On the night of Apr 11/12, 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 ). There was light cirrus over much of the sky during the entire run, so the data is somewhat noisy.
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 high sky value early is mostly due to large airmass, I think.
The FWHM rose slightly, but not a big deal. I used an aperture of radius 3 pixels.
Image adjustment factor shows the passing of light clouds.
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 0.012 mag overall. The brightest outlier is a saturated star, and 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. Note, as usual, that star B (and presumably the variable) creep up in brightness as they rise out of the East, due to differential extinction and their blue color.
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. I've removed the measurements taken during the bright sky at dawn, after JD 820.890.
# Measurements of MAXIJ1820+070 made at RIT Obs, UT 2018 Apr 12, # in mediocre conditions (and high airmass), # 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 Apr12.25105 2458220.75105 2458220.75244 12.217 0.031 Apr12.25120 2458220.75120 2458220.75259 12.213 0.030 Apr12.25130 2458220.75130 2458220.75269 12.190 0.029
Last modified 4/12/2018 by MWR.