuming power law emission with the photon index of 2 and HI column density of 3.07x10^20 cm^-2 this translates to the unabsorbed 0.3-10 keV flux limit of 1.4x10^-13 ergs/cm^2/s.

A faint ultraviolet source is detected at the position of the transient with the following UVOT magnitudes (Vega system, not corrected for the host galaxy light):

 
   JD        Band   Mag.  Err. 
 2458192.21  UVW2  19.50  0.20 
 2458192.00  UVW1  19.00  0.09 
 2458192.20    U   18.65  0.22 
 2458192.20    B   19.26  0.27 
 2458192.21    V  >18.68 

Taking into account the Galactic reddening of E(B-V)=0.032 (Schlafly et al. 2011 ApJ, 737, 103) these magnitudes suggest a black-body emission with the temperature ~9000 K (observer's frame). The low color temperature supports the classification of OGLE18act as a supernova. The VLT/FORS2 spectrum and OGLE-IV I-band lightcurve are similar to SN type IIn-P SN2011ht.

We thank the Neil Gehrels Swift Observatory Team and PI, Brad Cenko, for rapid execution of this ToO observation. We thank ESO and VLT staff; VLT/FORS2 ToO observations were carried under ESO programme 0100.B-0503(A), PI: Gromadzki. We acknowledge the OGLE-IV Transient Detection System presented in ATel #4495; Kozlowski et al. 2013, AcA, 63, 1, 1; Wyrzykowski et al. 2014, AcA, 64, 197; Klencki et al. 2016, AcA, 66,15).

OGLE-IV Transient Detection System