UT Jun 09, 2018: Photometry of ASASSN-18ey = MAXIJ1820+070

Michael Richmond
June 8, 2018

On the night of Jun 08/09, 2018, from about 10:40 PM until dawn, through mostly clear skies, I acquired a set of observations of the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ).

I also starting adjusting the azimuth position of the telescope mount; the corrections so far should reduce drift considerably.


ASASSN-18ey = MAXI J1820+070

The main setup was:

Notes from the night:


Photometry of ASASSN-18ey = MAXI J1820+070

This optical and X-ray and radio transient is likely a black hole accreting material at a higher-than-usual rate. It has been the subject of many observers over the past few months -- see the trail of telegrams that include

The object is located at



  RA = 18:20:21.9    Dec = +07:11:07.3

A chart of the field is shown below. The size of the chart is about 22 by 18 arcminutes.

I've marked the location of several comparison stars, which also appear in light curves below. Stars C, D, and E are mentioned by the Tomoe Gozen team in ATel 11426, but all three are rather red, with (B-V) ranging from 1.14 to 1.37. Star B is one of the bluest nearby bright stars, with (B-V) = 0.52.



  star        UCAC4              B          V
----------------------------------------------------
   B       486-079513        12.975     12.454
   C       486-079608        13.968     12.830
   D       486-079523        14.637     13.272
   E       487-077858        14.637     13.272 

----------------------------------------------------

 

The dark current was pretty ordinary.

The sky value shows clear skies until just before dawn, when light cirrus came.

Here's a record of the telescope's drift. Note little Dec drift early, when the field was near the Eastern horizon; that means the altitude is close to correct. As the field approached the meridian, note lots of Dec drift to south. I made several adjustments (CCW) to the azimuth position of the mount, which gradually decreased this drift. The total was about 5 notches CCW; I think that another 0.5 to 1.0 notches CCW might do the trick.

The number of objects detected -- I required 50 objects for an image to be included in the ensemble.

I used an aperture with radius 5.0 pixels. I adjusted the focus position from 0.809 -> 0.819 -> 0.829 as the temperature dropped, and that did improve the PSF. So, when temperature DECREASES, the focus position INCREASES.

Image adjustment factor shows a few light clouds, with one heavier bunch near the end.

Using aperture photometry with a radius of 5 pixels (binned 2x2, each pixel is 1.34 arcsec, so a radius of 6.7 arcsec), I measured the instrumental magnitudes of a number of reference stars and the target. Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame.

Sigma-vs-mag plots show that the floor was about 0.006 mag overall, which is pretty good! The outlier around instrumental magnitude 2.9 is MAXI J1820+070.

Here are light curves of the variable and the field stars.

I used the UCAC value for the V-band magnitude of star "B" = UCAC4 486-079513 to shift the ensemble magnitudes to the standard V-band scale -- but remember that these are UNFILTERED measurements.

Here's a closeup on the variable. I'll connect the dots to make its behavior a bit easier to see.

You can download my measurements below. A copy of the header of the file is shown to explain the format.

# Measurements of MAXIJ1820+070 made at RIT Obs, UT 2018 Jun 9, 
#    in good conditions, 
#    by Michael Richmond, 
#    using Meade 12-inch LX200 and ATIK 11000. 
# Exposures 25 seconds long, no filter. 
# Tabulated times are midexposure (FITS header time - half exposure length) 
#    and accurate only to +/- 1 second (??). 
# 'mag' is a differential magnitude based on ensemble photometry 
#    using a circular aperture of radius 5 pix = 6.6 arcseconds.  
#    which has been shifted so UCAC4 486-079513 has mag=12.454 
#    which is its V-band magnitude according to UCAC4.  
# 
# UT_day             JD            HJD        mag    uncert
Jun09.11426     2458278.61426  2458278.61907  13.724  0.020 
Jun09.11458     2458278.61458  2458278.61939  13.618  0.019 
Jun09.11492     2458278.61492  2458278.61973  13.619  0.019 


Last modified 6/09/2018 by MWR.