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The enigmatic star KIC 8462852 (hereafter KIC8462) was found to vary in brightness during the 4 years of the Kepler mission by up to 22% with no obvious periodicity (Boyajian et al. 2016).
This Research Note reports on a potential repeat interval of 1601 days of the Kepler day 1540 (D1540) dimming event in observations by the Hereford Arizona Observatory (HAO) during 2017 August.
Gary & Bourne (2017) (hereafter GB17), suggested a 1600 day repeat of a yearlong U-shaped fade in KIC8462, seen in Kepler full frame images (reported by Montet & Simon 2016) and also present in HAO data in 2017. In addition, GB17 point out that most dips occur during the later part of the U-shaped fades. Support for the U-shaped fades comes from Simon et al. (2017), who report a temporary increase in the brightness at KIC8462 after the Kepler mission, which is based on ASAS data. A repeat of a yearlong U-shaped fade in 2008 is not excluded by ASAS data.
Sacco et al. (2017) interpreted the 2017 dips as a repeat of the 2013 dips with a period of 1574 days by correlating dips in the 2013 and 2017 light curves.
Figure 1 presents the normalized flux for D1540 as well as a reversed display of egress overlaid on to the ingress portion of the graph. The reversed egress has been "stretched" by a factor of 1.08 to align the egress and ingress periods. This small adjustment is made to correct for the asymmetry between ingress and egress (due to a likely increasing transit velocity) during the 18 day transit caused by orbit eccentricity.
Figure 1. Kepler D1540 shifted forward 1601 days (black trace), HAO 2017 August V-band data (green diamonds) and a LCO TFN r'-band observation on JD2457974 obtained by Dr Boyajian (orange diamond). The red trace is a reverse of the Kepler egress observations.
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Transit simulation modeling of D1540 is the subject of a paper submitted to MNRAS by Bourne, Gary and Plakhov, that suggests the transiting object may be a brown dwarf (BD) with a ring system in an eccentric orbit with a transit velocity ∼25–30 km s−1. If this broad transit feature is a massive ring system, the system would be ∼0.2 au in diameter extending well beyond the Roche limit of a BD and would therefore be transient.
The HAO observation of KIC8462 during the two week period 2017 August 3–17 recorded a reduction in flux of up to 1.5% in V-band (Normalized Flux 1 = mag 11.9077). A comparison of the 33 hourly binned HAO observations with D1540 (shifted 1601 days) shows 25 of the observations align with D1540 egress (and the reverse egress curve for ingress period) within the HAO hourly bin error limits of ∼0.1% (1 mmag).
The observation on JD 2457974 was made by Dr Boyajian using the Las Cumbres Teide Observatory in Tenerife, Canary Islands, Spain (LCO—TFN), in the r'-band (from Dip Update 53n, WTF blog http://www.wherestheflux.com/single-post/2017/08/09/Dip-update-53n).
We note:
- (1)25 of 33 HAO observations in 2017 August match the Kepler data (and reverse egress) from 4.4 years earlier with a precision of ∼0.1%, and
- (2)the LCO TFN observation matches the brief central dip present in D1540, shifted by 1601 days.
The next transit of this object, in a 1601 day orbit scenario, is expected on 2021 December 27.
Ballesteros et al. (2017) found the standard deviation of the Kepler light intensities (one-week periods) exhibits a minimum near Kepler day 800, when our modeling of the hypothetical BD orbit places the object farthest from the star, and is maximum at a time that the model places the object closest. This indirectly supports the suggestion of a 1601 day period and the orientation for the proposed object's eccentric orbit.
An eccentric 1601 day orbital period for a substantial object, possibly a BD with an extensive ring system, is consistent with:
- 1.the suggested 1600 day period for the one year U-shaped fades,
- 2.the occurrence of dips in the latter part of the U-shaped fades,
- 3.light intensity variations peaking at the beginning and end of the Kepler observing period with a minima near day 800,
- 4.a possible repeat of D1540 in 2017 August, and
- 5.an apparent increase in transit velocity during D1540/2017 August being consistent with an eccentric orbit as the object approaches periapsis.
The presence of a BD in an eccentric 1601 day orbit could be detectable in 2017 October and November through radial velocity (RV) observations with a potential amplitude of ∼1–2 km s−1.
We thank Dr T Boyajian for making LCO observations available in the public domain, which allowed us to learn that the 2017 August dimming event exhibited a brief and deep central dip on JD 2457974 and for helpful comments during the drafting of this RN.