Ultraviolet Rebrightening of Superluminous Supernova ASASSN-15lh

ATel #8086; Peter J. Brown (Mitchell Institute, Texas A&M University)
on 23 Sep 2015; 23:53 UT
Credential Certification: Peter J. Brown (grbpeter@yahoo.com)

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The Swift satellite (Gehrels et al. 2004, Roming et al. 2005) has been observing the superluminous supernova (SLSN) ASASSN-15lh/SN2015L (Dong et al. 2015, CBET#4120) since the announcement of its discovery (Nicholls et al. 2015, ATEL#7642). It was brightest in the uvm2 during the first observation on MJD 57197.10 with a Vega magnitude of uvm2=15.24 +/- 0.06. Over the next two months it faded to uvm2=18.06 +/- 0.11 on MJD 57268.79. It has subsequently rebrightened to uvm2=17.27 +/- 0.07 on 57284.38. These magnitudes come from the Swift Optical/Ultraviolet Supernova Archive pipeline (SOUSA; Brown et al. 2014) including the revised zeropoints of Breeveld et al. (2011) and the newly-updated time-dependent sensitivity correction. They have not been corrected for galaxy contamination. The uvm2 filter has a central wavelength of 2246 Angstroms (Poole et al. 2008), corresponding to 1822 Angstroms in the rest-frame for a source redshift of 0.2326 (Dong et al. 2015). In the UVOT light curves, the rebrightening has a stronger effect at shorter wavelengths and may be caused by interaction of the SN ejecta with a circumstellar shell of ejected material. A slower decay was seen in the optical light curve of iPTF13ehe (Yan et al. 2015, arXiv:1508.04420) accompanied by late-time Hydrogen alpha emission. ASASSN-15lh may be another example of an SLSN with multiple energy sources (Wang et al. 2015, arXiv:1509.05543). These observations were performed as part of the Swift Guest Investigator Program "Ultraviolet Properties of Superluminous Supernovae over Ten Billion Years" (NNX15AR41G, PI: Brown) and Swift ToO requests from multiple others (Holoien, Quimby, Dong).

Preliminary UVOT Light Curve of ASASSN-15lh