PANCHROMATIC OBSERVATIONS OF SN 2011dh POINT TO A COMPACT PROGENITOR STAR

A key open question in the study of core-collapse supernovae (SNe) is the nature and diversity of their progenitor systems. High-resolution optical imaging of nearby galaxies has firmly established that the progenitors of Type IIP SNe are red supergiants (Smartt 2009), while pre-discovery imaging for Type II SNe 1987A (Gilmozzi et al. 1987), 1993J (Aldering et al. 1994), and 2005gl (Gal-Yam et al. 2007; Gal-Yam & Leonard 2009) point to a diverse set of massive stars. An alternative route to the nature of the progenitors is to obtain panchromatic follow-up observations within days of explosion. The shock breakout pulse and subsequent adiabatic cooling of the ejecta can yield information on the progenitor size since the duration and energy of these signals scales with the size of the progenitor star (Colgate 1974; Ensman & Burrows 1992; Waxman et al. 2007; Chevalier & Fransson 2008; Katz et al. 2010; Nakar & Sari 2010). Complementary follow-up observations at radio, millimeter, and X-ray bands provide unique diagnostics on the shock wave velocity which scale inversely with the progenitor radius. The utility of such a multi-wavelength technique was demonstrated by the serendipitous X-ray discovery and comprehensive follow-up study of SN 2008D (Soderberg et al. 2008; Chevalier & Fransson 2008).

On May 31.893 UT amateur astronomer Amadee Riou discovered an optical transient in M51 (d = 8.4 ± 0.6 Mpc; Feldmeier et al. 1997; see also Vinko et al. 2012). Multiple individuals and groups subsequently confirmed the transient using pre- and post-discovery imaging (Griga et al. 2011; Silverman et al. 2011). The Palomar Transient Factory (PTF; Law et al. 2009) reported a deep non-detection in pre-discovery data constraining the onset of the optical emission to be May 31.275–31.893 UT (Arcavi et al. 2011b). Based on an initial spectrum on June 3.3 UT, the transient was classified as a Type II SN, designated SN 2011dh (Silverman et al. 2011). Further spectroscopy revealed evidence for helium absorption features prompting the re-classification as Type IIb (Arcavi et al. 2011a; Marion et al. 2011).

A putative progenitor star has been identified in pre-explosion Hubble Space Telescope (HST) images with a spectral energy distribution consistent with a yellow supergiant (Maund et al. 2011; Van Dyk et al. 2011). The mass of the star is estimated to be between M_ZAMS  ≈  13 and 21 M_☉, but temperature-dependent bolometric corrections, discrepancies between evolutionary tracks, and treatments of rotation should also be carefully considered (Drout et al. 2009). Based on the estimated luminosity and temperature of the object, the stellar radius is R_* ≈ 10¹³ cm (Prieto & Hornoch 2011). Both Maund et al. (2011) and Van Dyk et al. (2011) discuss the possibility that the yellow supergiant is instead the binary companion to the SN 2011dh progenitor star. In this scenario, the actual progenitor star would have had a smaller radius.

Recently, Chevalier & Soderberg (2010) proposed that SNe IIb may be divided into two sub-classes based on the radius and mass-loss history of the progenitor star and the properties of the shock wave. In this framework, compact progenitors (R_* ∼ 10¹¹ cm) with modulated radio light curves and shock wave velocities of $\overline{v}\sim 0.1$c are identified as SNe cIIb with members including SNe 2001ig (Ryder et al. 2004), 2003bg (Soderberg et al. 2006a), and 2008ax (Roming et al. 2009). Meanwhile, extended progenitors (R_* ∼ 10¹³ cm) with smooth radio light curves and slower shock waves are identified as SNe eIIb (e.g., SN 1993J; Bartel et al. 2002; Weiler et al. 2007). The modulated radio emission points to an unusual (perhaps episodic) mass loss that may be unique to SNe cIIb.

SN 2011dh showed an initial peak magnitude of M_g ≈ −16.5 mag at Δt ≈ 1 day since explosion before fading quickly (Arcavi et al. 2011b; Prieto et al. 2011). The SN then re-brightened at Δt ≈ 5 days. The two light-curve components may be interpreted as cooling envelope emission followed by a re-brightening due to the radioactive decay of ⁵⁶Ni. Based on a comparison of the SN 2011dh light curve with that of SN 1993J and a photospheric temperature measurement, Arcavi et al. (2011b) proposed that SN 2011dh belongs to the Type cIIb class.

Here, we report the discovery and monitoring of X-ray emission associated with SN 2011dh and present radio- and mm-band detections from the first few weeks after explosion. We show that the radio and X-ray properties are consistent with those of SNe cIIb and dissimilar from those of SNe eIIb, suggesting that the progenitor was compact at the time of explosion. This is supported by our modeling of the early cooling envelope emission, which points to a progenitor radius of R_*  ≈  10¹¹ cm. Together, these diagnostics suggest that the putative yellow supergiant progenitor is instead a binary companion or unrelated to the SN. Finally, we present a detailed compilation of X-ray and gamma-ray observations from multiple satellites and instruments obtained during the purported explosion date range. We estimate that the shock breakout pulse was detectable with current high-energy instruments but likely missed due to their limited temporal/spatial coverage.

Following the optical discovery of SN 2011dh, we initiated a prompt panchromatic follow-up campaign to map the non-thermal properties of the ejecta.

2.1. Swift/XRT Observations

Thanks to multiple Target-of-Opportunity (ToO) requests, Swift/X-ray Telescope (XRT) promptly observed SN 2011dh on June 3.50, just ∼3 days after the explosion. As initially reported in Margutti & Soderberg (2011), we discovered a bright X-ray source (S/N ∼10) at coordinates, R.A._J2000 = 13^h30^m518, decl._J2000 = +47°10'1114 (uncertainty 4'' radius, 90% confidence) at 1'' from the optical SN position. This data analysis reported here supersedes the preliminary analysis presented in the circulars; we find a count rate of ∼0.015 counts s⁻¹ for this first epoch. We analyzed 69 ks of archival pre-SN Swift/XRT observations and these data reveal no X-ray source at the SN position with a 3σ upper limit of 5.6 × 10⁻⁴ counts s⁻¹. This fact, coupled to the spatial coincidence of the SN, strongly suggests that the new source represents the X-ray counterpart to SN 2011dh (Figure 1).

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Observations of SN 2011dh with Swift continued for the next several weeks. We retrieved and analyzed the XRT data from the HEASARC archive collected in the time period June 3 to July 3 UT (total exposure time of 137 ks). All XRT data were analyzed with the heasoft (version 6.10) software package and corresponding calibration files; standard filtering and screening criteria were applied. Due to the proximity of a nearby, steady X-ray source (Figure 1), we adopted a 12 pixel (∼28'') extraction region centered on the optical position; for lower count rates (<0.0025 counts s⁻¹) we reduced the extraction region to a radius of 6 pixels to increase the signal-to-noise ratio (S/N) and eliminate contamination from a nearby faint source. The background was estimated from the pre-explosion Swift/XRT data to properly account for the contamination from the extended X-ray emission associated with M51. A spectrum extracted over June 3–17 UT can be modeled by an absorbed power law with photon index Γ = 1.5 ± 0.2 (90% c.l.) assuming a Galactic foreground column density N_H = 1.81 × 10²⁰ cm⁻² (Kalberla et al. 2005) and no intrinsic absorption (χ²/dof = 70.7/73, P-val = 0.56). Alternatively, a thermal plasma spectral model with best-fitting temperature kT = 7.5^+9.7_{− 3.3} keV (90% confidence level (c.l.)) can adequately represent the data (χ²/dof = 68.8/73, P-val = 0.62). Both models give an average unabsorbed flux of F_X ≈ 1.65 × 10⁻¹³ erg cm⁻² s⁻¹ (0.3–8 keV) corresponding to a luminosity, L_X ≈ 2 × 10³⁹ erg s⁻¹.

Our resulting X-ray light curve is shown in Figure 2 and Table 1. Over the first 10 days the SN faded by a factor ≈8. Adopting a simple power-law model for the decay, we derive an index of α = −0.8 ± 0.2 (90% c.l.). The source shows some evidence for spectral softening with time, with the photon index evolving from Γ₁ = 0.9 ± 0.3 (90% c.l., Δt = 3–7 days) to Γ₂ = 1.8 ± 0.2 (90% c.l., Δt = 7–17 days). In comparison with X-ray observations of SN 1993J, the softening observed for SN 2011dh begins at an earlier epoch.

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Table 1. X-Ray Observations of SN 2011dh

Date	Time Range	Unabsorbed Fluxa	Error	Satellite
(UT)	(days)	(erg cm−2 s−1)	(erg cm−2 s−1)
June 3.512	0.004	1.12 × 10−12	2.8 × 10−13	Swift/XRT
June 3.526	0.003	6.28 × 10−13	2.8 × 10−13	Swift/XRT
June 3.585	0.007	7.45 × 10−13	1.7 × 10−13	Swift/XRT
June 3.943	0.152	5.01 × 10−13	2.2 × 10−13	Swift/XRT
June 4.753	0.252	4.25 × 10−13	1.8 × 10−13	Swift/XRT
June 5.326	0.033	3.80 × 10−13	1.1 × 10−13	Swift/XRT
June 6.536	0.571	1.29 × 10−13	4.9 × 10−14	Swift/XRT
June 8.042	0.181	1.34 × 10−13	6.0 × 10−14	Swift/XRT
June 8.911	0.253	3.69 × 10−13	1.1 × 10−13	Swift/XRT
June 10.020	0.301	1.96 × 10−13	6.8 × 10−14	Swift/XRT
June 10.954	0.165	2.16 × 10−13	8.9 × 10−14	Swift/XRT
June 12.027	0.370	1.87 × 10−13	6.7 × 10−14	Swift/XRT
June 13.127	0.179	2.03 × 10−13	7.4 × 10−14	Swift/XRT
June 13.887	0.200	1.89 × 10−13	6.6 × 10−14	Swift/XRT
June 15.493	0.601	1.80 × 10−13	6.4 × 10−14	Swift/XRT
June 16.969	0.136	1.23 × 10−13	4.8 × 10−14	Swift/XRT
June 21.345	2.848	1.06 × 10−13	2.5 × 10−14	Swift/XRT
June 29.278	4.227	6.57 × 10−14	2.2 × 10−14	Swift/XRT
June 12.300	0.115	1.00 × 10−13	3.0 × 10−14	Chandra/ACISb
July 3.400	0.115	2.75 × 10−14	5.5 × 10−15	Chandra/ACIS

Notes. ^aEnergy range (0.3–8) keV. ^bObservation reported by Pooley (2011) within energy range (0.5–8) keV.

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2.2. Chandra Observations

2.3. CARMA Observations

2.4. SMA Observations

2.5. EVLA Observations

On June 4.25 UT, a radio counterpart was detected with the Expanded Very Large Array (EVLA; Perley et al. 2009) with a flux density of F_ν = 2.68 ± 0.10 mJy at ν = 22.5 GHz (Horesh et al. 2011a). There is no coincident radio source in the catalog of M51 compact radio sources (Maddox et al. 2007). A comparison with the CARMA and SMA flux densities obtained contemporaneously indicates that the spectral peak lies between the EVLA and CARMA bands (Figure 3).

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We began monitoring SN 2011dh with the EVLA on June 17 UT (Δt ≈ 17 days after explosion) as part of a Rapid Response observing program for long-term monitoring of the SN (PI: Soderberg). Data were collected within the wide C, X, Ku, K, and Ka bands. Within each of these bands (except X), we selected two central frequencies enabling spectral coverage spanning ν = 5.0–36.0 GHz. Each central frequency has associated bandwidth of 0.8–1.0 GHz. All observations were obtained in the (most extended) A-array. We used J1327+4326 to monitor the phase while the absolute flux calibration was carried out using 3C286. Data were reduced using AIPS and the Common Astronomy Software Applications. We fit a Gaussian model to the radio SN emission in each observation to derive the integrated flux density (Table 2). The reported flux density errors include errors from Gaussian fitting, the map rms noise, and systematic errors of 1% at low frequencies (5–16 GHz) and 3% at high frequencies (20.5–36.0 GHz). At this epoch, the peak of the radio spectrum has clearly shifted to the centimeter (cm) band.

Additional observations with EVLA are ongoing and are the focus of a follow-up paper (Krauss et al. 2012). In Figure 3, we compare the radio spectrum of SN 2011dh with the newly available and unprecedented wide bands of EVLA which enable continuous spectral coverage from ∼1 to 40 GHz. SN 2011dh represents the first SN for which such detailed mapping of the spectrum has been possible in the EVLA era.

Early radio observations of SNe uniquely trace the shock wave as it races ahead of the bulk ejecta and shock-accelerates particles in the local circumstellar medium (CSM; Chevalier 1982). This environment was enriched by the progenitor star wind during the centuries leading up to the explosion. Through this dynamical interaction, the shock wave accelerates CSM electrons into a power-law distribution, N(γ)∝γ^−p, above a minimum Lorentz factor, γ_m. The accelerated electrons gyrate in amplified magnetic fields giving rise to non-thermal synchrotron emission. In the case of Type Ibc and cIIb SNe, the radio emission is quenched at low frequencies primarily due to synchrotron self-absorption (SSA), producing a spectral turnover that defines the peak of the radio spectrum, ν_p. The self-absorbed radio spectrum is described by F_ν∝ν^5/2 below ν_p and F_ν∝ν^{−(p − 1)/2} above ν_p. As shown in Figure 3, our EVLA, CARMA, and SMA observations of SN 2011dh on two separate epochs are well described by a synchrotron self-absorbed spectrum with p ≈ 3. We note that the modest disagreement between the SSA model and the measurements near the spectral peak may indicate asphericity of the emitting region (see Krauss et al. 2012 for a detailed discussion).

Chevalier (1998) showed that for radio SNe with minimal free–free absorption, the radius of the shock wave, R, and its time-averaged velocity, $\overline{v}$, can be robustly estimated from the observed values of ν_p and the associated peak spectral luminosity, L_{ν, p}. As the shock wave decelerates, the optical depth to absorption processes declines and ν_p cascades to lower frequencies; for the case of free expansion, we expect ν_p∝t⁻¹ and a nearly constant peak spectral luminosity (Chevalier 1998). For p ≈ 3, the shock wave radius is given by $R\approx 3.3\times 10^{15} (\epsilon _e/\epsilon _B)^{-1/19}(L_{\nu _p,26})^{9/19} \nu _{p,5}^{-1}\, \rm cm$, where $L_{\nu _p,26}$ is normalized to 10²⁶ erg s⁻¹ Hz⁻¹ and ν_{p, 5} is normalized to 5 GHz (Chevalier & Fransson 2006). The fractions of post-shock energy density shared by accelerated electrons and amplified magnetic fields are denoted by _e and _B, respectively, and we assume that the radio-emitting region is half of the total volume enclosed by a spherical shock wave.

As shown in Figure 3, for SN 2011dh we find ν_p ≈ 40 and ≈11 GHz on 2011 June 4 and 17 (Δt ≈ 4 and 17 days), respectively, with an associated peak luminosity of $L_{\nu _p,26}\approx 6.7$ on both epochs. These observables correspond to shock wave radii of R ≈ (1.0, 3.7) × 10¹⁵ cm assuming typical partition values of _e = _B = 0.1. The time-averaged shock wave velocity is thus $R/\Delta t\approx \overline{v}\approx 0.1c$ and thus a factor of ∼2 faster than the material at the optical photosphere at Δt ≈ 3 days (Silverman et al. 2011). The total internal energy required to power the observed radio signal can be estimated from the post-shock magnetic energy density, E = B²R³/12_B. As shown by Chevalier & Fransson (2006), the amplified magnetic field is directly determined from the spectral properties, B ≈ 0.70(_e/_B)^−4/19L^−2/19_{νp, 26}ν_{p, 5} G. At Δt ≈ 4 and 17 days, we find B ≈ 4.5 and 1.2 G. The total internal energy is thus E ≈ (1.7, 6.3) × 10⁴⁶ erg for the two epochs, respectively, by maintaining the assumption that _B = 0.1, i.e., _{B, −1}. The roughly linear temporal increase of internal energy is consistent with a slightly decelerated shock wave. As shown in Figure 4, the shock wave properties of SN 2011dh are similar to those of Type cIIb, which tend to also be characterized by shock wave velocities of $\overline{v}\gtrsim 0.1c$ while slower shock waves are inferred for SNe eIIb.

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The progenitor mass-loss rate is $\dot{M}\approx 0.39\times 10^{-5} \epsilon _{B,-1}^{-1} (\epsilon _e/\epsilon _B)^{-8/19} L_{\nu _p,26}^{-4/19} \nu _{p,5}^{2} t_{p,10}^2\,M_{\odot }\ \rm yr^{-1}$, where t_{p, 10} is the observed time of the spectral peak normalized to 10 days and we have assumed a wind velocity of v_w = 1000 km s⁻¹. Thus, we estimate a progenitor mass-loss rate for SN 2011dh of $\dot{M}\approx 3\times 10^{-5}\,M_{\odot }\ \rm yr^{-1}$, similar to the mass-loss rates derived for SNe Ibc and cIIb and also similar to the values observed for Galactic Wolf–Rayet stars (Cappa et al. 2004; Crowther 2007), and a factor of 100 lower than the mass-loss rate inferred for SN 1993J (Fransson et al. 1996). We note that in this framework, the radio data constrain the ratio, $(\dot{M}/v_w)$, such that a variation in the assumed value of v_w shifts the mass-loss rate estimate by the same factor.

On timescales of days after explosion, the X-ray emission observed from SNe Ibc and IIb may be dominated by a number of different emission processes including synchrotron, thermal, and inverse Compton (IC) scattering. Continued energy input from a compact remnant (black hole, magnetar) has also been invoked to explain the X-ray emission from some events (e.g., SN 2006aj, Mazzali et al. 2006; Soderberg et al. 2006b; SN 1979C, Patnaude et al. 2011). The temporal and spectral evolution of the radio and X-ray emission, together with their observed luminosity values, enables these different processes to be distinguished. Here, we consider the nature of the observed X-ray light curve for SN 2011dh.

The radio-to-X-ray spectral index is observed to be β_RX ≈ −0.7 on both June 4 and 17 UT. Thus, an extrapolation of the optically thin radio synchrotron spectrum with p ≈ 3 underestimates the X-ray flux at both epochs by a factor of ∼140. Similarly high radio-to-X-ray spectral indices are not atypical for SNe Ibc and IIb (see Chevalier & Fransson 2006 and references within). Attributing the X-rays to synchrotron emission would require efficient particle acceleration and a flattening of the electron distribution at high energies.

As shown in Chevalier & Fransson (2003), the thermal emission from shock-heated ejecta in the reverse shock region may give rise to strong free–free X-ray emission. If the material is fully ionized and in temperature equilibrium, the expected luminosity is broadly determined by the electron temperature and the mass-loss rate of the progenitor star with an expected linear decay in time. We use Equations (25)–(30) of Chevalier & Fransson (2006) with $\overline{v}\approx 0.1c$ and $\dot{M}\approx 3\times 10^{-5}\,M_{\odot }\ \rm yr^{-1}$ to estimate the electron temperature.²³ The expected free–free X-ray luminosity for SN 2011dh is then L_ff ≈ 2.5 × 10³⁷ erg s⁻¹ on June 4 UT. This is a factor of ∼200 lower than the Swift/XRT measurement at this epoch (Figure 2).

We next consider an IC scattering model in which the optical photons associated with both envelope cooling emission and the radioactive decay of ⁵⁶Ni are upscattered to the X-ray band by radio-emitting electrons (Chevalier et al. 2006). In this scenario, the X-ray decay should track the optical evolution through the cooling envelope decay to the re-brightening due to ⁵⁶Ni decay. The early optical emission indicates an average luminosity of L_bol ∼ few × 10⁴² erg s⁻¹ and a minimum near Δt ≈ 5 days. The X-ray light curve suggests a similar minimum near this epoch and an overall decay of roughly L_X∝t⁻¹ (Figure 2). Adopting the formalism of Chevalier & Fransson (2006) and assuming the relativistic electron population extends down to (γ_m = 1), the predicted IC emission is $L_{{\rm IC}}\approx 2.9\times 10^{36} \epsilon _{B,-1} \dot{M}_{-5} (\epsilon _e/\epsilon _B)^{11/19} (v/0.1c)^{-1} L_{\rm bol,42} \Delta t_{d,10}^{-1} \,\rm erg \,s^{-1}$, where $\dot{M}_{-5}$ is the mass-loss rate normalized to 10⁻⁵ M_☉ yr⁻¹ and we maintain the assumption of v_w = 10³ km s⁻¹. Here, Δt_{d, 10} is the time since explosion normalized to 10 days.

We find that the X-ray emission may be attributed to IC emission if the assumption of equipartition is relaxed and (_e/_B) ≈ 30 with _B ≈ 0.01. This deviation from equipartition implies modest adjustments to our physical parameter estimates, including a factor of ∼2 increase in the mass-loss rate and a minimal increase in the inferred total internal energy of the radio-emitting material. The modified values for the mass-loss rate and magnetic field are $\dot{M}_{-5}\approx 6$ and B ≈ 2.2 G, respectively. A prediction of this model is that the X-ray light curve will decay more steeply following the optical SN peak (Chevalier & Fransson 2006), which may be suggested by our Chandra measurement on July 3 UT.

The early optical emission from SNe is dominated by the adiabatic cooling of envelope material following the breakout of the shock wave through the stellar surface (Ensman & Burrows 1992). This component cascades through the optical band in the first few days following explosion. The radius and temperature associated with this component may be roughly approximated as thermal (although see Nakar & Sari 2010 for a more comprehensive discussion of the spectral evolution) and used together with estimates for the bulk SN parameters (including the ejecta kinetic energy, E_K, and mass, M_ej) leads to a determination of the progenitor radius, R_*. Such techniques have been used to derive the progenitor radius of SNe 1987A (Ensman & Burrows 1992), 1999ex and 2008ax (Chevalier & Soderberg 2010), 2008D (Chevalier & Fransson 2008; Soderberg et al. 2008), in addition to SN 2006aj associated with XRF 060218 (Campana et al. 2006; Waxman et al. 2007). Here, we adopt the formalism of Chevalier & Fransson (2008) in which the temperature of the photosphere is given by T_ph ≈ 7800E^0.03_{K, 51}M^−0.04_{ej, ☉}R_{*, 11}^0.25Δt^−0.48_d K and the photospheric radius is R_ph ≈ 3 × 10¹⁴E^0.39_{K, 51}M^−0.28_{ej, ☉}Δt_d^0.78 cm. Here we have normalized E_{K, 51} to 10⁵¹ erg, M_{ej, ☉} to M_☉, and R_{*, 11} to 10¹¹ cm. Within this framework, it is assumed that the density and pressure profiles of the envelope are consistent with those of Matzner & McKee (1999), and that the bolometric luminosity decays as L_ph∝Δt^−0.34_d.

We model the first few optical observations (Δt ≲ 5 days) including the initial detection of L_ph ≈ 10⁴² erg s⁻¹ on June 1.191 UT (Arcavi et al. 2011b). Silverman et al. (2011) report a photospheric velocity of v_ph ≈ 17, 600 km s⁻¹ at Δt ≈ 3 days after explosion implying a ratio of (E_{K, 51}/M_{ej, ☉}) ∼ few. We estimate R_ph ≈ 5 × 10¹⁴ cm and T_ph ≈ 8000 K at Δt ≈ 1 day. This is roughly consistent with the photospheric temperature derived from optical spectroscopy (Arcavi et al. 2011b) and implies a compact progenitor since T_ph∝R^0.25_{*, 11}. The high luminosity of the initial detection requires a ratio, E_K/M_ej ∼few, in line with the ratio implied by the high photospheric velocity. Thus, the early optical emission points to a compact progenitor, similar to those of SNe Ibc and cIIb, and consistent with the earlier reported by Arcavi et al. (2011b) and the conclusions of Murphy et al. (2011) based on a study of the SN 2011dh environment.

We note, however, that the observed optical decay, L∝Δt⁻¹_d, is significantly steeper than the model prediction. This may point to an irregular density profile near the stellar surface, perhaps associated with an unusual mass-loss ejection in the final stage of the progenitor's evolution. Such an irregular density profile may affect some of the conclusions that we derived above based on the canonical model and this will be the focus of a future publication.

For compact progenitors such as SN 2011dh, the breakout pulse may be detectable at X-ray and gamma-ray energies (e.g., SN 2008D; Soderberg et al. 2008). To this end, we searched for evidence of a high-energy pulse associated with the shock breakout of SN 2011dh using data collected by the Swift Burst Alert Telescope (BAT), the Monitor of All-sky X-ray Image (MAXI) camera attached to the Japanese Experiment Module, and the Interplanetary Network (IPN: Mars Odyssey, Konus-Wind, RHESSI, INTEGRAL (SPectrometer on INTEGRAL-Advanced Camera for Surveys), Swift/BAT, Suzaku, AGILE, and Fermi/Gamma-ray Burst Monitor (GBM); we note that Messenger was in superior conjunction and off during this period).

The position of SN 2011dh was observed by Swift/BAT over many pointings during the estimated explosion date range, May 31.275–31.893 UT. No gamma-ray emission was detected from the SN. We compile the Swift/BAT observations in Figure 5. Each pointing had a duration of ∼500–1000 s, and the total time on source was 11,148 s or 20% of the full explosion date range. We estimate the count rate at the SN position and also in the background in each pointing and note that the sensitivity varies as a function of the off-axis angle, ranging from 10 to 57 deg. Assuming a power-law spectrum for the breakout flux with a photon index of Γ = 2, we infer 5σ upper limits on the gamma-ray emission from the SN of F_γ ≲ (1.1–3.9) × 10⁻⁹ erg cm⁻² s⁻¹ (15–195 keV) for each individual pointing. The combined upper limit is F_γ ≈ 2.8 × 10⁻¹⁰ erg cm⁻² s⁻¹ for the same energy range. At the distance of M51, this limit corresponds to a luminosity of L_γ ≲ 2.4 × 10⁴² erg s⁻¹. We further note that no emission was detected by Swift/BAT in the direction of the SN during spacecraft slews throughout this time period; the BAT Slew Survey (Grindlay 2007) places a ∼4σ upper limit within energy range (15–50) keV.

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MAXI (Matsuoka et al. 2009) is an X-ray all-sky monitor that scans most of the sky every 92 minutes with its Gas Slit Cameras (Mihara et al. 2011). Unfortunately, during the estimated period of the SN breakout, the direction of M51 was covered only by a camera with high background, resulting in less stringent limits than usual. The X-ray image taken at every scan transit (∼80 s), encoded in one dimension by the detector position and the other by transit timing, was fitted to the point-spread function with the fixed source position to evaluate the source count rate. The energy range of (4–10) keV was chosen to maximize the S/N. Then the count rate was converted to energy flux assuming a power-law model with a photon index of Γ = 2 resulting in a conversion factor of F_X ≈ 1.1 × 10⁻⁸ erg cm⁻² s⁻¹ (counts s⁻¹)⁻¹. We derive an average 5σ upper limit of L_X ≲ 3.9 × 10⁴² erg s⁻¹. In comparison, the observed breakout pulse from SN 2008D (d ≈ 30 Mpc) had a luminosity ∼25 times higher (L_X ≈ 10⁴⁴ erg s⁻¹; 0.3–10 keV) with a duration of ∼5 minutes.

While Swift/BAT and MAXI both have a limited temporal coverage, the IPN is full sky with temporal duty cycle of nearby 100%; it is sensitive to hard X-ray emission in the energy range (25–150) keV (Hurley et al. 2010). Within a two-day window centered on the estimated explosion date, a total of four triggered bursts were detected by the IPN. All four were confirmed through observations by multiple instruments or spacecrafts and thus could be localized. These detections include Fermi/GBM (three bursts), Konus (one), INTEGRAL (one), and Swift/BAT (one). In all cases the localizations were statistically inconsistent at a ≳ 3σ level with the position of SN 2011dh. Thus, we find no evidence for a detected gamma-ray transient in coincidence with SN 2011dh and adopt an upper limit of F_γ ≲ 6 × 10⁻⁷ erg cm⁻² s⁻¹ corresponding to an energy of E_γ ≲ 5 × 10⁴⁵ erg for SN 2011dh.

6.1. A Comparison to Breakout Predictions

We present multi-wavelength follow-up observations of SN 2011dh spanning the radio, millimeter, X-ray, and gamma-ray bands and obtained within the first few seconds to weeks following the explosion. The X-ray light curve for SN 2011dh is perhaps the best-sampled to date for an SN IIb and suggests that X-ray properties (luminosity, spectral evolution) may further distinguish compact and extended progenitors. Using the newly available wide bands of the EVLA, we show that the radio and millimeter data are consistent with a synchrotron self-absorbed spectrum, while the X-ray emission may be understood as IC upscattering of optical SN photons assuming a non-negligible deviation from equipartition of the shock partition fractions. These data offer unique diagnostics (shock wave velocity, $\overline{v}\approx 0.1c$) and point to a compact progenitor star. Through modeling of the early optical emission, we find that the photosphere is characterized by a low temperature, T_ph ≈ 8000 K, and the physical parameters of the ejecta are constrained to be E_{K, 51}/M_{ej, ☉} ∼ few. These properties point to a compact progenitor, R_* ≈ 10¹¹ cm, however, we find that the fast decay of the early optical emission at Δt ≲ 5 day is incompatible with canonical cooling envelope models and may suggest an irregular ejecta profile. A compact progenitor size appears inconsistent with the extended radius (R_* ∼ 10¹³ cm) of the coincident yellow supergiant identified in pre-explosion HST imaging, suggesting an unusual density profile for the outer layers of the progenitor star. It may be possible to explain the coincident object as a binary companion, however, it is estimated that the yellow supergiant phase lasts just ∼3000 years (Drout et al. 2009) making this scenario improbable. At the same time, the rarity of yellow supergiants makes a positional coincidence unlikely, suggesting the supergiant is indeed related to the progenitor system.

If the supergiant was characterized by a low-mass envelope and a low-density stellar wind, it may be possible to reconcile the radio and X-ray observations with the putative progenitor. Georgy (2012) recently suggested that the mass-loss rates for yellow supergiants have been theoretically overestimated. However, the early cooling envelope emission requires a substantially lower density envelope then those associated with supergiants; in this scenario, ejecta asymmetries may be the only way to reconcile the early optical data with the coincident supergiant as the progenitor.

We conclude that SN 2011dh is likely a member of the Type cIIb class of core-collapse explosions. Our long-term EVLA monitoring observations will reveal if the radio light curves are modulated (see Krauss et al. 2012 for details), indicative of a variable and/or episodic mass-loss history in the decades leading up to explosion—an observational characteristic shared by other SNe cIIb. In addition, our long-term monitoring with the Very Long Baseline Interferometer will provide a direct measurement of the shock radius and, in turn, an independent constraint on the shock partition fractions (Marti-Vidal et al. 2011; Bietenholz et al. 2012). Future observations of the explosion site with HST imaging will reveal whether the supergiant has disappeared, thereby directly linking it to the explosion.

Finally, we used the parameters derived above for the shock wave and progenitor to estimate that the shock breakout pulse from SN 2011dh was detectable in the X-ray and gamma-ray bands with current satellites. We attribute the lack of a detection as likely due to limited temporal/spatial coverage during the estimated explosion date. Looking forward, sensitive and wide-field X-ray experiments such as LOBSTER and JANUS will regularly discover shock breakout emission from similarly nearby and compact SN progenitors (see, e.g., Soderberg et al. 2009 for a discussion). Such detections will not only provide information on the progenitor size since the explosion date estimates will enable statistically significant searches for gravitational wave and neutrino counterparts (Ott 2009; Katz et al. 2012). Furthermore, these prompt discoveries will enable rapid low-frequency follow-up with sensitive radio and millimeter facilities thanks to the advent of EVLA, the Atacama Large Millimeter Array, ASKAP, and MeerKAT.

We thank Philip Massey, Edo Berger, Ryan Foley, Maria Drout, and Robert Kirshner for useful conversations. K.H. is grateful for IPN support from NASA grants NNX10AI23G, NNX09AU03G, NNX07AR71G, and NNX10AR12G. B.K. and E.M.L. are supported by NASA through Einstein Postdoctoral Fellowship awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. Support for CARMA construction was derived from the Gordon and Betty Moore Foundation, the Kenneth T. and Eileen L. Norris Foundation, the James S. McDonnell Foundation, the Associates of the California Institute of Technology, the University of Chicago, the states of California, Illinois, and Maryland, and the National Science Foundation. Ongoing CARMA development and operations are supported by the National Science Foundation under a cooperative agreement, and by the CARMA partner universities. The SMA is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics, and is funded by the Smithsonian Institution and the Academia Sinica. The EVLA is operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.