UT Jun 26, 2018: Photometry of ASASSN-18ey = MAXIJ1820+070

Michael Richmond
June 26, 2018

On the night of Jun 25/26, 2018, from about 10:40 PM until dawn, through clear (but moonlit) skies, I acquired a set of observations of the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ). High school student Sean Doran helped for the first half of the night, monitoring the slight drift of stars across the field of view.

We turned alt knob 1/2 notch CW, but the adjustment didn't have the desired effect -- see below. More small adjustments needed.

ASASSN-18ey = MAXI J1820+070

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ATIK 11000 CCD camera
• no filter (for short exposures)
• images binned 2x2
• focus with no filter best at 0.913 inches, yielding FWHM = 2.9 pix I increased the focus later, to account for the decreasing temperature.
• no guiding

Notes from the night:

• clear all night (though light cirrus on S horizon late)
• nearly full Moon made sky bright, shone directly on telescope for last two hours or so
• not so warm: T = 75 F early, 49 F at end; The camera ran at -20 C.
• I used a subframe of the entire CCD (556 x 602 pixels) to speed up readout; with 25-second exposure time, the cadence was about 29 seconds

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

• ATel 11399
• ATel 11426

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 pretty ordinary.

The sky value shows no clouds. The small jumps in the second half of the night were caused by moonlight striking the front of the telescope each time the dome moved a bit further west.

Here's a record of the telescope's drift. At the start of the night, I turned the altitude knob 0.5 notches CW, which should have decreased the small Dec drift early and late; but it increased slightly instead. Perhaps the mount shifted when I loosened the bolts, before making the adjustment.

• the target crosses the meridian at JD 91.72. The very small South-ward drift around that time suggests that the azimuth position needs a very small turn CCW
• the drift to North before, and South after, crossing the meridian suggests that the altitude may still be too high, requiring adjustment via CW turns of knob

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. I adjusted the focus position from 0.913 -> 0.933 at 12:18 PM, then to 0.944 at 3:15 AM. The results are reasonable.

I discarded images which had obvious trailed stellar images, (188 of the 708 raw 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 good for this field. I marked star "A" as variable in the ensemble, as it was slightly saturated. The big outlier around instrumental magnitude 2.1 is MAXI J1820+070.

Here are light curves of the variable and the field stars. Wow! ASASSN-18ey rises by 0.7 mag during this 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. This time, our run coincided with the entire rise of 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 2018 Jun 26, 
#    in good conditions, 
#    by Michael Richmond and Sean Doran,  
#    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
Jun26.11147     2458295.61147  2458295.61652  13.408  0.013 
Jun26.11181     2458295.61181  2458295.61686  13.431  0.013 
Jun26.11250     2458295.61250  2458295.61755  13.401  0.013 

• Download ASCII text file with photometry of MAXI J1820+070 from Jun 26 UT

Last modified 6/26/2018 by MWR.