UT Jul 04, 2018: Photometry of ASASSN-18ey = MAXIJ1820+070

Michael Richmond
Jul 04, 2018

On the night of Jul 03/04 2018, from about 10:04 PM until dawn, through mostly clear skies, I acquired a set of observations of the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ).


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 -15 C. Nothing out of the ordinary.

The sky value shows two brief episodes of light cloud.

Here's a record of the telescope's drift. Need to adjust alt axis a bit, I think.

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 0.891 -> 0.901 at 11:37 PM = JD 303.652; and again at 1:41 AM = JD 303.737 to 0.911.

I discarded images which had obvious trailed stellar images, using a threshold of "round > -0.40". (133 of the 702 raw images). I also removed all images with an image adjustment (zero-point) value more than 0.6 mag above the baseline (removing another 16 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 0.007 mag overall. I marked star "A" as variable in the ensemble, as it was slightly saturated. The middling outlier around instrumental magnitude 2.6 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.

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 4, 
#    in good conditions, 
#    by Michael Richmond,  
#    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
Jul04.11310     2458303.61310  2458303.61813  13.378  0.014 
Jul04.11345     2458303.61345  2458303.61848  13.365  0.014 
Jul04.11380     2458303.61380  2458303.61883  13.347  0.014 


Last modified 7/04/2018 by MWR.