KIC 8462852 Hereford Arizona Observatory Photometry Observations #6
Bruce Gary, Last updated: 2018.08.12 19 UT

This web page has been "retired" because it already has too many items for downloading, and any more additions to it would just make it even slower to download. The last observation date described on this web page is 2018.08.01. Later date observations are described at ts7.

Since I have probably observed KIC846 more than anyone, I may have earned the right to comment about the suggestion of "alien mega-structures" as a possible explanation for the unexplained dimming behavior before natural explanations had been exhausted. I'm going to repeat a complaint by the Dutch astronomer Ignas Snellen of Leiden Observatory, but first I want to present a brief justification for my right to complain by describing my pro-SETI credentials: 1) In the 1950s I demonstrated the feasibility of an alien civilization in our interstellar neighborhood to broadcast its location by transmitting a map of how constellations appeared from their location, and 2) when I led the JPL Radio Astronomy Group in 1968 I suggested that SETI should be a group goal with a Deep Space Station radio telescope, which my successor eventually did, using DSS-14, until it was killed by a Nevada senator who complained about this NASA project. (As an aside, I was hired at JPL in 1964 by Frank Drake who was apparently impressed by my Jupiter observations with the radio telescope he used for Project Ozma). OK, here's what Dr. Snellen wrote (from link): 
"...there is no place for alien civilizations in a scientific discussion on new astrophysical phenomena, in the same way as there is no place for divine intervention as a possible solution. One may view it as harmless fun, but I see parallels in athletes taking banned substances. It may lead to short-term fame and medals, but in the long run it harms the sport. Same for astronomy: we should be very careful not to be ridiculed. I really hope we can stop mentioning SETI for every unexplained phenomenon. The article (by Elizabeth Howell) ends with a quote from Morris Jones, an Australian space observer: "The media is under pressure to deliver attention-grabbing news, but its hard to expect them to judge fringe SETI as spurious when it comes from reputable institutions and qualified researchers. The best way to reduce these reports is to stop the production of questionable scientific papers in the first place.  Amen!
_____________________________________________________________________________________________________________________________________ 

Links on this web page

    g', r' & i' Magnitude vs. date  
    V-mag and g'-mag vs. date 
 
    Comparison with AAVSO observations   
    List of observing sessions   
   
Finder image showing new set of reference stars
    HAO precision explained (580 ppm)  
    DASCH comment  
    My collaboration policy
    References    

  Go to 7th of seven web pages. 
  This is the 6th web page. 
  Go to 5th of seven web pages  (for dates 2017.11.13 to 2018.01.03)

  Go to 4th of seven web pages  (for dates 2017.09.21 to 2017.11.13)
  Go to 3rd of seven web pages  (for dates 2017.08.29 to 2017.09.18)
  Go to 2nd of seven web pages  (for dates 2017.06.18 to 2017.08.28)
  Go to 1st of seven web pages  (for dates 2014.05.02 to 2017.06.17)

    Reference Star Quality Assessment  (the 10 best stars out of 25 evaluated)  

This is the 6th web pages devoted to my observations of Tabby's Star. When a web page has many images the download times are long, so this is the latest "split."

g', r' and i' Mag's vs. Date


Figure 1.
HAO magnitudes at g'-band for the past 2.5 months. The "OOT only" trace is the second "U-Shaped" component needed to account for the upper-boundary of measurements (presumably the out-of-transit, or OOT, level).

V and g'-Mag vs. Date


Figure 2. HAO magnitudes at g'-band for the past 7 months. The "OOT only" trace is the second "U-Shaped" component needed to account for the upper-boundary of measurements (presumably the out-of-transit, or OOT, level).


Figure 3. This shows the last 2.5 years of C-, V- and g'-band HAO measurements (C- and V-mag's adjusted to match g'-mag's).


Figure 4. Normalized flux light curve for the past 2 months, based on the OOT model shown in the previous two graphs.


Figure 5. A one-year plot of "normalized flux" based on the previously displayed OOT model (with two U-shaped features). The HAO data for April are too sparse for showing dip structure well; the graphs in the next section do a better job of that sine they include AAVSO measurements.

Comparison of HAO with AAVSO Observations (Demonstration of the value of combining AAVSO data with other data with emphasis on the biggest ground-based observed dip.)

The following figure shows a selection of AAVSO observer V-band magnitudes vs. date for a 2.6-year interval.


Figure 6a. Selected AAVSO plus HAO V-band (and g'-band) magnitudes for a 2.6-year interval. Offsets for each observer were applied to achieve agreement with HAO data. Two U-shaped features are included in the OOT Model. Observers:  OAR = Arto Oksanen (Finland), HBB = Barbara Harris (Florida, USA), HJW = John Hall (Colorado, USA), LDJ = David Lane (Nova Scotia, Canada). 

During the first 2 years a steady fade is present (confirming last year's interpretation of HAO data). It is interrupted by a U-shaped fade feature in mid-2017, and is followed by a second U-shaped fade in early 2018. The "OOT model" trace is meant to show what would be observed if dips did not occur, so it can serve as a reference level for determining dip depth and shape. The deepest dip occurs at ~ JD4 = 8203 and is ~ 67 mmag (~ 6 %) deep.


Figure 6b. Same data as above, but showing just the last year of data.

David Lane (AAVSO Observer Code LDJ) has reprocessed his 2.6 year's of KIC846 observations (lot's of work!), and has re-submitted them to the AAVSO database. The LDJ V-band data for times of known dips were excluded, and the remaining OOT (out-of-transit) data was averaged in groups of 7 observing sessions. An arbitrary offset was applied to achieve maximum agreement (this is legitimate since every observer has a unique and unknown absolute calibration uncertainty). The purpose for performing these adjustments was to allow for an assessment of the presence of a U-shaped OOT pattern with an approximate 1-year timescale that has been derived using HAO data. The next figure is a light curve showing how the LDJ V-mag measurements compare with HAO data.


Figure 7. Six-month light curve for two observers, LDJ (David Lane) and HAO (Bruce Gary). The LDJ symbols are averages of 4 observing sessions (~ 25 images per session) while the HAO symbols are for individual observing sessions (>200 images per session, typically). Dip data for both LDJ and HAO have been excluded from this LC. The LDJ and HAO symbols can therefore be viewedd as OOT only. Both data sets confirm the U-shaped OOT model (centered on mid-2017). 

This figure shows agreement of both LDJ and HAO data with the main U-shaped OOT Model. 

I conclude that the U-shaped OOT model has support from both data sets. The depth of the U-shape is ~ 1.0 % and the duration of the U-shape is ~ 1.0 year. So far, the LDJ and HAO data agree in suggesting that since the time of maximum brightness at the end of the U-shape (near JD4 = 8050) a steep fade has been underway. LDJ B-band measurements will be presented here that show a greater U-shape depth than for V-band! This will provide a significant constraint on modeling the physical mechanism for the U-shape (large dust cloud vs. reflection of starlight by the dip clouds).

The next graph is a comparison of downloaded AAVSO data for March and April. Data for 5 observers is shown with offsets chosen to maximize agreement with HAO and each other. 


Figure 8. Comparison of g'-mag (and V-mag, adjusted) for 5 AAVSO observers and HAO during March, April and May, 2018. Arbitrary offsets have been applied to each observer data set to achieve agreement with HAO data, where overlap exists, and with each other (i.e., internal consistency). The green trace is the sum of a slowly varying OOT model plus a short-term dip modeled using "asymmetric hyper-secant" (AHS) functions. The 60 mmag dip ( ~ 5.3 %, dip at JD4 = 8203.4) is fitted well by the AHS function. (The data in this figure was downloaded from the AAVSO web site: https://www.aavso.org/lcg.) 


Figure 9. Comparison of B-mag for two AAVSO observers during March, April and May, 2018. Arbitrary offsets have been applied to each observer data set to achieve agreement with HBB data, where overlap exists, and with each other (i.e., internal consistency). The green trace is the sum of a slowly varying OOT model plus a short-term dip modeled using "asymmetric hyper-secant" (AHS) functions. The 82 mmag dip (6.6 %, at JD4 = 8203.4) is fitted well by the AHS function. (The data in this figure was downloaded from the AAVSO web site: https://www.aavso.org/lcg.)  [Still working with this graph, adding data from other observers.]

Both V-band and B-band exhibit a significant brightening from early April to early May. There appears to be a small but significant difference in the brightening amount at the two bands. The brightening from 8208 - 8215 to 8237 - 8253 is 13.7 +/- 3.0 mmag at V-band and 20.5 +/- 1.7 mmag at B-band (using a sum of chi squares analysis). The difference in brightening is 6.8 +/- 3.5 mmag, which is significant at the 2.0-sigma level. In other words, there's a 95 % probability that the difference in amount of brightening at B- and V-band is real, with a greater amount of brightening at B-band. If this finding holds up then we would have to invoke a dust cloud model to explain the slow variation (i.e., the U-shaped variations), and that dust cloud would have to include a significant component of small particles (<0.3 micron radius).

The deep dip in late March is also deeper at B-band than V-band. Again, this is evidence for the presence of a significant component of small dust particles in the dust cloud producing this dip.


Figure 10.  Same as Fig. 7, but showing only HBB, LDJ and HAO data. The good agreement between three independent data sets (showing a dramatic 2 % brightening) suggests that the variations shown are real. A similar graph for B-band shows the same brightening for different observers, again giving credence to the reality of the brightening.


Figure 12. Rafik Bourne's comparison of dip depth for different wavelengths. B-band has the deepest depth while Ic-band has the smallest depth. (The LCO data are manually read off graphs at Tabby's blog, made necessary because these data values are not in the public domain.) Clearly, the dip at JD4 = 8203.5 is due to a dust cloud dominated by a component of small particles (< 0.3 micron radius).

Transit Pattern and Speculation about Model for KIC846 Dust Cloud Geometry (not Physical Mechanism Model)

A thorough description of this matter is given in the 5th web page of this 6-part series of web pages: link.

 
List of Observations (for all earlier observations, before 2018 Feb 25, go to link)

2018.08.01  
2018.07.24  
2018.07.19  
2018.07.12  
2018.07.07  
2018.07.06  
2018.07.03  
2018.07.01  
2018.06.24  
2018.06.23  
2018.06.22  
2018.06.19  
2018.06.12  
2018.06.10  
2018.06.08  
2018.06.07  
2018.06.06  
2018.06.05  
2018.06.04  
2018.06.03  
2018.06.02  
2018.06.01  
2018.05.31 
2018.05.28 
2018.05.21  
2018.05.20  
2018.05.19  
2018.05.18  
2018.05.17  
2018.05.16  
2018.05.15  
2018.05.14  
2018.05.13  
2018.05.11  
2018.05.10   
2018.05.09  
2018.05.08  
2018.05.07  
2018.05.06  
2018.05.05  
2018.05.04  
2018.05.03  
2018.04.29  
2018.04.28   
2018.04.24  
2018.04.23  
2018.04.21  
2018.04.18  
2018.04.10  
2018.04.08  
2018.04.05  
2018.04.04  
2018.04.02  
2018.03.30  
2018.03.28  
2018.03.27  
2018.03.21 
2018.03.20  
2018.03.19  
2018.03.16  
2018.03.15  
2018.03.02  
2018.02.25  
 
Daily Observing Session Information (most recent at top)

2018.08.01  preliminary (still observing)





2018.07.24  





2018.07.19  





2018.07.12  





2018.07.07  





2018.07.06  





2018.07.03  





2018.07.01  





2018.06.24  





2018.06.23  





2018.06.22  





2018.06.19  





2018.06.12  





2018.06.10  





2018.06.08  





2018.06.07  





2018.06.06  





2018.06.05  





2018.06.04  





2018.06.03  





2018.06.02  





2018.06.01 





2018.05.31  g-data  r-data  i-data 





2018.05.28  





2018.05.21   g-data  r-data  i-data 





2018.05.20   g-data  r-data  i-data 





2018.05.19  g-data  r-data  i-data   
(graphs for r' and i' available by request)
  




2018.05.18  g'-data  r'-data  i'-data   (graphs for r' and i' available by request)





2018.05.17, g'-band, 5.2 hrs   g'-data  r'-data  i'-data 

Note: I have r' and i' graphs & data available for anyone who wants them.





2018.05.16, g'-band, 4.8 hrs  data 

I'm now observing with 3 filters: g', r' and i'.





2018.05.15, g'-band, 5.3 hrs  data 





2018.05.14, g'-band, 2.8 hrs  data 





2018.05.13, g' band, 2.7 hrs  data


Windy, so large PSFs.



2018.05.11, g'-band, 2.7 useable hrs  data 


This LC illustrates the importance of using only the subset of good quality data from an observing session, i.e., not an image set from a pre-determined, scheduled observing time.





2018.05.10, g'-band, 5.2 hrs  data  





2018.05.09, g'-band, 3.8 hrs (of useable data)  data  




I finally tweaked adopted mag's for two stars that had persistent opposite sign locations with respect to the fit line.

2018.05.08, g'-band, 2.9 hrs  data 





2018.05.07, g'-band, 1.4 hrs (bad focus tracking)  data





2018.05.06, g'-band, 4.6 hrs  data  





2018.05.05, g'-band, 4.9 hrs  data





2018.05.04, g'-band, 4.9 hrs  data  





2018.05.03, g'-band, 4.4 hrs, data 





2018.04.29, g'-band, 6.3 hrs, data 






2018.04.28, g'-band, 2.6 hrs  data  





2018.04.24, g'-band, 3.1 hrs  data 





2018.04.23, g'-band, 3.5 hrs  data 





2018.04.21, g'-band, 4.1 hrs, data  

Excellent observing conditions, good focus all night - so I have no explanation for the unprecedented brightness.





2018.04.18, g'-band, 1.3 hrs, data 





2018.04.10, g'-band, 4.5 hrs data





2018.04.08, g'-band, 3.5 hrs  data 





2018.04.05, g'-band, 4.6 hrs  data 





2018.04.04, g'-band, 4.6 hrs data    





2018.04.02, g'-band, 1.7 hr data 





2018.03.30, g'-band, 4.6 hrs data 





2018.03.28, g'-band, 3.7 hrs, data  





2018.03.27, g'-band, 3.1 hrs, data  





2018.03.21, g'-band, 2.5 hrs, data  





2018.03.20, g'-band, 2.8 hrs, data  





2018.03.19, g'-band, 2.2 hrs, data    





2018.03.16, g'-band,  1.4 hrs, data  

Changed optical configuration: inserted Optec focal reducer. Determined empirical correction to apply to this and subsequent data to be +0.0007 mag.





2018.03.15  g'-band, 2.6 hrs, Data  

"Atmospheric seeing" (PSF's FWHM) was bad, and variable, due to wind.





2018.03.02, g'-band, 2.1 hrs, Data
 




2018.02.25, g'-band, 1.2 hrs   DataExchangeFile  






Finder Chart

It has taken over a year to figure out which stars are reliably stable. I now use 17 stars for reference, some of which are shown in the next image.


Figure F1. Finder image, FOV = 12.5 x 9.6 'arc, north-east at upper-left, showing the 17 stars that I have determined are stable at about the 1 mmag level, out of 34 that have been evaluated. KIC846 is inside the blue square. 

I have evaluated stability of 34 stars near KIC846, and have accepted 17 for calibration use and have rejected 17. My acceptance criterion is based on RMS of daily averages, and also trend, during several months of data. My use of the 17 accepted stars began with observing date 2017.09.07. Some star behavior is described at link. Here's an assessment of the 17 stars during a 2-month interval in 2018.


Figure F2. Difference from average (plus an offset for clarity) for each of the 17 accepted stars used for reference.

RMS scatter and trend are used to create a "weight" that is later used in fitting an observing session ("star color sensitivity" fit, comparable to "CCD transformation coefficients", but better). The steepest trend is Star#2, (-12 mmag/2.2 months). That star also has a large RMS. It therefore has a low weight, ~8 % of Star#1's weight. Each star's "weight" is calculated using the subjective equation  (1/RMS) * (1/(3+|trend [mmag/2.2months]|). The highest weight star is Star#10, which has low RMS and no trend, as shown in the next figure.


Figure F3. Stability properties for 17 reference stars, showing final "weight" adopted for each.

HAO Precision Explained  

Some people may have noticed that my HAO measurements of KIC846 exhibit better precision than other observers. In this section I'll explain why. First, let's review the distinction between "accuracy" and "precision."

Accuracy vs. Precision

Precision is related to an observer's measurement-to-measurement RMS. It makes no claims regarding calibration. Accuracy is the orthogonal sum of precision and calibration accuracy. For example, I claim that my KIC846 g'-mag observing session precision and calibration accuracies are 0.0007 and 0.0200 magnitude. Their orthogonal sum (SQRT of the sum of squares) is 0.0201 magnitude. The task of describing a dip's depth and structure depends only on precision, whereas any comparison of HAO r'-mag with Tabby's LCO r'-mag, for example, would be evaluated in terms of our two estimated accuracies.

Measurement Precision
 
Ground-based measurements of a single image exhibit a noise level (i.e., precision per image) that is influenced by three principal factors: 1) thermal noise (CCD noise per pixel), 2) Poisson noise (fundamental limit set by sampling theory), and 3) atmospheric scintillation (produced mostly at the tropopause). I describe these, and other smaller sources of noise, in my photometry book Exoplanet Observing for Amateurs (Chapter 20). The most important of these noise sources, and probably the least appreciated, is scintillation! 
Scintillation noise can be approximated using the following equation (pg. 132):



where sigma = fractional RMS intensity fluctuation, D is telescope diameter [cm], sec(Z) is atmospheric air mass, h is observatory site altitude above sea level [m], ho = 8000 m, and g = exposure time [seconds]. Substituting D = 35 cm (14-inch), sec(Z) = 1.8, h = 1420 m and g = 30 seconds yields (these are typical HAO values) yields sigma = 0.0025 mag. This fluctuation occurs for all stars in an image, regardless of brightness. Note that this includes reference stars, which will scintillate differently (assuming they're > ~ 10 "arc apart). Use of just one reference star will cause the target star's scintillation noise to be root-2 greater (which is an argument for using many reference stars). Scintillation noise level can vary by a factor of two on hourly timescales; some nights can be significantly different from others (approximately 1/3 of nominal to 3 times nominal).

Poisson noise (for HAO) is typically 0.0024 mag (for KIC846). Thermal noise is typically 0.0015 mag (for KIC846). Scintillation noise, at 0.0025 mag typically, can easily become the most important noise component.

Its my impression that few observers realize that scintillation is often the dominant source of noise per image for bright stars. There are two ways to overcome scintillation noise: 1) use a very large aperture telescope (i.e., > 1 or 2 meters), and 2) average many images to obtain a more precise observing session average. It's possible that scintillation is too often overlooked because it's unimportant for faint stars. For professional astronomers it may be overlooked because it's unimportant when using large telescopes. However, with large telescopes short exposures are needed (to avoid CCD saturation), and this is only feasible when download times are short (to preserve duty cycle). Some professional observatories don't emphasize short download times (e.g., LCO). Every telescope system has an optimum observing strategy for a specific target and observing goal, and it is often the case that for a specific target and observing goal some telescope systems are closer to optimum than others. HAO can observe unbinned with exposure times of 120 seconds and download plus autoguide adjust time of 10 seconds. If the same CCD was on a 1-meter telescope, such as the LCO for example, exposure time would be limited to 15 seconds and download time would be 40 seconds. The improvement in scintillation by using a larger telescope would be lost by having to use a shorter exposure time with a much reduced duty cycle. The HAO may be within that "sweet spot."

Empirical RMS Noise Level: a Dose of Reality

Here's a plot of empirically-measured RMS noise level vs. star brightness for two air mass segments of one observing session.


Figure H1. Empirically-measured RMS noise level vs. star brightness for two air mass portions of one observing session when scintillation was about twice as bad as normal.

The brighter the star the more it departs from a simple SNR model. This is because scintillation is the same for all stars, regardless of brightness, so for brighter stars scintillation becomes the dominant source of noise.
The constant term for airmass = 1.7 is ~ 5.0 mmag, and since the Poisson noise level (for KIC846) is ~ 2.4 mmag and the thermal level (for KIC846) is ~ 2.0 mmag, we can estimate that the scintillation component was ~ 3.9 mmag (SQRT(5.0^2 - 2.4^2 - 2.0^2)). This is greater than the nominal 2.5 mmag (using the above equation).


Figure H2. Empirically-measured RMS noise level vs. star brightness for air mass = 1.6 for an observing session when scintillation was about normal.

This plot of RMS vs. g'-mag is for a date with an apparently lower level of scintillation noise. The constant term is ~ 3.5 mmag, and since the Poisson noise level (for KIC846) is ~ 2.4 mmag and the thermal level (for KIC846) is ~ 1.1 mmag, we can estimate that the scintillation component was ~ 2.3 mmag (SQRT(3.5^2 - 2.4^2 - 1.1^2)). This is slightly lower than the nominal 2.5 mmag (using the above equation).

Implications for KIC846 Observing

What are the implications for all of this when only a few images are made during an observing session? The above figure shows that if I took just one image the KIC846 g'-mag could have a SE uncertainty (precision) of 0.004 magnitude (for air mass = 1.6). That corresponds to a normalized flux SE = 0.37 %. If, for example, 4 images are taken, and their g'-mags were averaged, the expected observing session SE would be 0.002 magnitude (0.19 % normalized flux). I typically obtain > 200 images per observing session, so I can expect a precision that could be as low as 0.00028 mag (0.03 % normalized flux). Other systematics will become important at these high precision levels, so what's actually achievable from a 200-image observing session will be higher than the above estimates (as shown below).

It's possible to estimate actual observing session precision by comparing average magnitude for an observing session of one date with neighbor date values (when KIC846 is thought to be constant, or slowly varying). For example, consider the following light curve segment:


Figure H3. Normalized flux light curve for dates when KIC846 appeared to be either stable or changing slowly (late Sep and Nov).
 
This light curve was used to determine observing date precision using a "neighbor difference method." (It relies on the fact that a set of values that are the difference of a value from the average of its immediate neighbors, i.e., "neighbor difference" values, will have a STDEV = sqrt(1.5) x STDEV of the original values. The sequence of "neighbor differences" will be unaffected by real variations provided they are slow compared with the interval between values.) The data after mid-December can't be used because all observations at those dates were
short observing sessions (fewer than 100 images per night) at high air mass. There are 34 dates within the two intervals of apparent stability or slow variation, and the precision per measurement for both intervals is ~ 580 ppm, or 0.6 ppt, or 0.06 % of normalized flux. This corresponds to 0.00058 magnitude, which is (unsurprisingly) somewhat greater than the theoretically predicted best possible 0.00035 magnitude. The fact that the measured precision is greater is due to systematic errors that apparently were ~ 460 ppm (0.46 ppt, 0.046 %) for the two date intervals analyzed. I think I know what they are, but it is difficult proving any such speculation, and for present purposes it's not important.

The message of this sub-section is that when an observing session includes ~ 200 or more images, and when care is taken in their photometric processing, it is possible to achieve a precision for KIC846 of ~ 580 ppm.

Miscellaneous Photometry Practices

At the start of each observing session I carefully position the star field at a chosen standard location on the pixel field. This is meant to minimize the effect of flat field imperfections.

The CCD is kept at the same temperature for all observing sessions (until a seasonal temperature change requires, or permits, a change). Ghosting becomes noticeable for temperatures colder than about - 27 C (for my SBIG ST-10XME CCD), so I never observe colder than that. 

My master dark frame is made from a couple dozen dark images with the same exposure times and CCD temperature as all subsequent KIC846 imaging.

I use many reference stars (17) for calibration. The SNR penalty for using just one reference star is root-2. As discussed in the previous section these 17 reference stars have been carefully chosen to be stable over a timescale of many months.

I have adopted a standard photometry aperture size set for all observing sessions (currently 4 pixels for signal aperture radius, 4 pixels for gap annulus width, 12 pixels for sky background annulus width). This minimizes the effect of changing atmospheric "seeing" (causing PSF FWHM to vary). If I allowed the signal circle size to vary with "seeing" (as some observers do) then when seeing degrades (i.e., PSF broadens) the photometry signal circle for the reference stars (or target star) could expand into an area that includes a nearby star, thus affecting the flux reading of that star. 

Each observing session is calibrated with the chosen (17) reference stars using a "star color sensitivity" plot (which is a better practice than using "CCD transformation equations"). Visual inspection of this plot can identify whenever a reference star is "misbehaving" (due to unusually bad "seeing," for example), which allows outlier reference stars to be identified and toggled off for use.

Outlier KIC846 magnitudes are identified on a per image basis and any that exceed nearby median values by 3.3-sigma are rejected. Visual inspection is also employed for outlier identification.

Other quality assurance practices are described in my photometry book Exoplanet Observing for Amateurs.

Conclusion for this Section

The message of this section (for observers using telescopes of aperture smaller than ~ 1 meter) is that:

    To achieve a date-averaged precision of 1 mmag (0.1 % normalized flux), for example, scintillation noise requires that an observing session must consist of at least 50 images!

Observers who take only 4 images per date, for example, can expect their date-averaged precision to be no better than ~ 2 mmag (0.2 % normalized flux).

DASCH Comment

Someone recently asked me if brightness changes like the 0.02 mag brightening seen last April are present in the DASCH data. To answer this I created the following set of graphs. They illustrate two things: 1) a "conservative" interpretation of the (Harvard plates) DASCH data can be explained by "no big changes in brightness" during each of the two date intervals corresponding to pre-Menzel-gap and post-Menzel-gap, and 2) The measurement noise of DASCH data is much larger than current ground-based measurements (100 times larger). The first interpretation argues against the presence of a constant secular fade of ~ 0.2 magnitude (20 %) during the past century, and the second interpretation suggests that the kind of dips and OOT brightness changes that have been observed during past year cannot be expected to be seen in the DASCH data.


Figure D1. DASCH data with horizontal line fits for date intervals corresponding to before and after the "Menzel gap." The STDEV for these two date intervals is 0.19 and 0.12 mag. The DASCH measurements are of plates with mostly blue response. New plates were used after the Menzel gap, with a different spectral sensitivity; also, some lenses were changed. The set of dots on the right are HAO V-band (and g'-band) measurements (N = 227) with a 0.32 mag adjustment (to place them close to the DASCH B-mag values).


Figure D2. Same data, but showing only the last 54 years (same magnitude range as the previous graph). 


Figure D3. Another date scale change (same magnitude range) showing HAO measurements. The variations of brightness is much smaller than the noise level of the DASCH measurements.


Figure D4. Same date coverage, but a change of magnitude scale.


Figure D5. Same data with an even greater expansion of the magnitude scale.

My point is that DASCH data is only useful for the detection of very large brightness changes (e.g., 0.20 mag), none of which have been seen during the past 2.8 years. We also can't answer the question "Is there evidence for any counterpart in the DASCH data for the 2018 April
brightening of ~ 0.02 mag?"
 
My Collaboration Policy

At my age of almost 79 I'm entitled to have fun and avoid work. Photometric observing and figuring things out is fun. Writing papers is work. Besides, most people really don't care about what others have done. So, if anyone wants to use any of my observations for a publication, you're welcome to do so. But please don't invite me for co-authorship!

My light curve observations are "in the public domain." This means anyone can and may download my LC observations, and use (or misuse) any of that data for whatever purpose. If my data is essential to any publication just mention this in the acknowledgement section.
 

References

    Schaefer, Bradely E., Rory O. Bentley, Tabetha S. Boyajian and 19 others, 2018, "The KIC 8462852 Light Curve From 2015.75 to 2018.18 Shows a Variable Secular Decline," submitted to MNRAS, arXiv 
    Bodman, Eva, Jason Wright, Tabetha Boyajian, Tyler Ellis, 2018, "The Variable Wavelength Dependence of the Dipping event of KIC 8462852," submitted to AJ, arXiv.
    Bodman, Eva, 2018, "The Transiting Dust of Boyajian's Star," AAS presentation, link 
    Yin, Yao and Alejandro Wilcox, 2018, "Multiband Lightcurve of Tabby's Star: Observations & Modeling," AAS presentation, link (navigate down, etc)
    Sacco, Gary, Linh D. Ngo and Julien Modolo, 2018, "A 1574-Day Periodicity of Transits Orbiting KIC 8462552," JAAVSO, #3327, link
    Boyajian, Tabetha S. and 198 others, 2018, "The First Post-Kepler Brightness Dips of KIC 8462852," arXiv 
    Deeg, H. J., R. Alonso, D. Nespral & Tabetha Boyajian, 2018, "Non-grey dimming events of KIC 8462852 from GTC spectrophotometry" arXiv 
    Bourne, R., B. L. Gary and A. Plakhov, 2017, "Recent Photometric Monitoring of KIC 8462852, the Detection of a Potential Repeat of the Kepler Day 1540 Dip and a Plausible Model," arXiv:1711.10612     
    Bourne, Rafik and Bruce Gary, 2017, "KIC 8462852: Potential repeat of the Kepler day 1540 dip in August 2017," submitted to AAS Research Notes, preprint: arXiv:1711.07472
    Xu, S., S. Rappaport, R. van Lieshout & 35 others, 2017, "A dearth of small particles in the transiting material around the white dwarf WD 1145+017," approved for publication by MNRAS link, preprint arXiv: 1711.06960 
    Gary, Bruce and Rafik Bourne, 2017, "KIC 8462852 Brightness Pattern Repeating Every 1600 Days," published by Research Notes of the AAS at link; preprint at arXiv:1711.04205
    Gary, B. L., S. Rappaport, T. G. Kaye, R. Alonso, J.-F. Hambsch, 2017, "WD 1145+017 Photometric Observations During Eight Months of High Activity", MNRAS, 2017, 465, 3267-3280; arXiv
    Neslusan, L. and J. & Budaj, 2016, "Mysterious Eclipses in the Light Curve of KIC8462852: a Possible Explanation, arXiv: 1612.06121v2  (a "tour de force"; I highly recommend this publication)
    Neslusan & Budaj web site with animation of their way of explaining Kepler D1540 dip:  http://www.astro.sk/~budaj/kic8462.html
    Wyatt, W. C., R. van Lieshout, G. M. Kennedy, T. S. Boyajian, 2017, "Modeling the KIC8462852 light curves: compatibility of the dips and secular dimming with an exocomet interpretation," submitted to MNRAS, arXiv  
    Grindlay interview about Schaefer's assertion that KIC846 exhibited a century long fade using DASCH data: link
    Hippke, Michael and Daniel Angerhausen, 2017, "The year-long flux variations in Boyajian's star are asymmetric or aperiodic," submitted to ApJL, arXiv 
    Sacco, G., L. Ngo and J. Modolo, 2017, "A 1574-day Periodicity of Transits Orbiting KIC 8462852," arXiv
    Rappaport, S., B. L. Gary, A. Vanerdurg, S. Xu, D. Pooley and K. Mukai, 2017, "WD 1145+017: Optical Activity During 2016-2017 and Limits on the X-Ray Flux," arXiv, Mon. Not. Royal Astron. Soc.
    Steele, I. A. & 4 others, 2017, "Optical Polarimetry of KIC 8462852 in May-August 2017,"MNRAS (accepted), arXiv.
    Simon, Joshua D., Benjamen J. Shappee and 6 others, "Where is the Flux Going? The Long-Term Photometric Variability of Boyajian's Star," arXiv:1708.07822 
    Meng, Huan Y. A., G. Rieke and 12 others (including Boyajian), "Extinction and the Dimming of KIC 8462852," arXiv: 1708.07556  
    Sucerquita, M., Alvarado-Montes, J.A. and two others, "Anomalous Lightcurves of Young Tilted Exorings," arXiv: 1708.04600   Also: New Scientist link and Universe Today link.
    Rappaport, S., A. Vanderburg and 9 others, "Likely Transiting Exocomets Detected by Kepler," arXiv: 1708.06069 
    Montet, Benjamin T. and Joshua D. Simon, 2016, arXiv 
    Boyajian et al, 2015, MNRAS, "Planet Hunters X. KIC 8462852 - Where's the flux?" link
    Ballesteros, F. J., P. Arnalte-Mur, A. Fernandez-Soto and V. J. Martinez, 2017, "KIC8462852: Will the Trojans Return in 2011?", arXiv
    Washington Post article, 2015.10.15: link
    AAVSO Campaign Notice requesting KIC646 observations
    AAVSO LC Generator https://www.aavso.org/data/lcg (enter KIC 8462852)
    Web page tutorial: Tips for amateurs observating faint asteroids (useful for any photometry observing)
    Book: Exoplanet Observing for Amateurs, Gary (2014): link (useful for any photometry observing) 
    wikipedia description of Tabby's Star  
    My web pages master list, resume


    B L G a r y at u m i c h dot e d u    Hereford Arizona Observatory    resume 
 
This site opened: 
2018.02.25. 
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