UT May 08, 2018: Photometry of ASASSN-18ey = MAXIJ1820+070

Michael Richmond
May 08, 2018

On the night of May 07/08, 2018, in the wee hours of the morning, I acquired a set of observations of the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ). Conditions were good.

The period of variation has changed! It's now shorter than it was a week ago.

ASASSN-18ey = MAXI J1820+070

The main setup was:

Notes from the night:

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 nothing but the bright sky at dawn. Good!

Here's a record of the telescope's drift. Guiding was turned off, and the telescope drifted quite a bit. I re-centered the star around 2:12 AM, causing the jump in Dec.

I used an aperture with radius 4.0 pixels, since the focus was off at the start tonight.

Image adjustment factor shows a jump after the re-focusing, but no clouds.

Using aperture photometry with a radius of 4 pixels (binned 3x3, each pixel is 1.98 arcsec, so a radius of 7.9 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 0.010 mag overall. The bright outlier around instrumental magnitude 2 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. The time between the two minima is roughly 0.11 days, which is much shorter than the 0.17 days seen last week.

A very short section of the light curve shows some of the variations more clearly.

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 8,
#    in good conditions,
#    by Michael Richmond,
#    using Meade 12-inch LX200 and ATIK 11000.
# Exposures 5 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
May08.17582     2458246.67582  2458246.67914  12.709  0.026
May08.17589     2458246.67589  2458246.67921  12.757  0.027
May08.17598     2458246.67598  2458246.67930  12.709  0.026

Last modified 5/09/2018 by MWR.