CI Aql: Eclipsing recurrent nova!


On April 28, K. Takamizawa discovered a possible nova. The position was very close to the "old nova" CI Aql (1917). At the position of the literature CI Aql, an eclipsing binary showing HeII and CIII/NIII lines was discovered (IBVS 4338, 4332) (vsnet-alert 4695, vsnet-alert 4698). In the same night, M. Yamamoto also independently discovered the event at 9.8mag (vsnet-alert 4696, vsnet-alert 4697). On April 29, M. Uemura and T. Kato performed a spectroscopic observation at Okayama Astrophysical Observatory and detected strong H-alpha and (weaker) H-beta emission lines from which we confirmed the object is a genuine nova (vsnet-alert 4699).

(CCD image by Yasuo Sano)

(Spectrum taken by Makoto Uemura and Taichi Kato on Apr. 29 UT, Okayama Astrophysical Observatory)

A spectrum observed by M. Fujii can be seen at: (vsnet-alert 4719).
A spectrum observed by G. Cole can be seen at: (vsnet-alert 4730).
W. Liller's spectroscopic results: vsnet-alert 4731.
S. Kiyota's spectra and CCD image:
(vsnet-alert 4735).

With a CCD image taken by S. Kiyota and at Bisei by K. Ayani and K. Shirakami, H. Yamaoka performed astrometry and the calculated accurate position of the nova strongly indicated that it is identical with the eclipsing binary with strange spectrum (IBVS 4232, 4338), which is thought to be a quiescent counterpart of Nova Aql 1917 = CI Aql (vsnet-alert 4717, vsnet-alert 4712, vsnet-alert 4725). The quiescent identification with a 0.6-day eclipsing binary with HeII and CIII/NIII emission lines would then be interpreted as a system closely related to supersoft X-ray source (SSXS) which repeatedly undergo nova explosions. The eclipse light curve and the orbital period strongly resembles those of known SSXS and V Sge stars. The presence of high excitation emission lines are common among them, and CV-type recurrent novae like U Sco (vsnet-alert 4714).

Follow-up observations

R. Novak performed time-series observation and reported there are some profiles in the light curve on April 30 (vsnet-alert 4723). Time-resolved CCD photometry reported to the VSNET collaboration team until May 3 has shown an evidence of the presence of eclipses as predicted in [vsnet-alert 4742]. Since classical novae (even the fastest) usually do not show prominent eclipses until they fade more than 3-4 mag below the maximum, the likely detection of eclipses at this stage may suggest that the underlying binary is emerging from the nova photosphere. This may indicate either the true maximum, a few magnitude brighter, could have been missed, or the ejecta was thin compared to classical novae (vsnet-alert 4756). From the data on 5 May at Kyoto, it was suggested a relatively large phase offset may be necessary from proposed eclipse ephemeris (vsnet-campaign 7). A possible secondary eclipse was observed on 8 May at Kyoto and Tsukuba (vsnet-campaign 16). The object kept the roughly constant magnitude of 9.1 in early May, and then, began decline (vsnet-alert 4802). A. Pearce reported an appreciable short-term variability (vsnet-campaign 80). The observation by Kyoto team on May 25 also confirmed the presence of short-term variability, whose characteristics seem to be different from that reported in early post-maximum stage (vsnet-campaign 129). The object began fading after the peak on May 23 (vsnet-campaign 160).

The analysis of combined data by S. Kiyota (Tsukuba, Japan) and the Kyoto team on Jun. 15 has yielded a detection of the primary eclipse minimum. The eclipse was 0.3mag deep. A preliminary analysis yielded a mid-eclipse time of 15.597 UT, while the prediction (based on the epehemeris in IBVS 4232, Mennickent and Honeycutt) was 15.586 UT (vsnet-campaign 220). On June 20, T. Kato reported that the fading of CI Aql has become slower and short-term light variations have become less prominent. This may resemble a "plateau" stage observed in the later stage of recurrent nova outbursts (vsnet-campaign 237). Interrupting the plateau stage, the slightly rising was confirmed through the data on June 30 from Kyoto observations (vsnet-campaign-nova 15). The observation on July 1 at Kyoto has again caught the fading of CI Aql (vsnet-campaign-nova 16). The observation on July 3 by S. Kiyota confirmed the secure appearance of primary eclipses (vsnet-campaign-nova 33). L. Kral caught the whole primary eclipse on July 5/6 and reported the eclipse depth of about 0.2mag and the duration of about 2.3 hours (vsnet-campaign 314). The Kyoto observation on July 9 succeeded in very clearly record the predicted primary eclipse. The eclipse is V-shaped around the minimum, and the eclipse center was determined as July 9.713 UT, giving an O-C of +0.012 to the ephemeris (vsnet-campaign-nova 39). The fading trend stopped since the early August at the magnitude of about 13.6mag (vsnet-campaign-nova 79). As reported in IAUC 7528 (K. Matsumoto and T. Kato), the object has apparently entered the final rapid-decline stage (vsnet-campaign-nova 93).

General Information for This Object
VSNET Collaboration page