UT Aug 26, 2019: Photometry of ASASSN-18ey = MAXIJ1820+070

Michael Richmond
Aug 27, 2019

On the night of Aug 25/26, 2019, through dark skies, I acquired a set of observations of the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ). It has brightened slightly, to V = 13.8, with considerable variations on short time scales.


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 main setup was:

Notes from the night:

No problems with hardware or software tonight.

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 that the sky was clear and dark.

Here's a record of the telescope's drift. For the most part, it tracked well with 5-second exposures; I've kept the X aggressiveness at 0.5. Note the break when I had to slew a bit to get a new guide star.

The number of objects detected.

I used an aperture with radius 4.0 pixels. No changes in focus.

I discarded images which had obvious trailed stellar images, and also images which were outliers in the "image adjustment" plot. (discarded a total of 31 of the 332 raw images).

Using aperture photometry with a radius of 4 pixels (binned 2x2, each pixel is 1.34 arcsec, so a radius of 5.4 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. I did NOT mark star "A" as variable in the ensemble. MAXI is the outlier at ensemble mag of about 3.21.

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.

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 2019 Aug 26, 
#    in good conditions, 
#    by Michael Richmond, 
#    using Meade 12-inch LX200 and ATIK 11000. 
# Exposures 30 seconds long, clear 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 = 5.3 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
Aug26.04178     2458721.54178  2458721.54456  13.942  0.018 
Aug26.04228     2458721.54228  2458721.54506  13.712  0.015 
Aug26.04279     2458721.54279  2458721.54557  13.693  0.015 


Last modified 8/27/2019 by MWR.