UT Aug 01, 2020: Photometry of ASASSN-20jl

Michael Richmond
Aug 01, 2020

On the night of Jul 31/Aug 01, 2020, under fair conditions, I acquired images of the star ASASSN-20jl. A recent report on VSNET-alert by Denis Denisenko noted that this star had risen by over 2 magnitudes recently, despite its apparently ordinary nature; and Tonny Vanmuster confirmed this on cba-chat.

My measurements show that the star was indeed brighter than normal, in line with the reports, and that it was slowly fading.


ASASSN-20jl

You can find some basic information on this star at the ASAS-SN Transients page.

The main setup was:

Notes from the night:

The object is at



  RA = 16:46:14.62 	Dec = +17:00:17.4    (J2000)

A chart of the field is shown below. The size of the chart is about 19 x 14 arcminutes.

I've marked the location of several comparison stars.



  star       ID                  B          V         
-------------------------------------------------------------------------

   A     BD+17 3086            10.82      10.33                         

   B     TYC 1524-1170-1       11.64      10.45                      

--------------------------------------------------------------------------

 

I took a photo of the finder TV's screen when pointing to ASASSN-20jl; this could be a useful reference for the future:

I ran the camera at the slightly-higher-than-normal temp of -16 C. Nothing out of the ordinary.

I took a series of 369 exposures of the field, using the V-band filter and an exposure time of 20 seconds. I wasn't able to analyze the images during the run, due to some computer problems, which was unfortunate. The signal-to-noise of the target is considerably lower than I was hoping to acquire, so I could have increased the exposure time to switched to unfiltered images had I known. Oh, well.

The sky value shows that the sky was mostly clear, with just a couple of clouds passing late in the run. The sky value rose as the target set and the Moon rose in the sky.

Here's a record of the telescope's drift. The big jump in Dec is due to a small re-centering I made after a few early observations. Note the drift in Dec becomes pretty rapid later in the night, as the field moves into the western sky. I did not allow Dec corrections in the guider, fearing backlash issues. Maybe I should give them a try next time.

The number of objects detected. It drops at the end of the run due to extinction at higher airmass.

I used an aperture with radius 4.0 pixels tonight.

Using aperture photometry with a radius of 4 pixels (binned 2x2, each pixel is 1.25 arcsec, so a radius of 5.0 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 0.015 mag overall, which is not very good. Longer exposures would have helped, as would Moon-less skies.

Here are light curves of the variable and the field stars.

I used the SIMBAD value for the V-band magnitude of star "A" = BD+17 3086 to shift the ensemble magnitudes to the standard V-band scale.

Here's a closeup on the variable. It appears to fade by about 0.1 mag over the 0.15 days of the run.

You can download my measurements below. A copy of the header of the file is shown to explain the format.

# Measurements of ASAS20jl made at RIT Obs, UT 2020 Aug 1, 
#    in fair conditions, 
#    by Michael Richmond, 
#    using Meade 12-inch LX200 and ATIK 11000. 
# Exposures 20 seconds long, v 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 = 5.0 arcseconds.  
#    which has been shifted so BD+17 3086 has mag=10.33 
#    which is its V-band magnitude according to SIMBAD.  
# 
# UT_day             JD            HJD        mag    uncert
Aug01.08381     2459062.58381  2459062.58595  12.987  0.026 
Aug01.08414     2459062.58414  2459062.58628  13.009  0.027 
Aug01.08449     2459062.58449  2459062.58663  12.993  0.026 


Last modified 8/01/2020 by MWR.