On the night of May 04/05, 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 pretty good: only a few bits of cirrus most of the time, and one half-hour stretch of clouds near the end of the run.
I'm still working on the vertical stripes ...
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 interrupting observations near the end of the run, but leaving before dawn.
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 4.0 pixels, since the seeing (or focus) weren't great tonight.
Image adjustment factor shows a few small cloud banks early, then a big one near the end of the run.
Using aperture photometry with a radius of 4 pixels (binned 3x3, each pixel is 1.98 arcsec, so a radius of 7.9 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 brighter outlier around instrumental magnitude 2 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 the sinusoidal pattern superimposed on a fading trend.
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 5, # in fair conditions (light cirrus), # 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 4 pix = 7.9 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 May05.20060 2458243.70060 2458243.70373 12.565 0.027 May05.20069 2458243.70069 2458243.70382 12.529 0.025 May05.20078 2458243.70078 2458243.70391 12.464 0.022
Last modified 5/05/2018 by MWR.