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Swift and LT UV and optical observations of type IIn superluminous supernova 2017gir

ATel #10784; Zach Cano (IAA-CSIC), Paul Kuin (UCL/MSSL), Poonam Chandra (NCRA-TIFR and Stockholm University), Chris Ashall (LJMU), Daniele Malesani (DARK/NBI and DTU Space?), Andrea Pastorello (Padova)
on 26 Sep 2017; 14:39 UT
Credential Certification: Zach Cano (zewcano@gmail.com)

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

We observed the field of the type IIn superluminous supernova 2017gir (ATLAS17jsb, Tonry et al. 2017; Lyman et al. 2017, ATel 10674) with Swift via a target-of-opportunity for three epochs (6th, 16th and 19th of September, 2017) in the three UVOT UV filters (w1, m1, w2). The SN is clearly detected in all three filters, and it is seen that its brightness fades over this timescale. For the third and final epoch (a mean Julian date of 2458016.38), we measure AB magnitudes of w2=19.08 ± 0.10, m2=18.76 ± 0.10 and w1=18.61 ± 0.13, which are uncorrected for extinction.

We initiated follow-up with the 2-m Liverpool Telescope to obtain optical imaging in sloan filters griz the same night it was classified by ePESSTO (Lyman et al. 2017). Since the morning of the 30th of August, its apparent AB magnitudes in riz have brightened by roughly 0.2 magnitudes, while in g it has brighten by approximately 0.1 magnitudes. Since the 10th of September, its apparent magnitudes have remained unchanged, suggesting it is at peak light, and is evolving very slowly. On the 19th of September (a mean Julian date of 2458016.56), it had AB magnitudes of g=17.79 ± 0.02, r=17.77 ± 0.01, i=17.83 ± 0.02 and z=18.00 ± 0.03. These magnitudes are uncorrected for extinction.

Assuming SN 2017gir is at peak light, and for a spectroscopic redshift of z=0.163 determined from our GTC optical spectra, for a generic flat Lambda CDM cosmological model its inferred distance luminosity is ∼ 780 Mpc and its distance modulus is 39.47 mag. At z=0.163, observer-frame r-band is roughly rest-frame V-band, which suggests SN 2017gir has a peak absolute, V-band magnitude of MV ≈ -21.5, thus confirming its superluminous classification. As this value does not consider the dimming effects arising from sight-lines through the MW galaxy, or reddening arising from its host galaxy, this is formally an upper limit to its absolute magnitude.

Swift simultaneously observed the field with XRT, however no new source was detected at the SN position. Combining the three observations (for a total exposure time of 1345 s) we find a 3-sigma upper limit at the SN position of 8.31×10-03 c s-1. Assuming a temperature of 1 keV, and Galactic absorption of ~1.2×1020 cm-2 in this direction, this corresponds to an upper limit in 0.3-10 keV unabsorbed flux to be 2×10-13 erg cm-2 s-1. For its redshift of z=0.163, this corresponds to an unabsorbed 0.3-10 keV luminosity of 1.5× 1043 erg s-1.

We thank the Swift team for executing our requested ToO. Further Swift ToO observations have been requested, and future observations are planned as part of a comprehensive monitoring campaign of this event.