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Swift and ATLAS observations of ASASSN-17fy/SN2017dwq

ATel #10500; K. Sokolovsky (IAASARS NOA/ASC Lebedev/SAI MSU), L. Wyrzykowski, A. Hamanowicz, M. Gromadzki (Warsaw Observatory), P. Chandra (NCRA), J. Tonry, B. Stalder, L. Denneau, A. Heinze, H. Weiland (IfA, Univ. of Hawaii), A. Rest (STScI), K. W. Smith, S. J. Smartt, D. Young (QUB)
on 16 Jun 2017; 20:25 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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

ASASSN-17fy was discovered on 2017-05-04 near the emission-line galaxy 6dFGS gJ090332.4-212002 (ATel #10358) that is also associated with a faint (6.3 mJy at 1.4 GHz) radio source NVSS 090332-212002. Spectroscopic observations with NOT classified the object as a young Type IIn supernova at z=0.018 (ATel #10391). SALT spectroscopy on 2017-05-19 revealed spectral features resembling SN Type Ib/c and confirming the redshift (ATel #10424).

The transient position suggested by ATLAS, SALT and Swift/UVOT imaging is 09:03:32.49 -21:20:02.83 J2000 which is 6.2" off the position currently reported by the Transient Name Server (but 0.3" from the one reported in the discovery ATel #10358). Forced photometry on ATLAS imaging shows that the transient has been slowly rising for 40 days. It has a first detection (o = 18.3 +/-0.2) on 2017-04-16 (JD2457859.8) and a non-detection 4 days before (o < 19.2). The lightcurve now looks close to peak at o = 16.3, corresponding to M_r ~ -18.6. The ATLAS o magnitudes are measured through an r+i combined filter and defined in the AB system.

Swift observed ASASSN-17fy for 4.3ks on 2017-05-23 (JD2457897.2). The XRT detected no X-ray source at the position of the transient with an upper limit of 0.0005 cts/s. Assuming power law emission with the photon index of 2 and HI column density of 1.0x10^21 cm^-2 this translates to the unabsorbed 0.3-10 keV flux limit of 2.3x10^-14 ergs/cm^2/s.

An ultraviolet source is detected by UVOT at the position of the transient. The measured AB magnitudes include the host galaxy light falling within the 5" diameter aperture. This likely explains the discrepancy with the ATLAS photometry, which have the host subtracted:

 
Band  Mag.  Err. 
 W2  18.87  0.04 
 M2  18.65  0.04 
 W1  17.92  0.04 
 U   16.85  0.04 
 B   16.05  0.03 
 V   15.46  0.04 

Taking into account the Galactic reddening of E(B-V)=0.140 (Schlafly et al. 2011 ApJ, 737, 103) these magnitudes suggest a black-body emission with the temperature ~ 8300 K, consistent with the transient being a supernova.

We thank Brad Cenko and the Swift team for this ToO observation.