UT Apr 21, 2018: Photometry of MAXIJ1820+070

Michael Richmond
Apr 22, 2018

On the night of Apr 21/22, 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 very good -- dark and clear.

I may have some ideas for fixing or alleviating the persistant vertical bands that appear in all ATIK 11000 data. Will work on possible fixes.

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 "dark5_a" image was taken at dawn, after all the images. It contained only the small sub-region I used for images of MAXI, so it has fewer pixels than the full image.

The smooth sky value indicates that any clouds were very thin. However, I did not a semi-periodic variation in sky value with amplitude around 10 counts/2 and spacing 13-20 images. Does it have something to do with the tracking???

Speaking of tracking, here's the tail of the trail. Guiding was turned off, and the telescope drifted quite a bit.

I used an aperture with radius 4.0 pixels, larger than usual, due to the poor focusing at start of night.

Image adjustment factor shows basically just the decrease in zeropoint as the field rose in the East.

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 0.008 mag overall -- that's about the best I've seen with 5-second exposures. The brightest outlier is a saturated star, and the outlier around instrumental magnitude 4 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. Note, as usual, that star B (and presumably the variable) creep up in brightness as they rise out of the East, due to differential extinction and their blue color.

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 Apr 22, 
#    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
Apr22.26228     2458230.76228  2458230.76447  12.752  0.023 
Apr22.26236     2458230.76236  2458230.76455  12.781  0.023 
Apr22.26245     2458230.76245  2458230.76464  12.678  0.022 

Last modified 4/22/2018 by MWR.