PESSTO classification and characterisation of AT 2016jbu / Gaia16cfr

ATel #9938; M. Fraser (University College Dublin), A. Pastorello (Padova), R. Cartier (Southampton), J. Johansson (Weizmann), R. Kotak (QUB), K. W. Smith (QUB), A. Cikota (ESO), M. Magee (QUB), C. Inserra (QUB), T.-W. Chen (MPE), J. Lyman (Univ. of Warwick), E. Kankare (QUB), K. Maguire (QUB), S. J. Smartt (QUB), M. Sullivan (Southampton), S. Valenti (UC Davis), O. Yaron (Weizmann), D. Young (QUB), I. Manulis (Weizmann), J. Tonry (IfA, Univ. of Hawaii), B. Stalder (IfA, Univ. of Hawaii), L. Denneau (IfA, Univ. of Hawaii), A. Heinze (IfA, Univ. of Hawaii), H. Weiland (IfA, Univ. of Hawaii), A. Rest (STScI), L. Wyrzykowski (Warsaw Observatory, Poland)
on 4 Jan 2017; 19:23 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Morgan Fraser (morgan.fraser@ucd.ie)

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AT2016jbu (aka Gaia16cfr) was first announced by the Gaia Alerts team on 2016 Dec 27, based on detections at G=18.4 on Dec 26, and G=19.3 on Dec 1. The transient was independently discovered by B. Monard in late December, who kindly alerted us to the likely association with NGC 2442.

The field of NGC 2442 was extensively monitored during the follow-up campaign of SN 2015F (Cartier et al. 2017, MNRAS, 464, 4476). By inspecting archival images obtained with VLT+FORS2 on 2016 Sep 7 (JD=2457638.88), we do not detect AT2016bju to V>24.65, I>23.30. However, the transient is clearly detected on FORS2 images obtained on 2016 Oct 29 (JD=2457690.85) at the following magnitudes: V=22.48+/-0.10, R=21.30+/-0.07, I=21.17+/-0.08 (reference stars from Pastorello et al. 2009, A&A, 500, 1013 have been used for the calibration). We therefore suggest that the AT2016jbu light curve is experiencing a slow rise to maximum, lasting at least two months.

Riess et al. (2016, ApJ, 826, 56) find a distance m-M=31.51+/-0.05 to NGC 2442 based on Cepheids, while Cartier et al. (2017) find m-M=31.64+/-0.14 from SN 2015F; we adopt the former. There is substantial foreground extinction towards NGC 2442 (A_V=0.56 mag; from the Schlegel dust maps via NED). Using the most recent G-band Gaia photometry, we infer for AT2016jbu an absolute magnitude of G=-14.67 (neglecting any circumstellar or host-galaxy extinction). This absolute magnitude is consistent with that of a SN impostor (e.g. Smith et al. 2011, MNRAS, 415, 773; Tartaglia et al. 2016, ApJ, 823, L23).

We obtained a low resolution spectrum with NTT+EFOSC2+Gr13 (3985-9315A, 18A resolution) as part of the PESSTO survey on 2017 Jan 3. After a correction for foreground extinction, the spectrum is relatively blue. The dominant spectral feature is narrow Halpha emission. The line can be fit with a Lorentzian emission feature with FWHM 1900 km/s, and an absorption component blue-shifted by 800 km/s relative to the peak. Narrow NaD is also present with a P-Cygni profile, along with a large number of FeII lines at the blue wavelengths. Weak narrow Ca NIR triplet emission is possibly seen.

This spectrum is reminiscent of SN 2009ip-like events during the slow rise to their Event A peak (e.g. Pastorello et al. 2013, ApJ, 767, 1; Elias-Rosa et al. 2016, MNRAS, 463, 3894) or a few week after the Event B maximum (Fraser et al. 2013, MNRAS, 433, 1312). However, this spectrum is also similar to those of a number of SN impostors and LBV outbursts. The relatively high velocity of the absorption seen in Halpha implies a period of mass loss from a fast wind. Such velocities are typically associated with WR stars, and more rarely with LBV eruptions.

The site of AT2016jbu was observed by HST+ACS on 2006 Oct. 20 (~9 years before discovery). Images were taken with the F435W, F814W and F658N filters. By aligning to a deep R-band exposure obtained with the NTT+EFOSC2 on 2017 Nov 3, we identified the position of AT2016jbu on the HST images with an uncertainty of 1.97 pixels (99 mas). We find the transient to lie at pixel coordinates 2354.587 2818.917 in the drizzled F814W image from the Hubble Legacy Archive (hst_10803_01_acs_wfc_f814w_drz.fits).

There is a source ~1 pixel from the transient position in the ACS image (i.e. within the 1-sigma uncertainty). The pre-discovery counterpart is especially bright in Halpha. Using DOLPHOT, we measure magnitudes for the source of F435W=25.00+/-0.04, F658N=21.21+/-0.02 and 23.45+/-0.02 mag (in VEGAMAGs).

The absolute magnitudes of the pre-discovery source are B=-7.2 and I=-8.4. While bright, these are significantly fainter than the V=-10.3 mag reported for SN 2009ip (Foley et al. 2011, ApJ, 732, 32F), and could be consistent with a lower mass (<30 Msun) progenitor.

We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team.