UT Sep 20, 2018: Photometry of ASASSN-18ey = MAXIJ1820+070 in V-band

Michael Richmond
Sep 21, 2018

On the night of Sep 19/20 2018, I once acquired data through bright, cirrus-y skies on the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ).

The star has faded to V = 14.2, and is so faint that my measurements are probably too noisy to help constrain its behavior. Rats. I'll need to increase exposure time, or give up.

ASASSN-18ey = MAXI J1820+070

The main setup was:

Notes from the night:

Electrical outlets on the concrete pad are either working, or taped safely shut.

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 



I ran the camera at -20 C. Nothing out of the ordinary.

The sky value shows three brief episodes of thicker cirrus. Condensation due to 97% relative humidity caused me to stop observing at 11:25 PM.

Here's a record of the telescope's drift. A consistent drift of stars to the south as the telescope moved west of the meridian means the altitude needs a small turn CW.

The number of objects detected -- I required 35 objects for an image to be included in the ensemble.

I used an aperture with radius 5.0 pixels.

I discarded images which had obvious trailed stellar images, using a threshold of "round > -0.40", (removing 41 of the 149 raw images). I discarded the final 5 images as well, due to condensation on the dewar window.

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 0.010 mag overall, which is not very good.

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.

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 Sep 20, 
#    in mediocre conditions, 
#    by Michael Richmond,  
#    using Meade 12-inch LX200 and ATIK 11000. 
# Exposures 60 seconds long, V 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
Sep20.02788     2458381.52788  2458381.52867  14.152  0.032 
Sep20.02867     2458381.52867  2458381.52946  14.123  0.035 
Sep20.02946     2458381.52946  2458381.53025  14.119  0.035 

Last modified 9/21/2018 by MWR.