Photographic History and Suggested Nature of the Radio Source 3C 48

PHOTOGRAPHIC HISTORY AND SUGGESTED NATURE OF THE RADIO SOURCE 3C 48 Harlan J. Smith and Dorrit Hoffleit Yale University Observatory INTRODUCTION While seeking optical identifications of discrete radio sources in the principal catalogs, Mount Wilson and Palomar astronomers found what appears to be a peculiar star at α = 1^34^5(^5, δ = +32054'2 (1950) coinciding with the position of the Cambridge source 3C 48.1 They suggested the possibility that this object may be, apart from the sun, the first known radio star. While the object shows only a stellar image on 48-inch Schmidt plates, on 200-inch plates it appears to be centered in a faint nebulosity 5" X 12^ in extent. The spectrum is complex, containing absorption as well as emission lines. A broad emission feature has been identified provisionally as He ιι λ 4686, in which case the lines are not significantly red-shifted but are Doppler-broadened by about 3000 km/sec. Photoelectric magnitudes and colors are given as V = 16.04, Β — V = +0^40, and t/ - 5 = -0^61. The 2-meter radio flux is about 4.5 X 10-25 watts nr2 (c/s)"1, and the spectral index between 160 and 960 Mc is 0.42. The radio angular diame- ter is very small, less than a few seconds of arc. The above information, while of necessity preliminary and sketchy, nevertheless suggests certain speculations and possible verifications concerning the nature of 3C 48. Specifically, identified nonthermal radio sources in the galaxy are normally asso- ciated with rapidly moving gas clouds, more particularly with the remnants of supernova explosions. As a working hypothesis we may consider 3C 48 to be in this class of exploded stars, but in view of its unusual nature the initial possibilities might be broadened to include nova as well as supernova history, despite the fact that no nova has yet been identified as a radio source. general arguments bearing on the search The known post-nova stars (excluding recurrent novae) have 2 Mmin = +2.1 to +6.8. At galactic latitude —28° our line of 292 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System PHOTOGRAPHIC HISTORY OF 3C 48 293 sight to 3C 48 should pass through less than 500 parsecs of serious obscuration, giving an expected extinction of about 0^5 ; reason- able limits to the extinction are = 0Ψ1 to 1^0. These values, in the conventional relation M = m + 5 — 5 log d — A, set plau- sible limits on the distance to the object (Table I). TABLE I Distance in Parsecs as a Function of Assumed Absolute Magnitude and Absorption M \ 0™! lm0 +2.1 5900 3900 +6.8 670 440 If we assume the emission lines to be related to the nebulosity surrounding the stellar image, their half-width indicates a radial expansion velocity of the cloud of 1000 to 2000 km/sec. The present angular radius, 6", coupled with this velocity range and with the possible distances, then permits estimation (see Table II) of the range of intervals which may have elapsed since the outburst producing the cloud : τ _4.74 θ d where Τ is in years, the cloud radius Θ in seconds of arc, the distance d in parsecs, and the expansion velocity V in km/sec. Thus, TABLE II Time in Years Since Outburst, as a Function of Distance and Velocity of Expansion \ V d\ 1000 2000 440 13 6 5900 170 80 if the object now has an absolute magnitude commensurate with reasonably normal ex-novae, its explosion should have occurred since about 1790 and before 1955. The much more likely hypothesis of supernova origin is more difficult to evaluate since absolute magnitudes of supernova rem- © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 294 H. J. SMITH AND D. HOFFLEIT nants are an almost total enigma. Only one good case is available —the Crab Nebula (Ml), the residual star of which seems to have M*—' +5.5. This, and its expansion velocity of 1100 km/sec, fall well within the range of M and V spanned by the tables for novae above. As a nova, 3C 48 would have reached a peak apparent brightness approaching the visual limit ; in any event, it should have been brighter than m = 12 for at least a year during its slow decline. As a supernova of either type I or II, it would have been far above the visual limit, although perhaps only for several months while the sun was in this region of the sky; it would, anyhow, have remained brighter than 12th magnitude for several years. Such considerations seemed to warrant the effort of searching for an outburst on historical plate material. AVAILABLE HISTORICAL PHOTOGRAPHS A thorough search for past activity of 3C 48 was made possible with the kind permission and cooperation of the Harvard Observatory and with extensive help from C. Brooks, E. Dirks- wager, S. Gross, B. Marsden, and J. Rodman, all graduate students in our course on variable stars. Initially the writers checked all Harvard plates covering the region that were made with instruments having 8-inch and larger objectives. Those plates with limiting magnitudes reaching or approaching that of the 3C 48 object were taken to Yale for more careful examination. Later, with the students, we scanned the Harvard patrol plates taken with 4-inch and smaller objectives. In all, over 2500 plates were examined. Figure 1 summarizes the distribution in time of the plates (except the meteor patrols) with their approximate limiting magnitudes. Practically all the plates penetrated below magnitude 11.5, nearly 600 reached 13th, less than 100 reached 15th, and only 75 had the stellar image asso- ciated with 3C 48. Of course, each year has a gap in the plate material at least from late March through late June, often from February to September. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System PHOTOGRAPHIC HISTORY OF 3C 48 295 Fig. 1.—Distribution of plate material by year and limiting magnitude. Plate I, the finding chart, was kindly supplied from a 48-inch Schmidt plate by J. F. Bartlett. It shows the comparison stars which we used for estimating both the magnitude of the 3C 48 object and the limiting magnitudes of the plates not reaching it. Magnitudes for the comparison stars (Table III) were derived mainly by comparison with the Kapteyn Sequence in S. A. 45 at lh26m, +300^^4 but the brighter stars-were also compared with the Carte du Ciel Sequence at 2b00m, +31o30r,5 on patrol plates and confirmed the magnitudes derived on the basis of the Selected Area Sequence. Further approximate checks of the scale of the sequence were provided by interpolating star counts in the tables given by van Rhijn.6 Although these tables are based on the Selected Area sequences, the counts of numbers of stars per square degree to successive comparison stars indicated magnitudes systematically 0Ψ7 fainter than the direct comparisons with the Selected Area Sequence. The scale of magnitudes, however, is consistent by the two methods. TABLE III Adopted Magnitudes of the Comparison Stars A 11.3 13.0 13.7 —F' 14.2 Β 11.8 D' 13.3 E' 14.0 G 14.6 C Var? D 13.5 ^F Var? --G' 15.2 —H 15.7 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System PLATE I Ν -ι- AGK2 33-158 ΙΟ*. Β-· -D . ♦-Α C- -.t-E· Η^ν. ι ·~Ρ 6 # ^ΜΜΜ» * «ΜΜ9 F-'1 \ Ο'* ' Ε-· 30-48 The Field of 3C 48 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 296 H. J. SMITH AND D. HOFFLEIT RESULTS Of the nearly 2500 plates which failed to reach ISth magnitude, none showed a detectable image at the position of 3C 48. With slightly less conviction, the same statement of absence can be made for any other flaring object within several minutes of arc of the 3C 48 position, since each observer was also looking for novel features anywhere in the immediate vicinity. The 75 plates having an image of 3 C 48 were checked very carefully to investigate the further possibility of slight fluctuations or secular variation. Estimates of magnitude were made against the comparison stars on all of the plates independently by all seven observers. Table IV summarizes the results by plate series and Figure 2 the results by date. Among these observations only m 15.0 ·· ·· · • ·· ·Im · IS.2 • ·· · · — 15.4 J. (24)15000 20000 25000 30000 JD isioo ts'zo 19^40 Fig. 2.—Positive observations of 3C 48. seven are prior to 1925 : on JD 2414991 (1899), 2416085 (1902), 2416396 (1903), 2419306 (1911), 2423324 (1922), 2423997 (1924), and 2424034 (1924) ; all other positive observations are for the years 1933-49. On the Lick and Palomar charts the object appears to have about the same brightness as its average on the Harvard plates. Photometry of such plate-limit images is difficult, and the scatter found by individual observers and in the averages given in Figure 2 is no more than one would expect. We conclude that no evidence has been found for continuous or sporadic change of © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System PHOTOGRAPHIC HISTORY OF 3C 48 297 TABLE IV Summary of Positive Harvard Observations of 3C 48 Telescope Aperture Scatter No. of Scale Series (inches) (mag.) Plates ("/mm) A 24 15.0-15.2 0.2 3 60 MC 16 15.1-15.3 0.2 4 93 MA 12 15.1-15.2 0.1 24 93 IR 8 15.1-15.3 0.2 16 162 I 8 15.0-15.1 0.1 3 163 RL 4 15.3-15.3 0.0 3 290 BM 3 15.2-15.3 0.1 3 390 RH 3 15.0-15.3 0.3 19 390 brightness greater than 0Ψ3.

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