Swift observations of the fading transient ASASSN-V J060000.76-310027.83

ATel #13377; K. V. Sokolovsky, E. Aydi, L. Chomiuk, A. Kawash, J. Strader (MSU), K. Mukai (NASA/GSFC)
on 31 Dec 2019; 01:07 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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The ASAS-SN team reported an unusual decline in brightness by about 0.9 mag of a previously non-variable dwarf star ASASSN-V J060000.76-310027.83 (ATel #13346). The decline started around 2019-10-25 and the flux has partially recovered in the following month (ATel #13357). The spectrum obtained with the Southern African Large Telescope is of a normal K type star (ATel #13349). The archival UV-to-IR spectral energy distribution (SED) shows no infrared excess (ATel #13361).

Swift observed ASASSN-V J060000.76-310027.83 for 3.2ks between 2019-12-22 and 2019-12-25. Swift/XRT detected no X-ray source at the position of the transient with a one sigma upper limit of 0.0007 cts/s. Assuming power law emission with the photon index of 2 and HI column density of 2.17x10^20 cm^-2 (Kalberla et al. 2005 A&A, 440, 775) this translates to the unabsorbed 0.3-10 keV flux limit of 3x10^-14 ergs/cm^2/s and luminosity below 10^29 ergs/s (assuming the distance of 157 pc).

The star is clearly visible in the UVOT images, however we treat the UV detections at the UVW1 and UVW2 bands as upper limits as these filters are affected by the read leak. The UVM2 band, less affected by the red leak, provides a tight upper limit on the UV flux density: 4x10^-17 erg/s/cm^2/A (central wavelength of the filter is 2246 A), consistent with the SED reported in ATel #13361. The measured magnitudes in the Vega system are:

 
   JD        Band   Mag.  Err. 
 2458843.04    V   13.88  0.03 
 2458840.27    B   15.22  0.03 
 2458843.03    B   15.14  0.03 
 2458840.27    U   16.59  0.06 
 2458843.03    U   16.53  0.05 
 2458840.27  UVW1 >17.77  
 2458843.03  UVW1 >17.47  
 2458843.04  UVM2 >20.07  
 2458840.27  UVW2 >18.87  
 2458843.04  UVW2 >18.78  

The aperture was placed on the UVOT images at the Gaia DR2 ICRS position of the source 06:00:00.74 -31:00:28.1 corresponding to the reference epoch 2015.5. Note, that the source name is based on the Gaia DR2 position recomputed to the epoch J2000 taking into account the source proper motion.

Among the classical variable stars, two types show irregular dimming events: R CrB stars - dust forming (super)giants and VY Scl type cataclysmic variables. The Gaia parallax establishes that ASASSN-V J060000.76-310027.83 is not a giant (ATel #13346). Our X-ray non-detection and red optical color disfavor interpretation of the object as a VY Scl type cataclysmic variable. An elevated X-ray flux might have been expected from a young stellar object ("dippers", e.g. Ansdell et al. 2019, MNRAS, 483, 3579) or a star with an exceptionally high chromospheric activity (in which case, the dimming could have been caused by starspots).

The asymmetric shape of the ASAS-SN lightcurve suggests that if ASASSN-V J060000.76-310027.83 is a very long period eclipsing binary, the eclipsing object is not a star, but may be something like a patchy dust ring. One could think of a dwarf analog to epsilon Aurigae system, which, however, shows a clear infrared excess (Stencel 2012, JAVSO, 40, 618 and references therein). It appears more likely that ASASSN-V J060000.76-310027.83 is similar to the Boyajian's Star (Boyajian et al. 2016, MNRAS, 457, 3988) and its suggested analogs (Schmidt 2019, ApJ, 880, L7).

We thank the Neil Gehrels Swift Observatory team and PI, Brad Cenko, for scheduling these ToO observations.