Astron. Astrophys. 324, 683-689 (1997)

4. The ⁴⁴ Ti line emission of historical supernovae

4.1. Interpretation of COMPTEL results

Due to the short lifetime of radioactive ⁴⁴ Ti, young SNR s are the best potential sources of line emission. The history of astronomy records six supernova events in the course of the second millenium (van den Bergh & Tammann 1991). For each SNR, the maximum-likelihood analysis provides a flux estimate and quantifies the confidence level for a source detection (Table 2).

Table 2. COMPTEL measurements of ⁴⁴ Ti line emission from Galactic supernovae of the second millenium. Supernova distances and types are gathered from van den Bergh (1990). An explosion date of 1680 is assumed for Cas A (Fesen, Becker & Goodrich 1988) according to Flamsteed's testimony (Ashworth 1980). From the line fluxes and upper limits, two estimates of the ⁴⁴ Ti yield are inferred depending on the ⁴⁴ Ti lifetime (Sect. 1). Formal uncertainties are given for Cas A; upper limits are quoted at the confidence level. Systematic uncertainties are estimated to be less than 25%. No line flux is reported for the Crab which is a strong continuum source. Since its progenitor was a star (Nomoto et al. 1982), SN 1054 released less than of ⁴⁴ Ti (Woosley & Weaver 1995) so that the present-day flux is more than a hundred times below the sensitivity threshold of COMPTEL. A search for gamma-ray lines from the Crab is currently underway (van der Meulen et al. 1996).

Let D (kpc) denote the distance to the SNR, () the ejected mass of ⁴⁴ Ti, and t (yr) the time elapsed since the explosion. If is the mean ⁴⁴ Ti lifetime, the flux expected to be measured today reads:

At present, is loosely determined between 75 yr and 95 yr (Sect. 1). Rather than assuming any arbitrary lifetime, we convert each COMPTEL flux into two ⁴⁴ Ti yields according to the values of proposed by Adelberger & Harbottle (1990) and Frekers et al. (1983):

(Table 2). The inaccurate knowledge of translates into a large uncertainty on . For a old supernova, these yield estimates differ by a factor of two.

Out of the six historical supernovae recorded in the last millenium, only Cassiopeia A is detected by COMPTEL. Upper bounds of are derived for the other objects at the confidence level. Whether this actually constrains the yields depends on the SN event date. Supernovae from the 11th and 12th centuries - SN 1006 (Lupus), SN 1054 (Crab), SN 1181 (3C58) - are much too old. Upper limits of appear well above nucleosynthesis predictions by any standard model (Sect. 1). In the following we only discuss the three 16th- and 17th-century supernovae.

4.2. SN 1572 and SN 1604

From its low brightness and the shape of its light curve, SN 1572 (Tycho's SN) is classified as a Type-Ib supernova (Strom 1988). Type II-L is not excluded however (Doggett & Branch 1985) while van den Bergh (1993) even suggests that it may be a subluminous Type-Ia supernova. SN 1604 (Kepler's SN) is of equally uncertain type (van den Bergh 1990). Its high Galactic latitude favours a progenitor from the old stellar population (SN Ia), but the core collapse of a massive 'runaway star' (Bandiera 1987) now seems more plausible (Bandiera & van den Bergh 1991).

Our results set marginal constraints on the ⁴⁴ Ti mass released by these supernovae. If distance uncertainties are taken into account, (Strom 1988) to 4.5 kpc (Schwarz et al. 1995) for SN 1572 and 3-5 kpc (Rothenflug et al. 1994) for SN 1604, our observations imply ⁴⁴ Ti yields of - . These values are tantalizingly close to model predictions (Sect. 1; see Fig. 3), which suggests that both SN 1572 and SN 1604 are potential targets for the planned INTEGRAL mission in the line or for operational X-ray missions (XTE and SAX) in the two ⁴⁴ Ti lines at 67.9 and 78.4 keV, with fluxes in the range to .

Fig. 3. Age-distance detectability of a supernova through 44 Ti decay. The curves correspond to a present-day line flux for a mean lifetime (solid line) or 78.2 yr (dashed line). A yield is assumed. If this flux level is adopted as the detection threshold, COMPTEL has access to SNR s located in the lower left region delimited by these curves. Note that for a given age t. Zero-age supernovae are detectable by COMPTEL up to 15-20 kpc, a range excluding SN 1987A in the LMC.

4.3. Cassiopeia A

Cas A is considered to be a Type-Ib supernova involving a Wolf-Rayet star of initial mass 25- (e.g. Brecher & Wasserman 1980, Fesen & Becker 1991). Its low luminosity supports this hypothesis, since there is no historical record of any bright event around 1680, when Flamsteed observed it as a 6th-magnitude star only (Ashworth 1980). Models of Type-Ib supernovae (Hashimoto, Nomoto & Shigeyama 1989, Woosley, Langer & Weaver 1996) predict that they typically produce to of ⁴⁴ Ti.

Cas A was first seen by COMPTEL with a line flux of (Iyudin et al. 1994). In the most favourable case (event date 1680, , ), this converts into a ⁴⁴ Ti yield of which implies that substantial amounts of ⁵⁶ Ni powering the light curve were ejected by the supernova. As stressed by Hoffman et al. (1995), this result is difficult to reconcile with ⁴⁴ Ti nucleosynthesis predictions and observational evidence that Cas A was not a bright supernova.

Using COMPTEL data spanning three years, we detect Cas A in the 1.07-1.25 MeV band at the significance level with . Continuum emission is negligible towards the supernova: measurements in the neighbouring energy bands are below the noise level. Emission in the 1.07-1.25 MeV band is then attributable to ⁴⁴ Ti decay.

We derive an updated line flux for Cas A of , which is in agreement with the latest OSSE findings, (The et al. 1996). This value is obtained using background model (ii) (Sect. 2.2). In order to assess systematic effects, we have also performed a study of Cas A with method (i) and found . The difference between the two values is within statistical fluctuations as well as the estimated systematic uncertainty inherent to the analysis.

Depending on the ⁴⁴ Ti lifetime, between 75 and 95 yr, and the SNR distance, -3.4 kpc (Reed et al. 1995), the supernova ejected 0.9 to of ⁴⁴ Ti assuming a flux of . This yield is lower than the earlier values and seems to match our theoretical picture of Cas A. Yet it implies that the supernova was intrinsically bright (Timmes et al. 1996). That the event was hardly attested requires a larger visual extinction than the mag measured toward Cas A (Peimbert & van den Bergh 1971), presumably due to the dust and gas shell which surrounded the Wolf-Rayet star at the time of its explosion (Hartmann et al. 1996).

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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