VLT, SALT and Swift observations confirm the blazar nature of OGLE18wc = AT2018wc

ATel #11434; K. Sokolovsky (IAASARS NOA/ASC Lebedev/SAI MSU), M. Gromadzki, L. Wyrzykowski, P. Zielinski, K. A. Rybicki, A. Udalski, P. Mroz (Warsaw Observatory), N. Ihanec (Univ. Nova Gorica), D. Buckley (SAAO), J. C. Beamin (Univ. de Valparaiso, Millennium Institute of Astrophysics)
on 16 Mar 2018; 17:23 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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The optical transient OGLE18wc = AT2018wc was detected on 2018-02-16 during a dedicated search for fast transients CORTE (Chile-OGLE Rapid Transients Experiment). The flare lasted three days, reached about 19.1 mag in OGLE I-band at maximum (host subtracted) and coincided with the nucleus of a small galaxy.

The galaxy coincided with WISE J051811.72-514404.0, a candidate BL Lac type object identified by D'Abrusco et al. (2014, ApJS, 215, 14) on the basis of its WISE colors and identification with the radio source SUMSS J051811-514404 (76.0 mJy at 843 MHz; Mauch et al. 2003, MNRAS, 342, 1117). The radio source is also detected with astrometric VLBI: 05:18:11.727160, -51:44:03.97925 +/-2.6 mas J2000 (30 mJy at 8.4 GHz; Petrov and Kovalev, in preparation http://astrogeo.org/ ). The X-ray source XMMSL2 J051811.6-514354 is located 10" away and, despite the reported 4.3" uncertainty in its position, is likely associated with the transient.

We obtained a spectrum of OGLE18wc on 2018-02-19 using the VLT/FORS2 with grism 600V (3500-6300A). Additional low resolution spectra were obtained with SALT/RSS (3500-9200A) on 2018-02-20 and 2018-03-04. The spectra suggest rapid evolution, but the difference between the VLT and SALT spectra can be also partly attributed to the slit alignment. The first (VLT) spectrum shown Hb, Hg, [OIII] and [OII] emission lines and absorption lines of Ca II H&K, G-band and Mg I at z=0.22. In the second (SALT) spectrum Hb disappears, strong Iron complexes 4500A and 5500A are present. A broad Ha alpha is visible having FWHM~4000km/s. The third (SALT) spectrum is similar to the second one but flux in red part has decreased by about 25%.

Swift observed OGLE18wc for 2.0ks on 2018-02-21. The XRT detected an X-ray source with the net count rate of 0.013 +/-0.003 cts/s located 3.5" (error radius is 5.8") from the optical transient. Fixing the absorbing column density to the Galactic value in the direction of the source, n_H1 = 1.63x10^20 cm^-2 (Kalberla et al. 2005 A&A, 440, 775), the spectrum can be fit with the absorbed power law with the photon index of 2.15 +/-0.40 and the unabsorbed 0.3-10 keV flux of 4.3x10^-13 ergs/cm^2/s.

An UV source is detected at the position of the transient with the following UVOT Vega magnitudes:

 
 Band   Mag. Err. 
 UVW2  19.46 0.18 
 UVM2  19.74 0.29 
 UVW1  19.47 0.34 
   U   19.18 0.31 
   B  >19.86 
   V   18.76 0.24 

The above magnitudes are not corrected for the Galactic reddening of E(B-V)=0.013 (Schlafly et al. 2011 ApJ, 737, 103) and are not host-subtracted.

The Swift X-ray detection and blue UV colors together with the irregular lightcurve observed by OGLE-IV, WISE colors and archival VLBI detection suggest that OGLE18wc is a flaring blazar. The optical spectra of OGLE18wc resemble the spectrum of Mrk 501, a high-synchrotron peaked BL Lac type object that has a significant contribution from its host galaxy light in the optical band (Abdo et al. 2011, ApJ, 727, 129). OGLE18wc shows similarities to our spectra of the Fermi-detected transient ASASSN-17gs/AT2017egv (ATel #10642) recently identified as a radio-weak BL Lac by Bruni et al. (2018, ApJ, 854L, 23). We encourage the search for a GeV counterpart to OGLE18wc.

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. The SALT observations were conducted in the framework of the Large Programme on transients (2016-2-LSP-001). 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