Optical follow-up of MAXI J1820+070 and possible identity with ASASSN-18ey
ATel #11400; D. Denisenko (Sternberg Astronomical Institute, Lomonosov Moscow State University)
on 11 Mar 2018; 20:31 UT
Credential Certification: Denis Denisenko (d.v.denisenko@gmail.com)
Subjects: Optical, X-ray, Cataclysmic Variable, Transient, Variables
Referred to by ATel #: 11403, 11404, 11406, 11418, 11421, 11423, 11424, 11426, 11427, 11432, 11437, 11439, 11440, 11445, 11451, 11458, 11478, 11480, 11481, 11488, 11510, 11576, 11723, 11820, 11831, 11833, 11887, 11899, 12057, 12061, 12064, 12128, 12988, 13066, 13502, 13530, 14492
Following the report by T. Kawamuro et al. (ATel #11399), the error region of MAXI/GSC X-ray transient source MAXI J1820+070 was observed with iTelescope.Net T32 instrument (0.43-m f/6.8 reflector + CCD) in Siding Spring, Australia. A series of unfiltered 60-sec exposures was taken at 17:47-18:12 UT on 2018 March 11 with the T32 field of view (0.72x0.72 degree) covering the entire 90% confidence level uncertainty area. The combined images were compared to the scanned DSS Red plates taken on 1950-07-10, 1993-05-27 and 1993-07-15. The only object that stands out in the inspected field is the bright optical transient coinciding with ASASSN-18ey discovered by ASAS-SN project (B. Shappee et al., 2014) on 2018 Mar. 06.59 UT. See the list of ASAS-SN Transients. The available photometry is given in the table below.
| Date, UT | Magnitude | Instrument |
| 2018-03-02.59 | >16.7 V | ASAS-SN (last non-detection) |
| 2018-03-06.59 | 14.9 V | ASAS-SN |
| 2018-03-07.57 | 14.6 V | ASAS-SN |
| 2018-03-07.59 | 14.5 V | ASAS-SN |
| 2018-03-08.591 | 14.2 V | ASAS-SN |
| 2018-03-08.593 | 14.3 V | ASAS-SN |
| 2018-03-11.742 | 13.32 CV | iTelescope T32 |
| 2018-03-11.756 | 13.31 CV | iTelescope T32 |
Note: T32 magnitudes were measured using GSC 0444-2282 as reference star with V=12.80 in APASS DR9 (A. Henden et al., 2016).
As noted by S. Otero in AAVSO VSX entry, ASASSN-18ey is related to the faint ROSAT X-ray source 1RXS J182022.9+071125 formally 23" away with 21" error circle. The source has ROSAT flux of 0.0239+/-0.0108 cnts/s and hardness ratios HR1=0.71+/-0.39, HR2=0.29+/-0.47 which are rather usual for cataclysmic variables. Outburst amplitude from quiescent Gaia Gmag=17.8 is also typical for dwarf novae. GALEX magnitudes FUV=20.53, NUV=19.73 are somewhat faint resulting in rather large (FUV-W1) color index of 5.5-5.6 (WISE J182021.94+071107.2 has W1=14.76+/-0.04, AllWISE J182021.94+071107.2 - W1=14.91+/-0.4). Objects with (FUV-W1) above 5-5.5 are normally considered to be extragalactic, but it will be shown that some cataclysmic variables may have (FUV-W1) value larger than 8 magnitudes. J-K=0.75 and W1-W2=0.33 values are also telling in favor of the stellar origin.
ASASSN-18ey is showing variability on DSS plates, being brighter on POSS-I Blue and Red images. Animation of 1950 and 1993 DSS Red plates against 2018-03-11 iTelescope image is uploaded to http://scan.sai.msu.ru/~denis/ASASSN-18ey-anim.gif and color-combined DSS finder chart of ASASSN-18ey to http://scan.sai.msu.ru/~denis/ASASSN-18ey-BRIR5x5.jpg (2x zoom).
X-ray position improvement of MAXI J1820+070, as well as spectroscopy and photometric time series of ASASSN-18ey are encouraged to verify the proposed identity and clarify the nature of both objects.
