UT May 01, 2018: Photometry of MAXIJ1820+070

Michael Richmond
Apr 30, 2018

On the night of Apr 30/May 01, 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 ). Conditions were fair: the sky was clear, but a very bright full moon lit up the sky.

I'm still working on the vertical stripes ... I did take a set of 1x1-binned frames to send to Atik.

MAXI J1820+070

The main setup was:

Notes from the night:

Recovering from "emergency" power off: Even after I set the time properly later, the telescope slewed wildly when I asked it to go to Arcturus. So, I manually centered Arcturus and sync'd. Fine. I was able to move to my field and acquire images. So far, so good. But when I told the telescope to "park" at end of night, it again started to slew wildly to the west. To abort, I had to turn off power -- pressing the middle button on controller did not stop it.

So, after re-starting, the telescope said it needed to be aligned. I performed a "pseudo-alignment" to placate it:

  1. unhook all cables, including focuser phone-jack cable
  2. turn on power to telescope
  3. choose option "0" = Align
  4. enter Daylight Savings Time value, Time, Date
  5. move scope to "polar home" using keypad
  6. scope slews itself to Polaris, lots of rotation in RA
  7. when asked to center Polaris in field, make no adjustments; just press "Enter" immediately
  8. telescope slews itself to a bright star -- Altair in this case; the star did appear in low-power finder, but off-center
  9. center Altair in finder's crosshairs using keypad
  10. press "Enter"
  11. telescope declares "success!"
  12. put all cables back in place

I then moved the telescope to a vertical position, re-set the Park Position, and parked it. I hope this means that it will know roughly where it is pointing on my next observing session.

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 dark current was ordinary.

The sky value shows no clouds, a bright background when target low in the East.

Here's a record of the telescope's drift. Guiding was turned off, and the telescope drifted quite a bit.

I used an aperture with radius 3.0 pixels, as usual.

Image adjustment factor shows only small variations -- another sign of clear skies.

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 0.010 mag overall. 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. Looks like it faded in second half of the run.

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.

# Measurements of MAXIJ1820+070 made at RIT Obs, UT 2018 May 1, 
#    in fair conditions (bright Moon), 
#    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
May01.23667     2458239.73667  2458239.73952  12.773  0.026 
May01.23683     2458239.73683  2458239.73968  12.728  0.025 
May01.23700     2458239.73700  2458239.73985  12.764  0.026 

Last modified 4/30/2018 by MWR.