UT Aug 24, 2018: Photometry of ASASSN-18ey = MAXIJ1820+070

Michael Richmond
Aug 25, 2018
Aug 27, 2018

On the night of Aug 23/24 2018, after a month-long hiatus due to travel, I once again acquired data on the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ). I was assisted during the setup and early stages by RIT physics major Evan Muskopf and high school student Sean Doran. Thanks!

Tonight was the first time the "Swann" filter was used; just for flats, though.

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
• images binned 2x2
• focus with no filter best at 1.194 inches, yielding FWHM 4-5 pix; I changed the focus later, but it didn't help
• guiding with 15-sec exposures (bright Moon)

Notes from the night:

• the lamp in the dome used for dome flats appears to have been tilted down, so that it points at the floor rather than horizontally. That will mean that exposure times for dome flats will have to be longer, until we re-orient the lamp.
• evidence for only a few brief clouds
• near full Moon made sky very bright, alas
• T = 79 F at opening, dropping only to 63 at 2 AM The camera ran at -15 C
• I used a subframe of the entire CCD (550 x 538 pixels) to speed up readout.
• The guider had trouble tonight, due to the poor seeing and the very bright Moon. I was forced to use long guider exposure times of 12-15 seconds; eventually, as the target sank low in the west, the guider completely failed.
• strong skunk odor, but no visual sighting.
• first test of the Swann comet filter. An exposure time of 2.3 seconds yields dome flats with around 19,000 counts. The arrangement of filters in the filterwheel is currently (Note added 8/27/2018: this is incorrect: the Swann filter is actually in the "R" filter's place, and the "B" filter is in the second position.)
  • empty
  • Swann
  • V
  • R
  • I
  I will likely put the "B" filter back in place, and move the Swann to the "R" filter's location.

Below is a picture showing a median domeflat taken with the Swann filter. The colors range from black at 7259 counts/2 to white at (7259 + 2419) counts/2. Note the "squashed" appearance. I'm not sure why it doesn't have the nice, circular shape shown by all the Bessell filters.

Note added 8/27/2018: the image below is that through the "B" filter, not the Swann filter. The "B" filter has some filmy material growing over the top surface, causing this pattern.

RIT's Facilities Management Services staff have been making some repairs to the observatory's equipment.

• the dome building's foundations were showing some cracking and flaking around the edges; new concrete has been poured
• there are 9 electrical outlets around the concrete pad in the fenced area. The outlets have suffered from corrosion and rust over the last 19 years, for the most part growing un-operable. FMS is replacing the outlets; as of this evening, they had replaced two completely and were in the process of replacing the others.

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 

----------------------------------------------------

 

I ran the camera at -15 C. Nothing out of the ordinary. I switched to 2.3-sec darks in order to match the exposure time needed for the Swann filter's dome flats.

The sky value shows only very limited evidence for clouds. I stopped at local time 1:22 AM = airmass 2.23, long before dawn.

Here's a record of the telescope's drift. A consistent drift of stars to the south as the telescope moved west of the meridian means the altitude needs a small turn CW.

I adjusted the pointing manually at 354.69.

The number of objects detected -- I required 35 objects for an image to be included in the ensemble.

I used an aperture with radius 5.0 pixels. That was barely large enough tonight.

• JD 318.662, foc 1.194 -> 1.214

I discarded images which had obvious trailed stellar images, using a threshold of "round > -0.45", a somewhat more extreme value than usual (removing 61 of the 159 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 0.009 mag overall, which is not very good. I marked star "A" as variable in the ensemble, as it was saturated. The outlier around instrumental magnitude 2.9 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; I marked it as variable in the ensemble.

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 Aug 24, 
#    in mediocre conditions, 
#    by Michael Richmond,  
#    using Meade 12-inch LX200 and ATIK 11000. 
# Exposures 60 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
Aug24.09867     2458354.59867  2458354.60157  13.406  0.019 
Aug24.09946     2458354.59946  2458354.60236  13.386  0.018 
Aug24.10024     2458354.60024  2458354.60314  13.410  0.020 

• Download ASCII text file with photometry of MAXI J1820+070 from Aug 24 UT

Last modified 8/27/2018 by MWR.