On the night of May 24/25, 2018, from about midnight to dawn, I acquired a set of observations of the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ). Earlier in the evening, Jen Connelly and a pair of students acquired measurements of the eclipsin binary system HH UMa.
The main setup was:
Notes from the night:
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 clouds early, then clear skies.
Here's a record of the telescope's drift. The target crossed the meridian at JD = 263.80.
The number of objects detected.
I used an aperture with radius 3.0 pixels.
Image adjustment factor shows clouds early, and then moonset around 263.8.
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 0.008 mag overall. One of the outliers around instrumental magnitude 2.4 is MAXI J1820+070; others are due to clouds and some dust donuts.
Here are light curves of the variable and the field stars. The bright star I usually call "A" was saturated, so I picked a slightly fainter star as "A" tonight.
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 May 25, # in fair 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 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 May25.21804 2458263.71804 2458263.72231 13.101 0.014 May25.21837 2458263.71837 2458263.72264 13.090 0.018 May25.21872 2458263.71872 2458263.72299 13.071 0.019
Last modified 5/25/2018 by MWR.