EARLY ULTRAVIOLET OBSERVATIONS of a TYPE Iin SUPERNOVA (2007Pk)

The Astrophysical Journal, 750:128 (8pp), 2012 May 10 doi:10.1088/0004-637X/750/2/128 C 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A. EARLY ULTRAVIOLET OBSERVATIONS OF A TYPE IIn SUPERNOVA (2007pk) T. A. Pritchard1,2,P.W.A.Roming1,2,P.J.Brown3,N.P.M.Kuin4, Amanda J. Bayless2, S. T. Holland5,6,S.Immler7,8,9,P.Milne10, and S. R. Oates4 1 Department of Astronomy & Astrophysics, Penn State University, 525 Davey Lab, University Park, PA 16802, USA; [email protected] 2 Southwest Research Institute, Department of Space Science, 6220 Culebra Rd, San Antonio, TX 78238, USA 3 Department of Physics & Astronomy, University of Utah, 115 South 1400 East 201, Salt Lake City, UT, USA 4 Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK 5 Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD 20771, USA 6 Code 660.1, NASA/GSFC, Greenbelt, MD 20771, USA 7 Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 8 Center for Research and Exploration in Space Science and Technology, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 9 Department of Astronomy, University of Maryland, College Park, MD 20742, USA 10 Steward Observatory, 933 North Cherry Avenue, RM N204, Tucson, AZ 85721, USA Received 2011 June 16; accepted 2012 March 1; published 2012 April 24 ABSTRACT We present some of the earliest UV observations of a Type IIn supernova (SN)—SN 2007pk, where UV and optical observations using Swift’s Ultra-Violet/Optical Telescope began 3 days after discovery or ∼5 days after shock breakout. The SN observations commence at approximately maximum light in the UV and u-band filters, suggesting that the UV light curve peaks begin very rapidly after the initial explosion, and subsequently exhibit −1 a linear decay of 0.20, 0.21, 0.16 mag day in the UVOT uvw2, uvm2, uvw1(λc = 1928, 2246, 2600 Å) filters. Meanwhile the b- and v-band light curves begin approximately seven days before v-band peak and exhibit a shallow rise followed by a subsequent decay. A series of optical/near-IR spectra taken with the Hobby–Eberly Telescope at days 3–26 after discovery show spectra similar to that of the peculiar Type IIn 1998S. The emission from 2007pk falls below detection ∼20 days after discovery in the UV and 50 days in the optical, showing no sign of the long duration emission seen in other Type IIn SNe. We examine the physical and spectral characteristics of 2007pk and compare its UV light curve and decay rate with other Type II SNe. Key words: supernovae: general – supernovae: individual (SN 2007pk) – ultraviolet: general Online-only material: color figures 1. INTRODUCTION the IIn SNe’s hydrogen-rich spectra and exceptional brightness (Gal-Yam et al. 2007). Supernovae (SNe) are observationally classified by their The Swift UV/Optical observations of 2007pk which we spectra and may be broadly placed into two categories: Type I present here are some of the earliest phase UV observations SNe which have no hydrogen lines in their spectra, and Type II of a IIn SN. Follow-up optical spectroscopic observations are SNe which do (Filippenko 1997). obtained to allow us to examine the physical characteristics of Type II subclasses include the Type IIP (“plateau”), the system in concert with the photometric data. We go on to IIL (“linear”), and IIn (“narrow”) SNe. Type IIP SNe are distin- offer comparison with other UV-bright Type II SNe observed in guished by the long plateau phase in their optical light curves the UV in order to both give context to how this observed IIn which have been observed out to hundreds of days. These are light curve fits in with the taxonomy of UV–SNe. arguably the most common subtype of all SNe, accounting for 2. OBSERVATIONS ∼40% of all SNe observed in a recent volume limited sample (Lietal.2011). Type IILs are distinguished by the linear decline SN 2007pk was discovered on 2007 November 10.31 UT in their optical light curves and are some of the most rare SNe. (JD 2454414.81; Parisky & Li 2007). Previous observations of They have been seen both with and without broad P-Cygni pro- this field on November 5.33 (JD 2454409.83) by the discovery files in their spectra. Current theory suggests that IIPs originate team yield only upper limits. We estimate the shock breakout from the core collapse of red supergiants, while the progenitors to have occurred between these dates and for the purposes for IILs are more uncertain; perhaps blue supergiants or binary of this paper define it as the midpoint of these observations systems (Smartt 2009). (JD 2454412.2 ± 2.5). The Type IIn SNe subclass (Schlegel 1990) are those core- Observations with the Swift (Gehrels et al. 2004) Ultra- collapse SNe whose expanding ejecta exhibits interaction with Violet/Optical Telescope (UVOT; Roming et al. 2000, 2004, a strong circumstellar wind given off by the SNe progenitor. 2005) began on November 13.16 (2.85 days after discovery) These objects are characterized by strong emission lines, most using three optical (u, b, v) and three UV filters (uvw2, uvm2, prominently Hα, and a lack of broad absorption lines. Typically uvw1: λc = 1928, 2246, 2600 Å, respectively; Poole et al. only narrow P-Cygni profiles are visible in the hydrogen lines 2008). SN 2007pk fell below the detection limit in the UV after that are superimposed on top of a broader emission component, day 23 of the campaign, and the u band was visible until day 47. and the continuum is well characterized by a blackbody. Type IIn Optical detections in the b and v bands were observed until day SN progenitors are also uncertain, but observations of the 56 where we had our first u-band upper limit which then caused brightest objects favor luminous blue variables (LBVs) due to us to terminate the campaign since we had lost detection of any 1 The Astrophysical Journal, 750:128 (8pp), 2012 May 10 Pritchard et al. 13 uvw2 uvm2 14 uvw1 u b 15 v 16 Vega m 17 18 Observed 19 1.5 1.0 0.5 vega 0.0 uvw2−uvm2 m uvm2−uvw1 Δ −0.5 uvw1−u u−b −1.0 b−v −1.5 0 10 20 30 40 50 60 Days Since Discovery Figure 1. Left top: observed UVOT light curves of SN 2007pk (galaxy subtracted, though no extinction or k-corrections have been applied). Left bottom: UV and optical colors—the dashed line corresponds to the best-fit line. (A color version of this figure is available in the online journal.) Table 1 Observed UVOT Photometry Time Vega Mag (JD 2450000+) uvw2 uvm2 uvw1 ub v 4417.66 14.50 ± 0.06 4418.64 14.54 ± 0.05 14.47 ± 0.05 14.65 ± 0.05 14.90 ± 0.05 16.27 ± 0.07 16.27 ± 0.05 4420.21 14.91 ± 0.06 14.70 ± 0.05 14.78 ± 0.05 14.94 ± 0.05 16.22 ± 0.06 16.20 ± 0.05 4420.68 15.14 ± 0.05 14.83 ± 0.05 14.87 ± 0.05 15.06 ± 0.05 16.21 ± 0.06 16.21 ± 0.05 4421.68 15.44 ± 0.06 15.11 ± 0.05 15.07 ± 0.05 15.07 ± 0.06 16.25 ± 0.06 16.14 ± 0.05 4425.07 16.03 ± 0.06 15.72 ± 0.05 15.48 ± 0.06 4429.65 16.91 ± 0.08 16.71 ± 0.08 16.32 ± 0.07 15.56 ± 0.05 16.39 ± 0.06 16.17 ± 0.08 4433.90 17.85 ± 0.14 17.57 ± 0.13 17.01 ± 0.09 16.00 ± 0.06 16.53 ± 0.06 16.39 ± 0.06 4436.95 18.42 ± 0.21 18.51 ± 0.27 17.64 ± 0.14 16.32 ± 0.07 16.69 ± 0.06 16.39 ± 0.27 4440.80 19.00 ± 0.34 18.07 ± 0.19 16.77 ± 0.08 16.81 ± 0.07 16.44 ± 0.08 4451.68 17.65 ± 0.15 17.39 ± 0.09 16.81 ± 0.10 4462.02 18.66 ± 0.30 17.89 ± 0.12 16.95 ± 0.11 4474.10 18.32 ± 0.17 17.18 ± 0.11 mpeak (mag) <14.54 <14.47 <14.50 <14.82 16.15 ± 0.07 15.95 ± 0.164 tpeak (JD 2450000 + ) <4418 ± 2.5 <4418 ± 2.5 <4418 ± 2.5 <4418 ± 2.5 4420.5 ± 1 4424.5 ± 1 trise (days) <5.8 ± 2.5 <5.8 ± 2.5 <5.8 ± 2.5 <5.8 ± 2.5 8.3 ± 3.7 12.3 ± 3.6 k-correction 0.04 0.03 0.02 −0.03 −0.03 0.01 Extinction 1.54 1.54 1.26 0.93 0.75 0.58 Host (LMC) + MW UV emission. A median cadence of four days was used with (2009). The data reduction pipeline used the HEASOFT 6.6.3 time between observations increasing after the non-detection of and Swift Release 3.3 analysis tools with UVOT zero points from UV emission (Table 1). A later observation on 2009 November Poole et al. (2008) and updated calibrations from Breeveld et al. 24 was made after the SN had faded and was no longer detected (2010). The corresponding light curve is shown in Figure 1. for use as a galaxy subtraction template. Photometry using a To determine the peak magnitude (mPeak) and time of peak 5 3 source aperture, including template galaxy flux subtraction, (tPeak) in each filter, 3 × 10 Monte Carlo simulations fitting a was performed following the method outlined in Brown et al.

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