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Astron. Astrophys. 333, 583-590 (1998)

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6. BH X/O transients masquerading as classical novae

Discoveries based on the optical outbursts have not yet been considered. As previously mentioned, X/O novae appear optically similar to classical novae with moderate amplitude, slow decay and post-maximum fluctuations (type Bb or similar). In fact, 2023+338/V404Cyg appeared in earlier nova catalogues as Nova Cyg38 and A0620-003/V616 Mon was detected in its 1917 outburst at [FORMULA] on archival plates and claimed to be a normal recurrent nova (Eachus, et al. 1976). Clearly outbursts can be detected in the optical, but the historical nova surveys must be very incomplete at these magnitudes. Again assuming that the X-ray selected, dynamically confirmed objects provide a fair sample of absolute luminosities at outburst, one can use the modeled distribution of low mass BHB to predict the rate of optical BHB transients as a function of magnitude and then estimate the number appearing in optical novae catalogues (eg Duerbeck 1987; Downes, Webbink & Shara 1997). It is particularly interesting to estimating the number of X/O novae recorded in the period between 1895 and 1975, when the nova discovery rate was fairly constant (Shafter 1997), but there was little or no X-ray sky coverage.

The absolute magnitudes of the recent X/O novae cluster around [FORMULA] with the exception of 2023+336 which was very luminous at [FORMULA]. This is likely a consequence of the fact that the disk area in this system is [FORMULA] times larger than that of the other binaries. This leads us to speculate that optical outburst selection may preferentially find BHB with large [FORMULA]. Certainly an appreciable number of X/O novae prior to 1975 should have [FORMULA] and may be detected on archival plates. Some fraction will have been noted at outburst as novae.

Estimating this fraction, the completeness of faint nova discovery, is surprisingly difficult. Duerbeck (1984) argues that for [FORMULA] the nova discoveries are `complete', while for fainter novae only [FORMULA] of the potentially observable systems are discovered. The completeness clearly drops off to even fainter magnitudes, with a modern sample collected by Liller (1987) suggesting a fraction [FORMULA] of novae (mostly brighter than [FORMULA]) are discovered. From the ([FORMULA] 1975) nova discovery rates as a function of magnitude listed in Warner (1995) and Shafter (1997) and a census of pre-1975 novae in Downes, et al. (1997) extending fainter than [FORMULA], one can attempt to use the population distribution and extinction model of Sect. 3 to estimate the discovery completeness. To do this, the absolute luminosity distribution is extracted from the bright ([FORMULA]) novae in Shafter (1997) with measured [FORMULA] and secure distances from either nova shell expansion or detailed observations at maximum. The model novae are drawn from this parent [FORMULA] distribution, distributed according to (3.2), subject to extinction and binned into apparent luminosity ranges. Since historical novae are discovered visually or on sky survey plates, here V magnitudes and extinctions are used to more closely approximate the selection effects. Comparison of the model outburst frequency with the observed detection rate, normalized to the bright nova rate, should give the completeness. In practice the model completeness is non-monotonic with estimated discovery fractions [FORMULA] at [FORMULA] and lower rates at [FORMULA] and at faint magnitudes. This is probably due to an insufficient number of clear sight-lines to the galactic bulge in the model (where novae at [FORMULA] start to appear) as a result of unmodeled small scale inhomogeneity in the ISM. However, it seems that discovery rates remain greater than [FORMULA] 20% to a limiting magnitude [FORMULA]. A rough check of the estimates was obtained by comparing the values of [FORMULA] and [FORMULA] for well-studied novae at [FORMULA] against the model predictions, finding generally good agreement. It will be very difficult to improve the completeness estimates, so for now a conservative efficiency is adopted, falling smoothly from 0.5 at [FORMULA] to 0.15 for [FORMULA] and dropping rapidly to near 0 at [FORMULA] (Fig. 1). This predicts, for example, a discovery fraction 0.25 in the bin [FORMULA]. This is lower than the discovery fraction of [FORMULA] 0.5 implied by the fact that of the two post-1975 X-ray selected X/O novae now known to be present on archival plates at [FORMULA], one was noticed as a classical nova. While the estimates above are designed to be conservative, they must still be considered uncertain by a factor of [FORMULA].

[FIGURE] Fig. 1. Differential and cumulative numbers of X/O transient low mass BH systems expected in the historical nova catalogues covering 1895-1975. The dashed line gives the presumed recovery efficiency as a function of magnitude (right hand scale).

The efficiency of nova discovery represents a substantial uncertainty, but the number of nova in historical catalogues depends on the nova rate and, like the predicted BATSE detection rate, is not subject to uncertainties in the low mass BHB recurrence time or total population. Fig. 1 also shows, for the adopted efficiency, the number of low mass BHB expected in Duerbeck (1987) in each magnitude bin over the 80y span (1895-1975). The histogram shows the cumulative number for the brighter limiting magnitudes; the total expected in the catalogue to [FORMULA] is 13. Again this total is probably conservative, since a naive extrapolation of two systems selected during the [FORMULA] y of X-ray coverage extrapolated to 80y ([FORMULA] 1975) and a recovery rate of 1/2 gives a prediction of 6 X/O novae detected brighter than 11.5. The adopted efficiencies predict 3 such X/O novae. Interestingly, more than half of the expected recoveries are from P -type systems (Table 4), which are relatively luminous and have short recurrence times. One can thus expect a modest number of X/O novae to be recovered in historical catalogues. These represent a substantial fraction ([FORMULA] %) of the recorded novae fainter than [FORMULA] at maximum.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998