X-ray Emission from SN 2012ca: A Type Ia-CSM Supernova Explosion in a Dense Surrounding Medium
C. D. Bochenek, Vikram. V. Dwarkadas, Jeffrey M. Silverman, Ori D. Fox, Roger A. Chevalier, Nathan Smith, Alexei V. Filippenko
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X-ray Emission from SN 2012ca: A Type Ia-CSMSupernova Explosion in a Dense Surrounding Medium
Christopher D. Bochenek, , ⋆ , V. V. Dwarkadas † , Jeffrey M. Silverman , Ori D. Fox ,Roger A. Chevalier , Nathan Smith , and Alexei V. Filippenko , Department of Astronomy and Astrophysics, University of Chicago, 5640 S Ellis Ave, Chicago, IL 60637, USA Current address: Astronomy Department, California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USA Department of Astronomy, University of Texas, Austin, TX 78712, USA Space telescope Science Institute, Baltimore, MD 21218, USA Department of Astronomy,University of Virginia, Charlottesville, VA 22903, USA Steward Observatory, 933 N. Cherry Ave., Tucson, AZ 85721, USA Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA Senior Miller Fellow, Miller Institute for Basic Research in Science, University of California, Berkeley, CA 94720, USA
ABSTRACT
X-ray emission is one of the signposts of circumstellar interaction in supernovae (SNe),but until now, it has been observed only in core-collapse SNe. The level of thermalX-ray emission is a direct measure of the density of the circumstellar medium (CSM),and the absence of X-ray emission from Type Ia SNe has been interpreted as a sign of avery low density CSM. In this paper, we report late-time (500–800 days after discovery)X-ray detections of SN 2012ca in
Chandra data. The presence of hydrogen in the initialspectrum led to a classification of Type Ia-CSM, ostensibly making it the first SN Iadetected with X-rays. Our analysis of the X-ray data favors an asymmetric medium,with a high-density component which supplies the X-ray emission. The data suggesta number density > cm − in the higher-density medium, which is consistent withthe large observed Balmer decrement if it arises from collisional excitation. This is highcompared to most core-collapse SNe, but it may be consistent with densities suggestedfor some Type IIn or superluminous SNe. If SN 2012ca is a thermonuclear SN, thelarge CSM density could imply clumps in the wind, or a dense torus or disk, consistentwith the single-degenerate channel. A remote possibility for a core-degenerate channelinvolves a white dwarf merging with the degenerate core of an asymptotic giant branchstar shortly before the explosion, leading to a common envelope around the SN. Key words: shock waves; circumstellar matter; stars: mass-loss; supernovae: general; supernovae:individual: SN 2012ca; X-rays: individual: SN 2012ca
Thermal X-ray emission is one of the clearest indicationsof circumstellar interaction in supernovae (SNe; Chevalier1982; Chevalier & Fransson 1994). The intensity of the emis-sion depends on the square of the density, and thus is a goodestimator of the density of the ambient medium, as long asthe emission is not absorbed by the medium. So far, over 60SNe have been detected in X-rays (Dwarkadas & Gruszko2012; Dwarkadas 2014). All of them are core-collapse SNe,where the circumstellar medium (CSM) is formed by mass ⋆ E-mail: [email protected] † E-mail: [email protected] loss from the progenitor star. Until now, no Type Ia SNhas been detected in X-rays. Deep limits on the emissionfrom SNe Ia in the radio (Chomiuk et al. 2016) and X-ray(Margutti et al. 2014) bands indicate a very low mass-lossrate ( − M ⊙ yr − ) for the stellar progenitor system,suggesting that SNe Ia are surrounded by a very low den-sity CSM, if they even have one.It is generally accepted that the progenitor of a SN Iamust be a white dwarf. In order to raise the mass of thiswhite dwarf to nearly the Chandrasekhar limit to producean explosion, it must have accreted mass transferred froma companion in a binary system. The nature of the com-panion is hotly debated (e.g., Ruiz-Lapuente 2014), withevidence existing for both double-degenerate and single- c (cid:13) Bochenek, C. D., Dwarkadas, V. V., Silverman, J. M., et al. degenerate systems (Maoz et al. 2014). In the former case,the companion is another white dwarf (Scalzo et al. 2010;Silverman et al. 2011; Nugent et al. 2011; Bloom et al.2012; Brown et al. 2012), while in the latter case it is a mainsequence or evolved star (Hamuy et al. 2003; Deng et al.2004). It is likely that both channels are present.Some SNe Ia exhibit narrow hydrogen lines super-imposed on a SN Ia-like spectrum (Hamuy et al. 2003;Deng et al. 2004). The narrow line width suggests velocitiesmuch lower than that of the expanding shock wave, and aretherefore presumed to arise in the surrounding medium. Thisindicates the presence of an ambient medium, perhaps aris-ing from mass loss from the companion star. These SNe com-prise the subclass of Type Ia-CSM. Silverman et al. (2013)composed a list of several common features of SNe Ia-CSM.The absolute magnitudes of SNe Ia-CSM are larger thanthose of normal SNe Ia. They even exceed those of mostSNe IIn, whose spectra show relatively narrow lines (hencethe “n” designation; see Filippenko (1997) for a review) ona broad base. The spectra of SNe Ia-CSM consist of a SN Iaspectrum diluted by relatively narrow hydrogen lines, a bluecontinuum created from many overlapping broad lines fromiron-group elements, and strong H α emission with width ∼ − . The H α profile varies for ∼
100 days post-explosion before steadily increasing. He I and H β emissionare observed, but are relatively weak. SNe Ia-CSM havelarger Balmer decrements (the ratio of the intensity of theH α to H β lines) than the typical value under interstellarconditions of ∼ .
86, most likely resulting from emission dueto collisional excitation rather than recombination, suggest-ing high-density CSM shells that are collisionally excitedwhen the faster-moving SN ejecta overtake them. SNe Ia-CSM have never been detected at radio wavelengths, andthis work represents the first detection of a SN Ia-CSM inX-rays. The host galaxies of SNe Ia-CSM appear to be spi-ral galaxies having Milky-Way-like luminosities with solarmetallicities, or irregular dwarf galaxies similar to the Mag-ellenic Clouds with subsolar metallicities.Since SNe Ia-CSM exhibit characteristic features ofSNe Ia spectra with H lines that are the defining aspect ofType II SNe, there is still debate on their exact nature. Somesuggest that they are odd core-collapse SNe masqueradingas a SN Ia (Benetti et al. 2006). However, there are indica-tions that some SNe Ia-CSM are likely thermonuclear ex-plosions interacting with a CSM. PTF11kx is by most ac-counts a SN Ia produced from the single-degenerate channel(Dilday et al. 2012; Silverman et al. 2013). Observations ofPTF11kx show the evolution of its spectrum to be similarto that of SN 1999aa, a SN Ia. It appears to have a multi-component CSM, with no signs of a CSM in the early-timeobservations. There exists faster-moving material closer tothe SN, and shells of CSM (Dilday et al. 2012) expandingoutward in radius from the SN. These features are inter-preted by the authors as being consistent with recurrentnova eruptions, and thus suggest a thermonuclear explosionof a white dwarf through the single-degenerate channel.SN 2012ca reignited the debate about whether allSNe Ia-CSM are thermonuclear explosions. While a ther-monuclear progenitor is debated for SN 2012ca, its spec-tral classification is Type Ia with superimposed relativelynarrow H lines. Inserra et al. (2014) argue that SN 2012cais a core-collapse SN based on the identification of several intermediate-mass elements such as C, Mg, and O in its neb-ular spectrum. Furthermore, Inserra et al. (2016) point outthat SN 2012ca is likely a core-collapse SN on the basis ofenergetics, suggesting that the conversion of kinetic energyinto luminosity must be between 20% and 70%, an unusu-ally large value. Fox et al. (2015) argue that the C, Mg, andO features seen by Inserra et al. (2014) are misidentified; in-stead, they suggest that those of Mg and O are actually ironlines, and that of C is the Ca II near-infrared (IR) triplet.Based on the lack of broad C, O, and Mg in the spectrumof SN 2012ca and the presence of broad iron lines, Fox et al.(2015) conclude that SN 2012ca is more consistent with athermonuclear rather than a core-collapse explosion. Theyfurther point out that the high efficiency required for SN2012ca is within the realm of possibility.The goal of this paper is to further study the nature ofSN 2012ca via its X-ray emission. Through the analysis ofthe X-ray data, we shed light on the density of the ambientmedium and therefore the nature of the progenitor system.This paper is structured as follows. In §
2, we summarizethe X-ray data and analysis. In §
3, we use the optical datato estimate the SN kinematics. In §
4, we elaborate on thereduction and analysis of the spectra. An estimate of thedensity of the CSM, for both a homogeneous and a clumpymedium, is presented in §
5. Finally, § SN 2012ca was discovered in the late-type spiral galaxy ESO336-G009 on 2012 April 25.6 (Drescher et al. 2012). TheNASA/IPAC Extragalactic Database (NED) gives the lu-minosity distance to the galaxy as 80.1 Mpc for a value ofH = 73 km s − Mpc − , Ω m = 0 .
27, and Ω Λ = 0 .
73. We ob-served SN 2012ca with the
Chandra
Advanced CCD ImagingSpectrometer (ACIS); see Table 1 for details. Two observa-tions of 20 ks each were made about 6 months apart: ObsID15632 (2013 September 17, hereafter epoch 1) and 15633(2014 March 27, hereafter epoch 2). Using an explosion dateof MJD 55998 . ±
20 (Inserra et al. 2016), these epochs occur554 days and 745 days after the explosion, respectively. Allepochs in this paper will be referenced from this explosiondate, and all dates are listed in Universal Time (UT).The data were analysed using
Chandra
InteractiveAnalysis of Observations (CIAO) version 4.7 and CalDB 4.8.The source and background regions were each taken to be4 ′′ radius, which contains 90% of the Chandra point-spreadfunction. Analysis and fitting were done using CIAO andSherpa. Because the number of counts in each observationis low compared to the background, fitting methods involv-ing minimizing the χ statistic are invalid. Instead, we usedthe “cstat” statistic, corresponding to the maximum likeli-hood function of the Poisson distribution, and fitted boththe data and background together (Cash 1979). While weused ungrouped spectra in our analysis, the grouped spectrafor epochs 1 and 2 are shown in Figure 1.We use the two optical spectra of SN 2012ca publishedby Fox et al. (2015) at 486 and 508 days after the explo-sion. The most recent spectra, on days 522.8, 548.9, 580.8,were obtained from WISeREP (Yaron & Gal-Yam 2012) aspart of the PESSTO SSDR2 (Smartt et al. 2015). We thank c (cid:13) , 1–11 -rays from SN 2012ca Table 1.
Summary of X-ray data on SN 2012ca, listing the satellite and instrument which took the obser-vation, the date, the age (days after explosion), the exposure time, the count rate, the column density, thederived temperature, and the unabsorbed flux, with 1 σ error bars where available.Instrument Obs Date Days After Exposure Count Rate N H kT − (10 (keV) Flux (10 − counts s − ) cm − ) erg s − cm − ) Chandra
ACIS 2013-09-17 554 20.0 1.4 ± .
29 6.0 ± . . ± . . +389 − . Chandra
ACIS 2014-03-27 745 20.0 0 . +0 . − . . ± . . ± . . +104 . − . Dr. Cosimo Inserra for providing the rest of the optical spec-tra of SN 2012ca published by Inserra et al. (2014).
The velocity of the fast-moving gas is required for sub-sequent analysis. To determine this, we fit a cubic splinefunction to each H α profile in the optical spectra. Thisspectral fitting algorithm is similar to the one used bySilverman et al. (2012). We specifically recorded the veloc-ity of the peak of the H α profile, as well as the blue-sidewidth at zero intensity (BSWZI). All of the H α peaks havemeasured velocities consistent with 0 km s − (in the restframe of the host galaxy), and the velocity of the peak doesnot appear to evolve with time. As shown in Figure 2, theBSWZI is also measured to be approximately constant withtime and has a value of 3200 ±
300 km s − .It is not clear if the BSWZI is representative of the shockvelocity, since other factors could contribute to the broad-ening of the line. Electron scattering is one such factor, andhas been shown to be relevant in SNe such as SN 2010jl(Fransson et al. 2014). However, it is unlikely that electronscattering could be important as late as 530 days, when thelast optical spectra were obtained. The BSWZI also does notappear to change substantially in the first 500 days. Fur-thermore, the column density that we infer from the X-rayobservations, although large, is still two orders of magni-tude lower than that inferred for SN 2010jl (Fransson et al.2014; Chandra et al. 2015), and would not lead to large elec-tron scattering depths. Finally, electron scattering results inLorentzian line shapes, whereas the line shapes here are bet-ter fitted by a Gaussian. Thus, we do not believe that elec-tron scattering plays a large role. Other factors such as lineblending or contamination from the reverse shock could bepresent. Therefore, although there is evidence of gas mov-ing at velocities as high as 3200 km s − , we cannot easilyassume that this represents the shock velocity.The Gaussian shape of the lines indicates that there ismaterial in the SN moving at all velocities from +3500 kms − to − − . We have also measured the full widthat half-maximum intensity (FWHM) of the line from thetwo high-resolution spectra that we have, on days 436.2 and458.1. The FWHM of the H α line is around 1300 km s − .The other spectra have low resolution, and measuring theFWHM is quite unreliable, even when corrected for instru-ment resolution. It is possible that the lower velocity is moreindicative of the velocity of the bulk of the gas. We addressthis issue again in § There are several indications of a high density in the sur-rounding medium: (1) the large Balmer decrement, whichranges from 3 to 20 for SN 2012ca, and can potentially beexplained by collisional excitation rather than recombina-tion, as for other Type Ia-CSM; (2) the slow velocity ofthe SN shock wave even at early times ( . − ),compared to general SN Ia velocities in excess of 10 kms − (Wang et al. 2009); and (3) the high observed X-rayluminosity (see below) at a deduced redshift of z = 0 . z = 0 . N H of 2 . × cm − and a kT of0.38 keV, while the latter has an N H of 4 . × cm − and a kT of 1.31 keV. To address this problem, we explorethe likelihood parameter space with a modified Levenberg-Marquardt algorithm (Mor´e 1979) to find many local min-ima which each provide a good fit to the data and are in-disinguishable on statistical grounds. All such statisticallyindistinguishable local minima are shown in Figure 3, whichwas computed in the following manner. We first calculate theobserved (absorbed) flux with 90% confidence error bars be-tween 0.5 and 7 keV directly from the data using the CIAO srcflux command. At the first epoch the absorbed flux is1 . . − . × − erg cm − s − , and at the second epochthe flux is 2 . +2 . − . × − erg cm − s − . We then at-tempted to fit the “vmekal” model to the X-ray spectrum7000 different times. Each fit had different initial guessesfor N H and kT . The initial guesses for N H ranged from0 . × cm − to 10 × cm − . The initial guessesfor kT ranged from 0.1 keV to 25 keV. We placed a lowerbound on the column density ( N H ) of 0 . × cm − ,which is the Galactic column density in the direction of SN c (cid:13) , 1–11 Bochenek, C. D., Dwarkadas, V. V., Silverman, J. M., et al.
Epoch 1
Energy (keV) C oun t s / s e c / k e V Epoch 2
Energy (keV) C oun t s / s e c / k e V −0.0001−5e−0505e−050.00010.000150.0002 Figure 1.
Left: X-ray spectrum at epoch 1. The data are grouped such that there are 4 counts per bin. Right: Spectrum for epoch 2,also grouped into 4 counts per bin. Although the grouped spectra are shown for clarity, ungrouped versions of these spectra were usedin the analysis.
100 200 300 400 500Observer−Frame Age (d)3000350040004500 B l u e − S i d e W i d t h a t Z e r o I n t e n s it y ( k m / s ) Figure 2.
Blue-side width at zero intensity (BSWZI) of the H α line vs. time since explosion. The mean velocity indicated is ∼ − . − .For each fit, the model-dependent absorbed flux wascomputed using CIAO’s calc energy flux function. We re-jected all fits for which the model-dependent absorbed fluxwas not within the 90% confidence interval of the model-independent flux. We then visually inspected the fits whichpassed this criteria to ensure that the model did not producefeatures which massively overestimated or underestimatedthe data in any energy range, and rejected all fits that gavea reduced test statistic exceeding 1. The final N H and kT values for acceptable fits are shown in Figure 3, which wetreated as the parameter space of suitable values of N H and kT . From Figure 3, fits to the X-ray spectrum for the secondepoch cluster at low kT and high N H . In contrast, the firstepoch appears to have values that occupy two distinct re-gions: a region with low N H and high kT , and a region withhigh N H and low kT . This provides a clue to understandingthe true best fits — since there is no obvious mechanism toadd more material to the CSM as the SN evolves, we would expect the second epoch to have a lower value of N H thanthe first, suggesting that the true N H at the first epoch mustbe high, combined with a low kT .Another clue to the correct fitting parameters is pro-vided by the observed data. We can compute the elec-tron number density n e directly from the X-ray luminos-ity L = n e Λ V , given a cooling function Λ and the volumeof the emitting region V . Here we compute the minimumpossible density and infer a constraint on N H . As a firstguess, the minimum unabsorbed luminosity L is calculatedfrom the unabsorbed flux, which we determine from the ob-served count rate using the Galactic N H , the lowest possi-ble value of N H . This is done with the Portable, InteractiveMulti-Mission Simulator (PIMMS ). Because unabsorbedflux and thus density decreases with temperature, a temper-ature of 12 keV is used as a conservative value to minimizethe density calculation. The estimated unabsorbed fluxes are2 . +1 . − . × − erg cm − s − for epoch 1 and 2 . +2 . − . × − erg cm − s − for epoch 2. This procedure gives luminositiesof 1 . +0 . − . × erg s − for epoch 1, and 2 . +1 . − . × erg s − for epoch 2. Note that Λ is an approximation of http://cxc.harvard.edu/toolkit/pimms.jspc (cid:13) , 1–11 -rays from SN 2012ca kT [keV] N H [ c m − ] Epoch OneEpoch Two
Figure 3.
The red points (epoch 2) and blue points (epoch 1)represent acceptable X-ray spectral fit values for the column den-sity ( N H ) and temperature ( kT ) for SN 2012ca, using the methodoutlined in the text. The points to the right of the dashed lineare excluded from parameter estimation for reasons outlined inthe text. the cooling curve taken from Chevalier & Fransson (1994).The highest observed optical velocity is 3200 km s − , takenhere to be the shock velocity, although it is possible thatfaster-moving material exists that is not observed at opticalwavelengths because it has a lower emissivity, owing per-haps to a lower density. The radius is calculated assuming ashock velocity of 3200 ±
300 km s − and time since explosion.Since the inferred densities decrease with increasing shockvelocity, by assuming the maximum observed gas velocitywe minimize the inferred density. The shock is assumed tobe strong and nonradiative to begin with because this is thesimplest scenario, which can be tested and if necessary mod-ified for consistency once we obtain a value for the density.We find that the minimum n e is ∼ . +4 . − . × cm − atepoch 1, depending on the exact temperature, width of theshocked region, and shock velocity. The unshocked materialhas a density 4 times smaller for a strong shock. At epoch 2the density of the shocked material is ∼ . +1 . − . × cm − .The cooling time for these densities and this temperature ismuch longer than the length of time between the explosionand observation, so our assumption of a nonradiative shockis self-consistent. This represents the absolute minimum lu-minosity and density. The density gives us an idea of whatthe minimum value of the column density should be, if in-tegrated along the radius from the first observation to thesecond. Clearly, this indicates a very high column > × cm − .Because we do not have spectra before 50 days afterthe explosion, there is a possibility that the velocity of theshock was much higher than assumed for these first 50 days.In the worst case, the velocity is 10 km s − , before sharplydropping to 3200 km s − . This would mean that the vol-ume of material in the density calculation was underesti-mated, and thus our densities overestimated. In this worst-case scenario, our inferred shocked density for epoch 1 dropsto 3 . +1 . − . × cm − . For epoch 2, our inferred shocked density drops to 9 . +5 . − . × cm − . The column densitymust still be & × cm − . Keeping the above discussion in mind, we proceed to deter-mine the actual fit parameters using Figure 3. We used akernel density estimator with Gaussian kernels on the fitsin Figure 3 to estimate the distributions for N H and kT foreach epoch. We excluded all fits to epoch 1 in the high- kT and low- N H regime, those with kT > . N H > × cm − . Wealso excluded the fits to epoch 2 in the high- kT and high- N H region because N H and kT must be higher for epoch1 than for epoch 2. To optimize the width of the Gaussiankernel, we performed the following procedure for a rangeof widths (Ivezi´c et al. 2014). First, we removed one pointfrom the dataset. Next, we calculated the log of the likeli-hood of the density at the removed point. We repeated thisstep for each point. The sum of each log-likelihood dividedby the number of fits was then used as a cost function. Thiscost function was minimized with respect to the width ofthe Gaussian kernel. We chose this cost function in orderto minimize the estimator’s error between points, as the pa-rameter space for epoch 1 is sparsely populated. For epoch1, the width of the kT kernel was 1.8 keV and the width ofthe N H kernel was 1 . × cm − . For epoch 2, the width ofthe kT kernel was 0.15 keV and the width of the N H kernelwas 0 . × cm − . From our distributions of tempera-ture and column density, we calculated the expectation valueand 68% confidence errors from the distribution. While theuncertainties may appear small, we emphasize that the fullrange of possible values is larger. A realistic value for epoch1 is kT = 2 . +1 . − . keV and N H = 6 . +3 . − . × cm − .Taking the variation in all quantities into account, forepoch 1, this gives a flux of 7 . +389 − . × − erg cm − s − , and thus a luminosity of 5 . +298 − . × erg s − . Weassume, despite not knowing the true shock velocity, a strongnonradiative shock with velocity 3200 km s − . We use thesame formula as in § . +16 . − . × cm − at epoch 1. The cooling time for this temperatureand density is ∼ kT = 1 . +0 . − . keV and N H =5 . +2 . − . × cm − , which gives an unabsorbed flux of3 . +104 . − . × − erg cm − s − . This results in a luminos-ity of 2 . +79 . − . × erg s − and an unshocked density of1 . +5 . − . × cm − . The cooling time for this temperatureand density is ∼ ∼ × cm − at epoch 1 and about a factor of 4 smaller at epoch 2.Given an approximate mean molecular weight µ of 1,we can derive a mass density ∼ . +28 . − . × − g cm − forepoch 1 and ∼ . +8 . − . × − g cm − for epoch 2. Taking c (cid:13) , 1–11 Bochenek, C. D., Dwarkadas, V. V., Silverman, J. M., et al. the value of n e at the radius at epoch 1 ( r ), and integrating n e along the line of sight from r to the radius at epoch 2( r ), assuming a constant density profile, we find that N H ≈ . × cm − . If we assume that n e drops rapidly after r to its value at r and take the value of n e at r as morerepresentative, then we get a value of N H lower by a factorof 3. In reality, it is probably somewhere between the two,or possibly greater due to material beyond r , and thus > × cm − . If we assume the density at r to be spreadover the region all the way to at least r , there is ∼ .
04 M ⊙ of material around the SN between r and r , a lower limiton the mass of the CSM. The density at r increases this to0.11 M ⊙ . The actual value is again probably somewhere inbetween, suggesting that at least ∼ . ± .
05 M ⊙ of materialis present around the SN.In an ionised medium, the lower limit on N H derivedabove may not be valid if some or all of the material isionised. It is therefore worthwhile to check the ionisationstate of the medium. Since there is no nearby photoion-ising source, it is the X-ray emission itself that tends toionise the medium. We compute the ionisation parametergiven by Kallman & McCray (1982), ξ = L X /n e r . An ion-isation parameter ξ &
100 indicates that elements suchas C, N, and O are completely ionised, whereas ionisationof heavier elements such as S and Fe requires ξ & ξ = 71 . +703 . − . .For epoch 2, ξ = 54 . +325 . − . . Our results show that the gasis far from being fully ionised and therefore this should notsignificantly affect the column density. Inferring the globalionisation state from ξ may be more difficult. For a spheri-cally symmetric medium and a steady-state wind (for exam-ple), n e ∝ r − , and ξ is a constant that describes the globalstate of the medium. In the present scenario, we have noknowledge of the actual density variation with radius, andthe geometry of the medium is likely not spherical as ar-gued in the next section. In fact, using the densities inferredfor the high-density component in the next section wouldreduce the ionisation parameter significantly, indicating analmost neutral medium, with the caveat that the coveringfraction of the high-density component is unknown.Since we are agnostic about the shock velocity for thefirst 50 days, we must assume it is 10 km s − during thattime and calculate the density. In this worst-case scenario,our inferred density for epoch 1 drops to 2 . +10 . − . × cm − . For epoch 2, our inferred density drops to 8 . +33 . − . × cm − . The inferred minimum column density must stillbe > × cm − . The above calculations assume a spherically symmetricmedium, which would be the simplest assumption. The cal-culations do, however, lead to a contradiction. The velocityindicated from the BSWZI, 3200 km s − , implies a post-shock proton temperature of 12–20 keV, depending on themean molecular weight. The X-ray fits give an electron tem-perature on the order of 2 keV or less. However, the highdensities implied by the above calculations would mean thatCoulomb equilibration should be important. For numberdensities greater than 10 cm − and T ≈ K ( ∼ <
10 days. The equilibration time for electrons and protons would be on the order of 2 days for epoch 1, sowe would expect the electron and proton temperatures tobe equilibrated.One way to resolve this is to assume that the X-rayemission is not coming from the fastest-moving gas. This isnot unreasonable, given that there is evidence in the Gaus-sian line profiles for gas moving at all velocities from − − to +3500 km s − . If most of the emission was comingfrom a thin shell of gas with a small velocity range, we wouldhave seen more of a boxy line profile. This is best exemplifiedby the case of SN 1993J (Fransson et al. 2005), where theemission is inferred to arise from a thin shell having a smallvelocity range. Instead, we see profiles that can be fit with aGaussian shape. Indeed, our high-resolution spectra suggesta FWHM of around 1300 km s − , indicating that there issufficient material going at several hundred to a thousandkm s − .This indicates the possible existence of a two-phasemedium. Part of the gas is traveling at speeds up to 3200 kms − , but is expanding into a lower-density medium. Giventhe temperatures from the X-ray fits, the X-ray emissionarises from gas moving at velocities around 1157 +346 − kms − during the first observation and 773 +114 − km s − duringthe second observation. Thus, the X-ray emission must bearising within a medium where the shock velocity is lowerthan that of the forward shock, and therefore the densityis correspondingly higher. In a single-degenerate scenario,the dense medium may be due to clumps in the surround-ing environment, or a dense torus. A clumpy medium hasbeen inferred for SN 2002ic from spectroscopic (Deng et al.2004) and spectropolarimetric (Wang et al. 2004) observa-tions. Most standard double-degenerate scenarios involvingthe merger of two white dwarfs would not produce a densemedium, but one special case proposed to explain H lines inSNe Ia via a double-degenerate scenario (Sparks & Stecher1974; Livio & Riess 2003) results in a circumbinary cloudor common envelope around the SN. A surrounding diskis a possibility in both the single- and double-degeneratescenarios (Mochkovitch & Livio 1990; Han & Podsiadlowski2006), although it is doubtful that the Mochkovitch & Livio(1990) model could explain the H lines. A circumstellar diskor torus is also an ingredient of binary evolution models ofSNe Ia computed by Hachisu et al. (2008). If the clumps,torus, envelope, or disk are in pressure equilibrium with thesurrounding material (which is not necessary in a transientsituation), the velocity ratios of a factor of 3–4 would in-dicate that this medium was about 9–16 times denser thanthe interclump/interdisk medium.The mass density of this high-density medium, ρ hd , canbe estimated. A ∼ − shock moving into a mediumof density ∼ cm − would be radiative. Therefore, inthis scenario we use the equation for a radiative shock toestimate the density, assuming that most of the emissionfrom the radiative shock arises at X-ray energies: L X = 0 . α × πr ) ρ hd v , (1)where we take v sh to be the velocity associated with thetemperature given by the X-ray fits, and α × πr is thearea of the emitting region, taken as a fraction α πr . For epoch 1, v sh =1157 +346 − km s − , and for epoch 2, v sh = 773 +114 − km s − . c (cid:13) , 1–11 -rays from SN 2012ca The radius, or the surface area, of the clumps/disk is notknown. Given the X-ray temperatures we obtain, we setthe radius assuming a constant velocity of 1157 +346 − kms − over 554 days. For the radius of the second observa-tion, we assume the same constant velocity for the first 554days and then a velocity of 773 +114 − km s − out to 745days. We find ρ hd = 1 . +1977 − . × − α − g cm − and ρ hd = 3 . +562 − . × − α − g cm − for the first and sec-ond observations, respectively. These density values corre-spond to number densities of 1 . +1181 − . × α − cm − and1 . +336 − . × α − cm − for the first and second observa-tions, respectively. If the area is larger, the density will de-crease correspondingly, and vice versa. The cooling time ofboth a 1157 km s − shock and a 773 km s − shock into aregion with such a density is less than twenty days, so ourassumption of a radiative shock is justified.This model fits some aspects of the data better, and somay be preferable. While the lack of detailed informationdoes not allow for accurate calculations, it is clear that thedensities are possibly even higher than in the symmetricalcase, as expected. However, they are consistent with den-sities suggested for other SNe Ia-CSM (Wang et al. 2004;Deng et al. 2004; Aldering et al. 2006). These high densitiesare also consistent with those required for collisional excita-tion to be responsible for the high Balmer decrement values(Drake & Ulrich 1980), which require n e > cm − . We describe here the X-ray emission from SN 2012ca, thefirst Type Ia-CSM SN to be detected in X-rays. Althoughthe statistics preclude an accurate fit, there are several indi-cators that the SN is expanding in an extremely high den-sity medium, with density exceeding 10 cm − , and > cm − in our preferred scenario. The high Balmer decre-ment seen in this SN, with values ranging from 3 to 20,has been interpreted in other SNe Ia-CSM as being mostlikely produced through excitation rather than recombina-tion (Silverman et al. 2013), which requires a very high den-sity medium, on the order of n e ≈ cm − . The spectralfits to the X-ray data imply a large column density > cm − , consistent with a large CSM density. The relativelylow shock velocity (3200 km s − at early times) also suggestsa density orders of magnitude higher than that of a normalSN Ia.Although the data could be fit by a spherically symmet-ric medium, this does lead to a contradiction in that the elec-tron and ion temperatures are quite different, whereas thelow Coulomb equilibration time suggests that they should besimilar. The difference between the post-shock proton tem-perature and the fitted electron temperatures could indicatethat the X-ray emission does not arise from the fast-movinggas, but from slower-velocity gas expanding into a densermedium. The fact that the optical spectra indicate there isa large amount of gas moving at low velocities suggests thatour low X-ray temperatures are correct. This dense mediumcould consist of high-density clumps, a dense torus, a denseequatorial disk, or a common envelope. The X-ray emissionwould arise from shocks, probably radiative, entering thisdense medium. The existence of this dense medium rendersmost double-degenerate scenarios unviable, except perhaps for one which involves the degenerate core of an asymptoticgiant branch (AGB) star that shed its H envelope in a mergerwith a companion white dwarf shortly before the explosion(Livio & Riess 2003).The BSWZI of the H α emission indicates that itarises from the main 3200 km s − shock. This, however,presents a problem: it is unlikely that a nonradiative shockcould give rise to such a level of Balmer emission. UsingChevalier & Raymond (1978), the ratio of H α power toshock power is of order 10 − , which is much smaller thanthe observed H α luminosity (Fox et al. 2015). The emissionis probably not due to a spherical shock— as noted earlier,this would result in a boxy line profile at the shock velocitythat is not seen. A radiative shock could be inferred, but ifwe assume the X-ray luminosity to be the shock luminosity,the density inferred due to the high shock velocity (fromEquation 1) is not enough to cool the shock in the requiredtime. Thus, none of these solutions is particularly attractive,and there is an inherent lack of self-consistency in some ofthem. This suggests that the interaction is probably morecomplex than noted here, occurring presumably at differentvelocities and with variable-density ambient medium. Wenote that a similar problem also occurs in many SNe IIn.Inserra et al. (2016) use the optical light curves to com-pute the mass of the CSM in SN 2012ca, finding 2.3–2.6M ⊙ . For the spherically symmetric case, our mass is only0.1 M ⊙ , much lower than their value. However, in the caseof an asymmetric or clumpy medium, the densities of thehigh-density component are about two orders of magnitudegreater. If this higher-density component has even a 10%filling factor, then we would expect a CSM mass in theasymmetric-medium model of ∼ ⊙ , which is more con-sistent with the value of Inserra et al. (2016). Thus, an ad-ditional factor in favor of the asymmetric-medium model isthe higher CSM mass, which is more likely able to power theoptical luminosity. The discrepancy in the computed massesis not significant, considering that Inserra et al. (2016) usean ejecta profile that is not appropriate for Type Ia SNe(Dwarkadas & Chevalier 1998), and they do not make al-lowances for deviations from spherical symmetry and aclumpy medium. Their model also requires a kinetic energyinput of 7–9 × erg, which is large even compared to thatof other SNe Ia-CSM modelled by them.While we are constrained by the available data, in § > cm − , and showclearly that it is still higher than is typical for most SNe after1.5 yr, and in the range deduced for SNe Ia-CSM in general.In order for all the observations to be consistent thereafter,the final deduced density ranges seem quite appropriate.The inferred densities from SN 2012ca are extremelyhigh compared to those of other SNe Ia, which typically ex-pand in much lower densities. However, they are consistentwith those inferred for other members of the SN Ia-CSMsubclass, with most showing indications of high-densitysurroundings (Silverman et al. 2013). Aldering et al. (2006)suggest that SN 2005gj, also identified as a SN Ia-CSM,had an ambient density n e > cm − . Wang et al. (2004)suggest that SN 2002ic had a dense, clumpy, disk-like en-vironment, with clumps of density > cm − and sizes c (cid:13) , 1–11 Bochenek, C. D., Dwarkadas, V. V., Silverman, J. M., et al.
Figure 4.
The X-ray light curves of most X-ray SNe, groupedby type. Adapted from Dwarkadas (2014), with SN 2012ca andsome other data points added. The stars represent the nominalX-ray luminosities at the two epochs for SN 2012ca. Note thatthese place it in the middle of the range for SNe IIn. × cm. This is quite similar to the picture envisionedhere for SN 2012ca. Deng et al. (2004) also suggest a dense,clumpy, and aspherical circumstellar medium for SN 2002ic,with mass-loss rates as high as 10 − v w M ⊙ yr − , wherev w = v w /
100 km s − , and v w is the stellar wind ve-locity. The density is much higher than that around typ-ical core-collapse SNe, and perhaps also the subclass ofType IIn SNe. Densities of up to n e ≈ cm − havebeen estimated for the Type IIn SN 2006jd (Chandra et al.2012), and between 3 × and 10 cm − for SN 2010jl(Fransson et al. 2014). High mass-loss rates have been foundfor SN 2005kd (Dwarkadas et al. 2016) and SN 2005ip(Katsuda et al. 2014). A clumpy medium has been suggestedfor Type IIn SNe such as SN 1988Z, SN 1978K, and SN1986J (Chugai 1993; Chugai & Danziger 1994; Chugai et al.1995), and a dense torus for SNe 2005kd, 2006jd, and2010jl (Katsuda et al. 2016). SNe IIn in general have highX-ray luminosities (Figure 4; Dwarkadas & Gruszko 2012;Dwarkadas 2014; Chandra et al. 2015; Dwarkadas et al.2016), which if due to thermal emission suggest high densi-ties. Note that the luminosity predicted for SN 2012ca, of or-der 10 erg s − , places it squarely within the range of the X-ray luminosities of SNe IIn (Figure 4; Dwarkadas & Gruszko2012; Dwarkadas et al. 2016). Despite these similarities, thetemperature of SN 2012ca is significantly lower than that ofmost SNe IIn at a similar epoch. It typically takes SNe IIna few thousand days to reach such low temperatures. Thehigh CSM density around SN 2012ca may allow it to coolmore quickly, explaining this discrepancy. Thus, while thespectra suggest a Type Ia SN, the X-ray luminosity, highdensity, and circumstellar interaction are typical of a core-collapse Type IIn SNe, suggesting a CSM similar to thatseen in SNe IIn.While SN 2012ca is the first Type Ia-CSM SN to bedetected in X-rays, others have been examined unsuccess-fully for signs of X-ray emission. Hughes et al. (2007) took deep X-ray observations of SN 2002ic and SN 2005gj andplaced upper limits on their X-ray flux. The limits on SN2005gj are more stringent. The redshift 0.019 of SN 2012caplaces it much closer than SN 2002ic ( z = 0 . z = 0 . ρ CSM = ˙ M/ πr v w , where ˙ M is the mass-loss rate and v w the wind velocity, then forepoch 1, ˙ M /v w must be larger than 2 . × − v w M ⊙ yr − in the spherically symmetric case, and two orders ofmagnitude larger in the two-component medium case (al-though asphericity may increase it further). Despite beingextremely high, this mass-loss rate is consistent with thosederived from light-curve modelling of SNe 2002ic and 1997cy(Chugai & Yungelson 2004), as well as that derived fromspectral modelling of SN 2002ic (Kotak et al. 2004). Thislower limit is orders of magnitude larger than upper lim-its derived from deep X-ray observations of nearby SNe Ia.Margutti et al. (2012) place an upper limit on ˙ M/v w for thenearby Type Ia SN 2011fe of 2 × − v w M ⊙ yr − , whileMargutti et al. (2014) posit an upper limit on SN 2014J of2 . × − v w M ⊙ yr − . Of course, the constant veloc-ity over at least 500 days suggests that if there is a windmedium, its density is not decreasing as steeply as r − butmore gently (Dwarkadas & Gruszko 2012).This mass-loss rate may only be satisfied by the higherend of red supergiant stars (Mauron & Josselin 2011) and byyellow hypergiant stars. If the surrounding velocity is higher,say 100-1000 km s − , then the mass-loss rate increases ac-cordingly. This would then require a luminous blue variable(LBV) undergoing eruptive mass loss (Smith 2014). OtherSN progenitors such as Wolf-Rayet (W-R) stars have windsof ∼ − , and would therefore require a mass-lossrate 100 times higher than that postulated above, which isorders of magnitude higher than those actually measuredfor W-R stars (Moffat 2015). In addition, the presence of anH-rich medium immediately around the star effectively rulesout W-R stars.It must be pointed out here that having a high-mass sec- c (cid:13) , 1–11 -rays from SN 2012ca ondary likely seems implausible around a white dwarf, giventhat one would have expected the higher-mass companionto have evolved faster and therefore be long gone before thewhite dwarf made an appearance. The only lower-mass ( < ⊙ ) progenitor that may be able to satisfy the high mass-loss rates for at least a short period would be an asymptoticgiant branch star.The ambient medium may not result from a freely ex-panding wind at all, but could be the result of a swept-upmedium due to interacting winds (Dwarkadas 2011), as sug-gested for SN 1996cr (Dwarkadas et al. 2010). In this case,the two winds do not need to have very high mass-loss rates,as long as there is sufficient mass for the later wind to sweepup. However, this would mean a low density very near thestar, yet the low velocity shortly after explosion seems toargue against this.An ambient medium owing to recurrent nova eruptionswas suggested by Dilday et al. (2012) for PTF11kx. In thatcase, the presence of multiple components, with fast-movingmaterial inside denser and slower-moving shells, could beexplained by the SN shock interacting with shells of materialresulting from episodic nova eruptions. It is not clear if sucha model could apply here, since the SN shock maintains alow velocity that does not vary much over the first 550 days(see Figure 1).Kepler’s SN remnant is a possible Galactic exampleof a SN Ia that requires dense mass loss. However, theCSM of Kepler’s SN remnant is much farther out than thatof SN 2012ca (Reynolds et al. 2007; Katsuda et al. 2015).Katsuda et al. (2015) argue that Kepler’s SN was a highlyoverluminous SN Ia, and that the CSM consists of tenu-ous gas with dense knots, similar to the model outlinedherein. Their analysis suggests that the knots were parsecsaway from the progenitor, and the medium shows evidenceof CNO processing — so, although both require a SN Iaprogenitor and dense surrounding medium, the two eventsare not directly comparable. They infer a high mass-lossrate of 10 − yr − , which is considerably larger than the up-per limits quoted above, although lower than that inferredfor SN 2012ca, especially in the asymmetric-medium sce-nario. Their models require a progenitor with a high mass-loss rate and no surviving companion; a recurrent-nova sce-nario is not favored for Kepler’s SN. The core-degeneratescenario laid out by Livio & Riess (2003) and elaboratedfurther by Tsebrenko & Soker (2013) could be the origin ofKepler’s SN remnant. The supersoft channel investigated byHan & Podsiadlowski (2006) also remains a possibility.Finally, as noted earlier, it is possible that this SN, andothers like it, are not part of the class of SNe Ia at all. Theymost closely resemble SNe IIn, which presumably have morethan one progenitor, all of the core-collapse variety. That,however, then leads to the question of why spectra of SNe Ia-CSM resemble those of the SN Ia class the most, and howthe explosion of a massive star could give rise to SN Ia-likespectral features.Notwithstanding its origin, the high density surround-ing SN 2012ca is indisputable. The SN is fading in X-rays,and unlikely to be detectable any more by current instru-ments. However, it makes a clear case for other SNe Ia-CSMto be observed in X-rays, and presumably at radio wave-lengths as well, opening up a new window to understandingthis class of intriguing objects and unraveling the mystery of their progenitors. Continued long-term observations wouldalso be useful to show if SNe of this class eventually evolveinto remnants resembling that of Kepler’s SN. ACKNOWLEDGMENTS
V.V.D.’s research is supported by NASA Astrophysics DataAnalysis program grant NNX14AR63G (PI Dwarkadas)awarded to the University of Chicago. J.M.S. is supported byan NSF Astronomy and Astrophysics Postdoctoral Fellow-ship under award AST-1302771. O.D.F. was partially sup-ported by
Chandra grant GO4-15052X provided by NASAthrough the
Chandra
X-ray Observatory center, operatedby SAO under NASA contract NAS8-03060. A.V.F. hasbeen supported by the Christopher R. Redlich Fund, theTABASGO Foundation, NSF grant AST-1211916, and theMiller Institute for Basic Research in Science (UC Berke-ley). His work was conducted in part at the Aspen Centerfor Physics, which is supported by NSF grant PHY-1607611;he thanks the Center for its hospitality during the neutronstars workshop in June and July 2017. This research hasmade use of data obtained from the
Chandra
Data Archive,and software provided by the
Chandra
X-ray Center (CXC)in the application packages CIAO, CHIPS, and SHERPA.We would like the thank the anonymous referee for a help-ful and thorough reading of this paper.
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