UT Jul 12, 2018: Photometry of ASASSN-18ey = MAXIJ1820+070

Michael Richmond
Jul 12, 2018

On the night of Jul 11/12 2018, from about 10:16 PM until dawn, through mostly clear, dark skies, Tracy Davis and I acquired a set of observations of the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ).

Thanks to Tracy Davis for help with the observations and with the restoration of an old refractor.

The target is now very low in west at dawn -- tonight, it reached airmass 2.4. This will only get worse as time goes by; I may have to stop making measurements while the sky is still dark.

ASASSN-18ey = MAXI J1820+070

The main setup was:

Notes from the night:

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 -16 C. Nothing out of the ordinary.

The sky value shows no clouds.

Here's a record of the telescope's drift. With the guider, there's no long-term drift in RA, but a slow drift in Dec remains.

The number of objects detected -- I required 50 objects for an image to be included in the ensemble.

I used an aperture with radius 5.0 pixels. Looks like the seeing got bad at the end of the run, but since the airmass reached 2.4 at that time, that's not surprising.

I discarded images which had obvious trailed stellar images, using a threshold of "round > -0.40". (76 of the 676 raw images). I also removed all images with an image adjustment (zero-point) value more than 0.30 mag above the baseline, removing another 2 images.

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 0.006 mag overall, which is very good. I marked star "A" as variable in the ensemble, as it was slightly saturated. The outlier around instrumental magnitude 2.3 is MAXI J1820+070. Note that star "B" has a larger-than-normal scatter due to its blue color and the second-order extinction at high airmass near the end of the run.

Here are light curves of the variable and the field stars. Note the change in the blue-ish star "B" due to second-order extinction at the end of the run.

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.

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 Jul 12, 
#    in very good conditions, 
#    by Michael Richmond and Tracy Davis,  
#    using Meade 12-inch LX200 and ATIK 11000. 
# Exposures 25 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 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
Jul12.09479     2458311.59479  2458311.59971  12.953  0.010 
Jul12.09519     2458311.59519  2458311.60011  12.958  0.010 
Jul12.09558     2458311.59558  2458311.60050  12.946  0.010 

Last modified 7/12/2018 by MWR.