UT Jun 16, 2018: Photometry of ASASSN-18ey = MAXIJ1820+070

Michael Richmond
June 15, 2018

On the night of Jun 15/16, 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 made some adjustments to the altitude position of the mount, but now I have to adjust the azimuth again. Oh, well.

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 a period of clouds in the middle of the night.

Here's a record of the telescope's drift. I made several adjustments to the altitude axis before the meridian crossing. I first turned the altitude knob CW by 2.5 bumps, which improved the Dec drift greatly. I then made small adjustments in each direction, quitting when the field approached the meridian.

The drift to North near meridian tells me that I need to adjust the azimuth again -- turn knob CW by small amounts.

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 0.857 -> 0.877 as the temperature dropped, and that did improve the PSF a bit. Later, I changed the focus 0.877 -> 0.897, which helped again.

I discarded images which had obvious trailed stellar images, (139 of the 700 raw images), and also removed outliers in the image adjustment graph. The resulting ensemble shows a reasonably clean image adjustment graph.

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; I had to mark star "A" as variable in the ensemble, since it showed _slight_ signs of saturation. The (larger) outlier around instrumental magnitude 2.7 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 16, 
#    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
Jun16.11168     2458285.61168  2458285.61664  13.396  0.014 
Jun16.11201     2458285.61201  2458285.61697  13.387  0.014 
Jun16.11359     2458285.61359  2458285.61855  13.367  0.013 

Last modified 6/16/2018 by MWR.