Astrometry of the Transients

Based on the image subtractions of the three ASASSN candidate flares, it is possible to determine the positions of the flares relative to the centres of their hosts. The centroiding uncertainty on the point source residual in each subtracted image is de- termined under the assumption of a gaussian profile with signal to noise determined from its photometry and the FWHM of the point spread function.

The centroid of the host is determined in two ways. The first is based on the galfit models, using the position of the most compact extended component

in each case, as it is assumed that any supermassive black hole would likely reside at the centre of this component. In the case of ASASSN14ae and ASASSN14li, this component is the compact bulge that dominates the central emission. In the case of ASASSN15lh, the difficulty in accurately modelling the host precluded this. However, by masking the entire central region of the host and fitting a simple S´ersic profile to the wings of the galaxy gives a first order estimate of the centroid position and is included in the analysis with due caution. The second method, which is less model dependent, makes use of the iraf task imexam to determine the centroid

position based on a simple gaussian model of the central region of the host (8 pixel radius). This method works under the assumption that the remaining transient emission in the late-time epoch does not dominate the central region of the host, the case by case evidence for which is outlined below.

First it is assumed that all of the F275W/F225W emission in the images of ASASSN14ae and ASASSN14li is associated with the transient flare8 and that the flare has not undergone any considerable change in spectral energy distribution. Thus, the ratio of the late and early time F275W or F225W emission represents the fraction of transient emission that remains with respect to the early epoch. In the case of ASASSN14ae, the F275W emission has decayed to a flux of 16.1_±1.1µJy, 8_{Note that if this assumption is incorrect and host emission is present in either the F275W or} F225W images of either candidate this would mean that the remaining transient emission constitutes a smaller fraction of the central region’s late-time emission and would apply a smaller skew on the

or 47_±3% of its early value. Assuming the thermal SED with a temperature of

∼₂₀₀₀₀±_{2000K determined in Holoien et al. (2014), this implies the remaining}

transient flux in the late F606W imaging is 8.7_±2.6µJy. Alternatively, assuming the F606W transient emission has decayed by the same fraction as the F275W transient emission, the remaining F606W transient would have a flux of 14.6±1.2µJy. Note that the fact that these two values do not agree may indicate either that the spectral energy distribution of the flare has changed to become somewhat redder (i.e. cooler), or that the original fit did not accurately represent the SED of the flare. In any case, both implied transient emission fluxes constitutes less than 10% of the flux within the iraf imexam fitting region of the late time image (based on 8 pixel

radius aperture photometry) and thus might not be expected to strongly skew the result. However, by adding fake point sources into the nuclear region of the late time image and determining the skew to the imexam fitted region, it was found

that the transient emission could produce sizeable skews to the fit, even when its total flux contributes less than 10% of the emission in the fitting region. As such theimexamresult for ASASSN14ae is presented with caution.

In the case of ASASSN14li, the F225W emission has decayed to a flux of 125_±1µJy, or 17.5_±0.1% of its original value. Assuming the admittedly poorly constrained thermal SED with a temperature of 35000K from Holoien et al. (2016), this implies the remaining F621M transient emission would have a flux of∼35µJy or, by assuming again that the F621M transient has decayed by the same fraction as the F225W transient, the remaining F621M yransient emission would have a flux of∼56µJy. In this case, the poor constraints on the SED of the flare mean it is not possible to determine if this difference is evidence of spectral evolution. Note also that both of these values are far below the remaining point source emission implied by thegalfit modelling (158µJy). These estimates constitute less than 25% of the

flux within the central region of the host (229µJy within an 8 pixel radius). Again, modelling shows that this could produce a considerable skew in theimexamfit and

radial profile of theimexamfit and thus again is considered with caution.

The radial profile of the central region of ASASSN15lh is extremely narrow, indicating point source emission may well be present in the late time image. Indeed, based on the model SED of the host produced by Leloudas et al. (2016), the V-band magnitude of the host emission is almost 0.5 mag fainter than that determined in the late-time F606W imaging, indicating the central region of the host may still be dominated by transient light. For completeness, however, the imexam result is

included in the astrometry of the host.

Source imexam imexam galfit galfit Offset (00) Offset (pc) Offset(00) Offset (pc)

ASASSN14ae 0.0034_±0.0028 2.9_±2.4 0.0130_±0.0030 11.2_±2.6 ASASSN14li 0.0148_±0.0080 6.2_±3.3 0.0260_±0.0080 10.8_±3.3 ASASSN15lh 0.0033±0.0048 12±18 0.0063±0.0048 23±18 Table 5.4: The angular distances and physical offsets of the two host centroid meth- ods for the three ASASSN candidate flares. In the cases of ASASSN14ae and ASASSN14li, the transient is located within ∼_{10 parsecs of the determined host}

centroid, making association with the supermassive black hole quite likely. The greater redshift of ASASSN15lh, and the consequently poorer physical resolution of the images of its host, make the astrometric ties somewhat poorer. Further the strong tie based on the imexam position may be as a result of skewing from re-

maining transient emission. However, while it is unclear how applicable thegalfit

result is in the case of such a complex central morphology, the transient emission appears to be consistent with the host centroid.

to be somewhat smaller in all cases. This may indicate that theimexammethod is

indeed skewed by transient emission, particularly in the case of ASASSN15lh. While ASASSN14ae and ASASSN14li are technically inconsistent with their host centroids as determined bygalfitto the 3-4σlevel, even those poorer matches are still within

∼_{10 pc of the host centroid. This is an extremely close tie, especially when compared}

with their inner bulge effective radii of 170 pc and 260 pc respectively. ASASSN15lh, being at a somewhat higher redshift, has a consequently poorer physical resolution. However, based on the wider galaxy model the transient appears to be consistent with the host centroid to∼_{20 pc.}

In the case of CSS100217, as in Drake et al. (2011), it was not possible to distinguish between the position of the transient and the underlying AGN point source emission in the HST imaging. In addition, while the CRTS lightcurve of the event implies that the transient constitutes approximately two thirds of the emission at the time of theHSTimaging, the true fraction of the light coming from the transient is somewhat uncertain. As a conservative estimate on the minimum resolvable separation of the two, the WFC3 UVIS minimum separation of faint companion stars determined in Gilliland and Rajan (2011) is extrapolated to a magnitude difference of∼1 mag resulting in an estimated separation of∼0.0500. This is equivalent to ∼_{100 pc at the redshift of CSS100217, comparable to the} ∼_{150 pc}