A strong ultraviolet pulse from a newborn type Ia supernova

Abstract

'Discovery'). No activity had been detected at the same location in the images taken on the previous night and earlier, indicating that the SN likely exploded between May 2.29 and 3.29. Our follow-up spectroscopic campaign (See Extended Data Table 1 for the observation log) established that iPTF14atg was a Type Ia supernova (SN Ia). 3Upon discovery we triggered observations with the Ultraviolet/Optical Telescope (UVOT) and the X-ray Telescope (XRT) onboard the Swift space observatory 11 (observation and data reduction is detailed in Methods subsection 'Data acquisition'; raw measurements are shown in Extended Data Table 2). As can be seen in Figure 1, the UV brightness of iPTF14atg declined substantially in the first two observations. A rough energy flux measure in the UV band is provided by ν f ν ≈ 3×10 −13 ergs cm −2 s −1 in the uvm2 band. Starting from the third epoch, the UV and optical emission began to rise again in a manner similar to that seen in other SNe Ia. The XRT did not detect any X-ray signal at any epoch (Methods subsection 'Data acquisition'). We thus conclude that iPTF 14atg emitted a pulse of radiation primarily in the UV band. This pulse with an observed luminosity of L UV ≈ 3×10 41 ergs s −1 was probably already declining by the first epoch of the Swift observations (within four days of its explosion).Figure 1 also illustrates that such an early UV pulse from a SN Ia within four days of its explosion is unprecedented 12,13 . We now seek an explanation for this early UV emission.As detailed in Methods subsection 'Spherical models for the early UV pulse', we explored models in which the UV emission is spherically symmetric with the SN explosion (such as shock cooling and circumstellar interaction). These models are unable to explain the observed UV pulse. Therefore we turn to asymmetric models in which the UV emission comes from particular directions.A reasonable physical model is UV emission arising in the ejecta as the ejecta encounters a companion 9,14 . When the rapidly moving ejecta slams into the companion, a strong 4 reverse shock is generated in the ejecta that heats up the surrounding material. Thermal radiation from the hot material, which peaks in the ultraviolet, can then be seen for a few days until the fast-moving ejecta engulfs the companion and hides the reverse shock region. We compare a semi-analytical model 9 to the Swift/UVOT lightcurves. For simplicity, we fix the explosion date at May 3. We assume that the exploding white dwarf is close to the Chandrasekhar mass limit (1.4 solar mass) and that the SN explosion energy is 10 51 ergs. These values lead to a mean expansion velocity of 10 4 km s −1 for the ejecta. Since the temperature at the collision location is so high that most atoms are ionized, the opacity is probably dominated by electron scattering. To further simplify the case, we assume that the emission from the reverse shock region is blackbody and isotropic. In order to explain the UV lightcurves, the companion star should be located 60 solar radii away from the w...

The Extremes of Thermonuclear Supernovae

“…What is used here is a somewhat wider reading of the term compared to White et al (2015), including also SNe that do not exhibit the same very low ejecta velocities as SN 2002es. With this definition the class encompasses SN 2002es itself and SN 1999bh (Li et al, 2001a;Ganeshalingam et al, 2012), SN 2010lp , Taubenberger et al 2013b), PTF 10ops (Maguire et al, 2011), iPTF 14atg (Cao et al, 2015;Kromer et al, 2016), PTF 10ujn and PTF 10acdh (White et al, 2015), and arguably SN 2006bt (Hicken et al, 2009;Ganeshalingam et al, 2010;Foley et al, 2010). White et al (2015) have shown that 02es-like SNe are clearly distinct from SNe Iax in a number of key properties.…”

Constraining the progenitor of the Type Ia Supernova SN 2012cg

“…Also, it should be kept in mind that we assume the actual explosion date of SN 2011fe was indeed on MJD=55796.696 (Nugent et al 2011) through this study. Given this estimated excess UV luminosity, following Cao et al (2015) one can further calculate a spherical radius of the CSM of R CSM ≈ 1.0 × 10 16 cm (or R CSM ≈ 9.0 × 10 15 cm), assuming a gas temperature of 15 000 K (or 20 000 K), leading to a CSM mass of…”

“…Therefore, the interaction of the SN shock with the CSM might provide an alternative explanation for the early blue colour seen in SN 2012cg (Marion et al 2016). Here, we adopt the analytical CSM-interaction model used in Cao et al (2015) to calculate the radius and mass of the CSM to examine the possibility of the early UV excess of SN 2012cg having CSM-interaction as its origin.…”

“…Therefore, early UV observations are proposed as a direct way to test progenitor models of SNe Ia. The analysis of observed early light-curves has been carried out for many nearby SNe Ia to look for evidence of these shock emissions (Hayden et al 2010;Brown et al 2012;Cao et al 2015;Marion et al 2016;Olling et al 2015), but only a sublumious SN Ia (iPTF14atg, Cao et al 2015) and one normal event (SN 2012cg, Marion et al 2016) have been reported as likely detections of the signatures of the interaction between SN ejecta and a stellar companion. Interestingly, Marion et al (2016) have shown that the analytical model of Kasen (2010) with a 6.0 M ⊙ MS companion star could provide an explanation for the excess blue luminosity detected in SN 2012cg at about three days after the explosion.…”

“…It has been widely proposed that very early-time observations of SNe Ia can be used to place strong constraints on their progenitor systems (e.g., Kasen 2010;Nugent et al 2011;Rabinak, Livne & Waxman 2012, hereafter RLW12;Brown et al 2012;Cao et al 2015;Olling et al 2015;Marion et al 2016). In particular, Kasen (2010) suggested that the strong excess emission can arise from the collision of SN ejecta with its companion star in the SD progenitor system, and it should be detectable in the ultraviolet (UV) and blue optical bands in the first few hours to days after the SN explosion under favorable viewing angles.…”