UT May 24, 2018: Photometry of ASASSN-18ey = MAXIJ1820+070

Michael Richmond
May 24, 2018

On the night of May 23/24, 2018, from about midnight to dawn, I acquired a set of observations of the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ).

These notes on the uncertainty in unfiltered magnitude measurements for different exposure times might be useful to someone.

     12-inch telescope + ATIK camera + no filter
                under good conditions
         based on observations Mar-May 2018

exptime         mag         mag_uncertainty     comment
   5 sec       V = 12           0.01           binned 2x2 or 3x3
               V = 15           0.07           binned 2x2

  25 sec       V = 12           0.006          binned 3x3
               V = 15           0.05           binned 3x3

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 two weeks -- 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 ordinary.

The sky value shows clear skies all night long.

Here's a record of the telescope's drift. I made a few adjustments to the mount's altitude axis, but may have gone in the wrong direction. The drift in Declination when pointing toward East horizon was worse tonight than earlier this month.

The number of objects detected went down at dawn, as sky brightened.

I used an aperture with radius 4.0 pixels, due to the large FWHM at early times.

Image adjustment factor shows no evidence for clouds.

Using aperture photometry with a radius of 3 pixels (binned 3x3, each pixel is 1.98 arcsec, so a radius of 5.9 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. The outlier around instrumental magnitude 2.4 is MAXI J1820+070.

Here are light curves of the variable and the field stars. The bright star "A" was saturated, so I marked it as "variable" in the ensemble.

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 May 24, 
#    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 4 pix = 7.9 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
May24.17855     2458262.67855  2458262.68277  13.263  0.014 
May24.17888     2458262.67888  2458262.68310  13.236  0.013 
May24.17920     2458262.67920  2458262.68342  13.291  0.015 

Last modified 5/24/2018 by MWR.