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Optical brightness and colours of V2492Cyg before, during and after the recent record peak in brightness

ATel #10259; Dirk Froebrich (University of Kent), Justyn Campbell-White (University of Kent), Tarik Zegmott (University of Kent), Samuel J. Billington (University of Kent), Sally V. Makin (University of Kent), Justin Donohoe (University of Kent)
on 12 Apr 2017; 17:56 UT
Credential Certification: Lynne Hillenbrand (lah@astro.caltech.edu)

Subjects: Optical, Young Stellar Object, Pre-Main-Sequence Star

The highly variable young star V2492Cyg (PTF 10nvg; VSX J205126.1+440523) underwent a record peak in brightness in early March 2017, as reported by Ibryamov and Semkov (ATel #10170). Recent observations by Munari et al. (ATel #10183) showed a significant drop in brightness and mildly redder colours on March 31st.

We have monitored the VRI brightness of V2492Cyg for the last year with the University of Kent's Beacon Observatory. It is equipped with a 17" Astrograph and 4kx4k CCD. Additional observations were made as part of the citizen science project HOYS-CAPS.

The object reached a period of minimum brightness from about JD2457500 to JD2457550. During that phase the detected brightness dropped as low as V=16.8mag (with several non-detections with the same magnitude as the detection limit), R=15.5mag, I=14.1mag. It then brightened to V=14.5mag, R=13.4mag, I=12.1mag on JD2457641.36 before fading by 1.3mag, 1.1mag, and 1.0mag in VRI, respectively, within the next 21days. It then brightened to an observed maximum of V=13.3mag, R=12.3mag, I=11.2mag on 2017-01-26 (JD2457780.25), slightly brighter than and about 5.5weeks prior to the reported maximum in Ibryamov and Semkov (ATel #10170), who did not cover this period in their observations. The maximum brightness that we recorded was V=13.5mag, R=12.2mag, I=11.4mag. Observations were also taken two days prior to and after the maximum, and the object was about 0.1mag fainter in all filters on both days. For a period of 57 days after the maximum V2492Cyg has faded by 1.6mag, 1.5mag, and 1.4mag in VRI, respectively. Since then it has entered a phase of rapid variations (up to 0.5mag) from night to night with a general increase of 0.5mag.

Our light curves and colour evolution plots show that the general brightness variations are also associated with changes in optical colours. Between the observed minimum and maximum brightness the object decreased its V-I colour from 2.90mag to 2.15mag while V-R dropped from 1.55mag to 1.00mag.

The phases of brightening or fading of the object can be put into two categories: i) colour and magnitude changes in agreement with changes in normal interstellar extinction; ii) changes in brightness with almost no discernible changes in colour. During the observed period, when fading at a particular magnitude the object always had an about 0.1-0.2mag bluer V-I colour than when brightening at the same magnitude.

link to plots and data