DRAFT VEHSIOl'i OCTOBER 13, 2011 Prcprint typeset using b"TEX style emulatcapj v. ll/1O/09 A SWIFT LOOK AT SN 201lfe: THE EARLIEST ULTRAVIOLET OBSERVATIONS OF A TYPE Ia SUPERNOVA PETER J. BROWNl KYLE S. DAWSON!, lVlUJlrl;'V DE PASQUALE2, CARYL GRONWALL:U , STEPHEN HOLLAND·,.I1.7, , PAUL KUlNlO, PAOLO l'vlAZZALI!l.l2, PETER , SA;\IANTHA OATESD. & MICHAEL SIEGEL2 Draft version October 13, 2011 ABSTRACT We present the earliest ultraviolet (UV) observations of the bright Type Ia supernova SN 201lfe/PTFllkly in the nearby galaxy ?vI101 at a distance of only 6.4 ?vIpc. It was discovered shortly after explosion by the Palomar Transient Factory and first observed by Swift/UVOT about a day after explosion. The early UV light is well-defined, with ~20 data points per filter in the 5 days after explosion. With these early UV observations, we extend the near-UV template of SNe Ia to earlier times for comparison with observations at low and high redshift and report fits from semi empirical models of the explosion. We find the early UV count rates to be well fit by the superposition of two parabolic curves. Finally, we use the early UV flux measurements to examine a possible shock interaction with a non-degenerate companion. \Ve find that even a solar mass companion at a distance of a few solar radii is unlikely at more than 95% confidence. Subject headings: galaxies: distances and redshifts~ supernovae: general~ultraviolet: general L EARLY OBSERVATIONS OF TYPE Ia SUPERNOVAE 060218 (Campana et aL 2006). Promptly announced SN discoveries can also result in early Swift UV IX-ray ob Serendipitous discoveries of core-collapse supernovae servations (Roming et aL 2009; Gal-Yam et aL 2011). (SNe) at very early times have been used to determine For SNe la, early observations also offer clues into the early temperature of ejecta, the size of the SN ejecta, the nature of the progenitor system and properties of and the shock breakout, providing strong constraints the SN explosion (e.g. Foley et aL 2011). While the on the progenitors. The shock breakout of SN 2008D shock breakout itself is expected to be extremely short (Soderberg et aL 2008) was observed in the X-ray, ultra (Piro, Chang, & Weinberg 2010), early observations do violet (UV), and optical bands by the Swift spacecraft constrain the size and separation of a companion star (Gehrels et al. 2004) and the shock breakouts of several (Kasen 2010; Brown et al. 2011, hereafter KlO and Bll). SNe IIP were found after the fact in UV observations In this letter we present results from very early Swift ob from GALEX (Gezari et aL 2008). Ra.pid response ob servations of SN 201lfe in the nearby galaxy M101, the servations of SN 2006aj by Swift were automatically trig earliest UV measurements to date for a SN la. gered by the corresponding Gamma Ray Burst (GRB) 2. OBSERVATIONS 1 Department of Physics & Astronomy, University of Gtah, SN 2011fe, also known as PTFllkly, was discovered 115 Sonth 1400 East #201, Salt Lake City, UT 84112, GSA in ?vIIOl at a magnitude g=17.2, classified as a proba 2 Department of Physics and Astronomy, University of ble young la, and promptly announced by the Palomar Nevada, Las Vegas - 4505 S. Maryland Parkway, Las Vegas, NV USA Transient Factory (PTF; Law et aL 2009) on 2011 Au Ast;rorlOIT1V and Astrophysics, The Pennsyl gust 24 (Nugent et aL 2011). It was not detected by PTF Laboratory, t'niversity Park, to a limiting magnitude of 20.6 one day before, strongly constraining the explosion date. X-ray and UV obser vations were promptly requested from the Swift space- and observations began August 24.9. Swift's Ul traviolet/Optical Telescope (UVOT; Roming et aL 2005) utilized the 6 broadband filters with the following cen- tra! and full-width half of 'ni,.lAr,,;tv of Arizona. Tucson. AZ 2 several observations we changed observing modes to use will still be absorbed, but larger abundances of Fe, Co, a smaller region of the CCD read out at a faster rate Cr, Ti will be present near the photosphere. The optical (3.6 ms compared to the normal 11.0 ms frame time) lines of Fell, III, Co II, III, Ti II, CrII are expected to sat so the effects of coincidence loss could be corrected to a urate, and fluorescence via UV lines should then become higher count rate (Poole et al. 2008). Observations with possible. As the SN approaches maximum optical light. more than 0.95 counts per frame were discarded. The a decrease in temperature leads again to a reddening in use of smaller hardware windows allowed us to follow it uvw1-v color. to magnitudes of 12.4, 12.4. and 10.8, in the u, b, and SN 201lfe is significantly bluer than SN 2009ig at early v filters, respectively. In the UV, count rates are much times and follows the blue end of the SN sample. Com lower, but near peak the SN still required significant cor bined with the detection of ClI in the early spectra rections to the UV rates and some frames were saturated (Cenko et al. 2011 b), this is consistent with the obser in the uvw1 filter. vation that SNe Ia with carbon usually have bluer NUV The adopted analysis uses a 5"c ircular aperture with optical color evolution (Thomas et al. 2011; Milne et al., which to measure the source counts, and otherwise fol 2012, in preparation). lows the procedure of Brown et al. (2009). Pre-explosion It is essential to model the time evolution of SN Ia images of :\1101 taken in 2007 March/April were used luminosity through template light curve to determine to subtract the underlying galaxy count rate. The co times of maximum light, interpolate light curves. dif incidence loss corrected count rates are given in Table ferentiate between typical and atypical SNe, and define 1 along with the observed apparent magnitudes. The normal behavior for comparison with theoretical models. final data set uses over 500 individual exposures, includ The first near-UV SN Ia template (F275W filter with ing ",20 points per filter in the first 5 days after ex peak wavelength 2740 A and FWHl\I=594 A) was plosion and rv50 pre-maximum points per filter in the generated from International Ultraviolet Explorer (IUE) UV. The photometry is based on the UVOT photomet and Hubble Space Telescope CHST) observations of SNe ric system of Poole et al. (2008) to be consistent with the 1990N and 1992A (Kirshner et al. 1993) . This served existing sample of UVOT SN photometry (Brown et al. as an excellent template for early Swift/UVOT obser 2009; Milne et al. 2010, hereafter B09;MW). A Cepheid vations (Brown et al. 2005) without the stretching usu based distance modulus of 29.04 ± 0.20 (6.4 Mpc; ally required in the optical to fit individual SNe. :\110 Shappee & Stanek 2011) is assumed for the absolute improved upon this template using normal events ob magnitudes. A small reddening of E(B-V)=O.01 is as served by Swift/UVOT. Only the rapid declining SNe sumed for the Milky Way (Schlegel et al. 1998) and the 2005ke (Immler et al. 2006) and 20070n (which were not host galaxy reddening is negligible (Li et al. 2011). included in the generation of the template) show signifi cant deviations from it (MIO). 3. A~ALYSIS The early observations of SN 2011fe give us the op The excellent sampling of this data enables a detailed portunity to extend that template to earlier times. To look at the early UV behavior for the extension of tem do so, we begin with the MlO templates for uvw1 and plate light curves, an examination of the physical behav uvw2 and shift SN 2011fe in time and magnitude to give ior, and constraints on single degenerate companions. the minimum X2 between the SNe and the template. A cubic polynomial is fit to the points to give the new pre maximum template. The new uvw1 and uvw2 templates 3.1. Early UV light wrves and colors are giwn in Table 1 and displayed in the left panel of Figure 1 displays the exquisitely sampled UVOT light Figure 1. This extension is based only on one SN, but is curves of SN 201lfe. vVhile the SN had already bright consistent with other SNe Ia observed before maximum. ened to 1.5. 7 mag in the optical 1 day after explosion, rv rv The previous earliest UV observations from SN 2009ig the first two exposures in uvm2 provided only 99% upper (Foley et al. 2011:Bll), can be stretched (i.e. scaling limits at mag 19.2 (corresponding to an absolute magni the time axis) to match this new template. The stretch- tude of -9.6 and a flux density of rv 5 X 10-17 erg s-l must be done independently before and after maxi A-I). mum as in Hayden et al. (2010), as SN 2009ig rises more Sl'<e Ia have been characterized their quickly but then fades more slowly. low UV flux relative to the optical at maximum for the uvm2 filter is more difficult (Holm, Wu. & Caldwell 1974: Kirshner et al. 1993; seen in the times and light curve 2003). The observations of SN 2011£e We leave a of the unn2 of UV flux. This behav- curves to future work. but the well uvm2 Nw,,,r"Drl in :\110 to times. curves of SN 201lfe here shows the uvw1-v color excellent for other SNe. to the Sl'<e A Swift look at SN 20llfe 3 of the time since explosion squared. If the tempera three LV filters. The later component may be the rise of ture and velocity are relatively constant compared to the the reverse flourescence emission seen as the photosphere rapidly changing time since explosion, then those other recedes to layers with a more favorable composition. In terms can be assumed into a constant of proportional an attempt to simulate a possible shock breakout, we also ity. Specifically, the flux relates to the time since explo tried a second model consisting of an early bump param sion approximately as f a(t to)2 (Riess et al. 1999; eterized as a parabola with a negative amplitude super Garg et al. 2007: Ganeshalingam et al. 20l1), where t is imposed on a fireball model. However, the fit gave a X2 the observation date, to is usually taken to be the date nearly triple that of the double fireball model and was re of explosion, and a is a constant that absorbs the dis jected. As discussed by Foley et al. (20l1) for SN 2009ig, tance, temperature, velocity, and other factors. The flux the reddening of the colors is also inconsistent with a is zero for t < to. The assumptions underlying the use of cooling shock. the fireball model in the optical are not as applicable in the UV. UV SN flux does not come from the Rayleigh 3.3. The 'unseen shock from a companion Jeans tail of a blackbody spectrum~ the little flux emit ted from the thermal photosphere is mostly absorbed by The early time UV data from SN 201lfe is also impor a dense forest of absorption lines from iron-peak elements tant for what is not seen excess UV emission arising (Pauldrach et al. 1996) and most of the UV light which from the interaction between the SN explosion and the is observed results from reverse flourescence (Mazzali companion (KlO). In the single degenerate Roche-lobe 2000). We will nevertheless use the fireball model as a overflow scenario, this interaction is predicted to produce starting point for comparisons. a shock that is very bright in the first few days after The conversion from observed count rate to flux the explosion, particularly in the UV. In Bll, we used requires a spectrum-dependent conversion factor numerical and analytic models from KI0 to predict the (Poole et al. 2008) but the extreme colors of SN 20llfe luminosity of this shock as a function of viewing angle at early times are beyond those previously determined and companion separation distance. The analytic mod (Brown et al. 20l0). Since the UV SN spectrum at such els give the time dependent luminosity and temperature early times is not well understood (Foley et al. 2011) we as a function of the separation distance. From these we postpone those calculations and work with the count calculate the expected brightness of the shock in the 6 rates from Table 1. The left panel of Figure 2 shows the UVOT filters. The peak luminosity of the shock emission count rate curves in the first 4 days of observations along increases for larger separation distances (and thus larger with the best fit parabolic curves. The fit parameters stellar radii of the companion, since it assumed to fill its are given in Table 2. The UVOT b and v curves can Roche-lobe). Thus, a 1 ;\l~ evolved red giant (RG) com be fit with explosion dates of August 23.8 ± 0.15 and panion at a separation distance of 2 x 1013 cm produces 23.6 ± 0.26 respectively. These times are consistent more UV shock emission than main sequence (MS) stars. with the explosion date of August 23.69 calculated by For all companions, the maximum shock emission occurs Nugent et al. (2011b) and reported via Horesh et al. for a viewing angle of 0 degrees, corresponding to a ge (20l1). Surprisingly the fireball model is a reasonable fit ometry in which the companion lies directly in the line to the rising UV flux despite the assumptions underlying of sight between the observer and SN la. the model (Riess et al. 1999) being less supported in the Following the method of Bll, we do not attribute any UV. observed LV flux to the SN la, but instead use it as an Our optical data is consistent with the fireball modeL upper limit on the early UV flux from the shock. This extending to approximately five, and ten days past is necessary because the independent UV templates of the explosion in the u, b, and v bands. respectively, be MlO do not begin as early as these observations and be fore being saturated. As the UV fits are expanded to cause numerical simulations do not adequately match the past five days after the explosion, the quality of the fits observed LV light of SNe la (Brown et al. 2010). \Ye are drastically reduced. as the count rate rises quicker determine 95% confidence lower limits on the viewing than the extrapolated model. For example, fitting the angle for each separation distance through Monte Carlo uvm2 count rates for the exposures less than four days realizations that model the errors in the explosion date. after explosion, a to of August 23.7 ± 1.05 is found, con distance modulus, and reddening. Further sistent with the other filters. If data between 5 and 10 are found in Bll. after explosion are used for ob- the very and servations of SNe a larger tions result in tighter limits on shock a much later of 27.2.5 any S;\' Ia in Bll. As with most of the SNe la that the strictest limits come from the first observa- tions in the uvm2 filter. In the SN 2011fe the absolute uvm2>-9.6 mag at 1.2 the 23. The we distances. 3, the Table 2 for the are 177 and 179 4 (6 MS) and 2 x 1013 cm (1 Mo RG) separation dis exclude separation distances corresponding to RG com tance models considered in B11. By simple geometric panions (Brown et aL 2011), the limits from SK 201lfe arguments, the probability of the SNe Ia occurring at begin constraining separation distances down to a few those viewing angles is negligible. For even smaller com solar radii. panions, we obtain lower limits of 169 and 172 degrees These early data are a great test for the theoreti for companions separated by 0.05 x 1013 (1 MS) and cal models of the early SN explosion itself. The time 0.03 x 1013 cm (2 Mo MS), with geometric probabili and magnitudes reached are comparable to that of ties of less than for both. Thus MS companions the shock heated, expanded envelope of the WD itself with a mass greater than 2-3.5 Mo, corresponding to (PifO, Chang, & Weinberg 2010), though a more detailed the super-soft x-ray sources (Li & van den Heuvel 1997; understanding of the early lJV light is needed to dis Podsiadlowski 2010), are extremely unlikely. These lim entangle different effects that may have been observed its on the companion are stricter than that obtained from for the first time. Combining these data with observa pre-explosion imaging by the Hubble Space Telescope tions across the electromagnetic spectrum (Kugent et al. (Li et aL 2011), showing the great power of these lJV 2011b; Horesh et aL 2011; Marion 2011; Smith et al. observations to studying the systems. The companion 2011) will make SN 201lfe the best studied SN Ia ever. star could still be non-degenerate if the companion can shrink well within its Roche-lobe limit before the time of We are especially grateful to the Palomar Transient explosion (Justham 2011). Further modeling is required Factory for promptly announcing this exciting object to determine the geometry of such systems that would and to Eran Ofek for initating the first Swift observa still be undetected with these deep, early UV data. tions. This work at the University of Utah is supported by NASA grant NNXlOAK43G, through the Swift Guest 4. SUMMARY Investigator Program. This work is sponsored at PSU by The early detection of SN 2011fe at such a close dis NASA contract NAS5-00136. The Institute for Gravita tance and the rapid response of Swift resulted in ex tion and the Cosmos is supported by the Eberly College tremely early, sensitive, and densely sampled UV mea of Science and the Office of the Senior Vice President surements. This allows us to extend the NUV template for Research at the Pennsylvania State University. SRO to a day after explosion. The early flux seems to fol and NPK gratefully acknowledge the support of the UK Iowa parabolic rise as suggested by the fireball model, Space Agency. This analysis was made possible by ac though separate rises can be fit to the first five days and cess to the public data in the Swift data archive and the period five to ten days after explosion. The low lJV the NASAjIPAC Extragalactic Database (KED). 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J. 525 Horesh. A., et 2006. ATEL 3612 A Swift look at SN 2011£e 5 o 10 20 30 Days ~rorr explosio'1 Julian Date (24S(]()(Xl+) FIG. 1. Left: UVOT light curves of SN 2011£e in Vega magnitudes. The MID templates for uvwl and uvw2 are overplotted with dashed lines and the new pre-maximum templates derived from these observations are marked with dotted lines. Right: uvwl-v color curves for a sample of normal SNe observed within 10 days of explosion. "0.00 , , .co 'C'<."Jl. 8 C'"" " u0 u0 6 N E O. .~u . 797 798 799 800 80' 802 803 5800 5802 580~ 58C6 JD 2115500 ~) 2"5000C FIG. 2. Left: Early count rates in the 6 UVOT filters along with the best fit parabolic curves. Right: Early count rates in the uvm2 filter fit with a two component fireball model. =-':::: soon. Right: legend in explosion). 6 TABLE 1 SN2011FE PHOTOMETRY Filter JD-2450000 Mag Count Rate uvn12 5798.44 >19.15 0.04 ± 0.04 uvn12 5798.51 >19.17 0.02 ± 0.04 tlvm2 5799.51 18.61 ± 0.35 0.19 ± 0.06 uvm2 5799.64 18.59 ± 0.31 0.2 ± 0.05 uvm2 5799.72 18.81 ± 0.36 0.16 ± 0.05 uvm2 5799.9 18.04 ± 0.28 0.33 ± 0.08 NOTE. The full table of photometry is available in the electronic version. TABLE 2 EARLY Comn RATE FITS Filter Range O!l to 0!2 to,2 uvw2 1-4 days 0,18 ± 0.04 5797,52 ± 0.50 uvm2 1-4 days 0,03 ± 0,02 5797,33 ± 1.05 uv\vl 1-4 days 0,78 ± 0,19 5797,44 ± 0.53 11 1-4 days 8.68 ± 1.77 5797,34 ± 0.42 b 1-4 days 18,50 ± 4,04 5797,29 ± 0.45 1-4 6,69 ± 0.8:3 5797,12 ± 0.26 uvw2 5-10 0.88 ± 0,09 5800.03 ± 0.23 uvm2 5-10 days 0,31 ± 0,06 5800,77 ± 0.44 uvwl ,5-10 days 4,50 ± 0.82 5800.09 ± 0.39 uvvv2 1-10 days 0.20 ± 0,02 5797,64 ± 0.23 0.96 ± 0,15 5801.70 ± 0.45 uvm2 1-10 days 0.03 ± 0,01 ,5797.38 ± 0.59 0.43 ± 0.09 5802.19 ± 0,57 nY'wl 1-10 da)·s 0.97 ± 0.10 5797.70 ± 0.25 5.11 ± 0,91 5801.59 0.51 the number of fit parameters (P)