UT Apr 12, 2018: Photometry of MAXIJ1820+070

Michael Richmond
Apr 12, 2018

On the night of Apr 11/12, 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 ). There was light cirrus over much of the sky during the entire run, so the data is somewhat noisy.


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 high sky value early is mostly due to large airmass, I think.

The FWHM rose slightly, but not a big deal. I used an aperture of radius 3 pixels.

Image adjustment factor shows the passing of light 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 0.012 mag overall. 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. I've removed the measurements taken during the bright sky at dawn, after JD 820.890.

# Measurements of MAXIJ1820+070 made at RIT Obs, UT 2018 Apr 12, 
#    in mediocre conditions (and high airmass), 
#    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 3 pix = 5.9 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
Apr12.25105     2458220.75105  2458220.75244  12.217  0.031 
Apr12.25120     2458220.75120  2458220.75259  12.213  0.030 
Apr12.25130     2458220.75130  2458220.75269  12.190  0.029 


Last modified 4/12/2018 by MWR.