Evolution of the Transients

The SwiftUVW1 and UVM2 filters have similar effective wavelengths to the HST

F275W and F225W filters respectively, making possible the production of long- term lightcurves of each event. The resulting lightcurves for ASASSN14ae and ASASSN14li are plotted in Figure 5.6 and 5.7 respectively.

ASASSN14ae

As noted in Holoien et al. (2014), ASASSN14ae has a somewhat peculiar evolution compared to other tidal disruption flare candidates. Based on the bolometric lu- minosity of the flare determined from fits to the Swift-UVOT observed SED, they were able to determine that an exponential decline produced a better fit to the early time data than the normal power law models with indices between -5/3 and -5/12, as are commonly seen in other flares (Strubbe and Quataert, 2009; Lodato and Rossi, 2011). Using these newHST observations, and including further Swift data from Brown et al. (2016b), it is possible to extend the lightcurve to a potentially host dominated epoch and thus the F275W/UVW1 data was fitted with power law (F = (t₋T0)α +const) and exponential (F =a×et/tdecay +const) models, each

with an additional constant component to represent the underlying host emission. The power law fits were completed both with free fits ofT0 andα and by holdingα

to the canonical value of -5/3 varying onlyT0.

If permitted to vary freely, the power law fits prefer early values ofT0 (40+

days before the ASASSN trigger on 2014 January 25, MJD 56682.5) and steep power law indices (α∼−_{3). Unfortunately, pre-flare imaging can only place a weak limit}

on the beginning of the flare at a factor of∼2 in flux below peak 24 days before the flare’s detection (Holoien et al., 2014). Further, some leeway does exist in the “true” T0 given that it represents the time of the return of the most bound material (Rees,

1988; Lodato and Rossi, 2011) and not necessarily the time of the first observable emission. As such it is difficult to exclude a slow rising transient. If on the other hand the flare was detected within a few days ofT0, this simple power law fit would

imply that the flare still dominates the ultraviolet emission, making the flat late time observations somewhat at odds with the fit. Ifα is held to the canonical value of -5/3, the best fitT0 occurs ∼17 days before the ASASSN trigger, consistent with

the 18 days determined in Brown et al. (2016b) who instead fitted theSwiftUVW2 photometry at early times. The exponential fit also shows a reasonable match to the data, particularly at late times, where the fit implies the flare has now reached host level.

101 102 103 Days since MJD 56665.5 101 102 103 Flux / µ Jy

Figure 5.6: The Swift UVW1 (crosses) and HST F275W (squares) lightcurve for ASASSN14ae. All photometry is presented without host subtraction. Also displayed are the canonical power law fit with a index of -5/3 (solid), upon which the value of T0 is determined, and the best fit exponential model (dashed), each model with

However, none of the fits adequately reproduce the lightcurve in its entirety, with the power law fits producing reduced chi-squared values (χ2_red) of between 3 and 9, depending on the value of T0 used, and the exponential fit having a χ2red =

5.3, implying that these simple fits do not fully represent the evolution of the flare. A more complicated fit with a broken power law improves the fit greatly (χ2_red= 1.7) with an early power law index of -0.9_±0.1 breaking to a much steeper decay of -2.5_±0.2 after ∼_{45 days. This is qualitatively similar to the models of Lodato and}

Rossi (2011) that predict that tidal disruption flare lightcurves follow a shallow decay at early times and steepen at late times.

Alternatively, a possible cause for the poor fits could be systematic offsets in the photometric calibration of the photometry, or the possibility that the small offset in the central wavelengths of the filters used produces a larger than expected offset in the output photometry. Visual inspection of the lightcurve shows the F275W and UVW1 emission exhibits little sign of any systematic offset except perhaps at late times where the UVW1 observation at ∼_{800 days from Brown et al. (2016b)}

is brighter (3.8σ) than the late F275W photometry at ∼_{500 days. However, given}

the existence of a red leak in Swift UVW1 filter9, it is likely that this is due to contamination from optical host emission in the presence of a reddening SED as the blue transient fades. Given that the exponential model fit agrees well with the∼₈₀₀

days UVW1 emission, whether or not it is included in the fitting process, and that the photometry is approximately consistent with the implied magnitude of the host fitting in Holoien et al. (2014), it is likely that the late time photometry represents host level emission.

ASASSN14li

As with ASASSN14ae, the lightcurve of ASASSN14li had previously been noted as being best fit by an unusual exponential fit (Holoien et al., 2016). Again, the combinedSwift-UVOT observations from Holoien et al. (2016) and the HST obser- vations from this study are fitted with the same power law and exponential models as ASASSN14ae, each with a constant factor to account for the underlying host emission.

As before, the free power law fits favour exceptionally early values of T0,

hundreds of days prior to the trigger on 2014 November 22 (MJD 56983.5). Due to the host being behind the Sun for a considerable period of time preceding the flare, the limit on early emission of 2014 July 13 (MJD 56851, 132 days before the ASASSN trigger Holoien et al., 2016) is not as constraining as for ASASSN14ae. Fixing

9

102 103 Days since MJD 56938.5 102 103 Flux / µ Jy

Figure 5.7: TheSwiftUVM2 andHSTF225W lightcurve of ASASSN14li. Again, the photometry is presented without host subtraction. Also plotted are thet−5/3 power law (solid) and exponential (dashed) fits, again with a constant component included to represent host level emission. The exponential model produces the superior fit to the data and shows that the flare has now reached host level.

the power law index to the canonical -5/3, however, produces a likely T0 around

MJD 56940, ∼_{45 days before the ASASSN detection. This fit would also imply}

that the flare still contributes considerably to the remaining ultraviolet emission. However, an exponential model with tdecay = 52 days produces a far superior fit

(χ2_red = 2.3 cf 5.9 for the canonical power law case). The same goodness of fit is only achieved in single power law fits with unfeasibly early values ofT0, more than

300 days before the trigger, which are precluded by the limits on early detection. The constant component of the exponential fit also agrees well with the late epochHST

photometry, indicating the flare has now decayed to host level. More recent Swift

UVOT observations that are not analysed in this work appears to support this, having plateaued at a magnitude of ∼_{18.7, consistent with the late-time F225W}

photometry10. Alternatively, a broken power law fit does produce a comparable fit to the data (χ2_red= 2.3) but indicates a very steep late time decay (∼_3.2).

CSS100217

A number of features of interest exist the long-term lightcurve of CSS100217 (Fig- ure 5.4). The first is, as mentioned above, a clear offset in the pre- and post-flare photometry. The most logical explanation for this, particularly given the known Seyfert nature of the host, is a change in output of the central AGN, either through a change in the accretion rate or through a phase change in the emission mecha- nism. The temporal coincidence of the flare and this decline, along with the spatial coincidence of the flare and the centre of the host, makes it plausible that there is causal connection between the two events, perhaps with the flare resulting in the decline of the AGN or some mechanism causing both emission features.

The second point of interest is the apparent lack of correlation between the X-ray and optical emission. During the flare, while the UV and optical observations showed a clear linear decline in magnitude with time, the combined Swift X-ray observations were only able to determine the presence of a weak X-ray source with an inferred luminosity of (4.1_±1.2)_×1042erg s−1, placing it at the low end of the X-ray luminosity function of AGN at low redshift, well belowL∗ at 1.5×1044erg s−1

(Aird et al., 2015). However, the lateSwiftobservations showed the source was going through a possible X-ray flare with bright, X-ray emission up to a factor of 10 more luminous than during the high optical state and showing factor∼_{3 variability over} ∼_{10 days. The Swift-UVOT observations obtained during the same epoch showed}

no evidence for evolution and were consistent with GALEX NUV (Bianchi et al., 2011) observations of the host made on 2004 January 24, 6 years before the flare

10

(UVW2= 19.4_±0.2 cf NUV= 19.3_±0.1) and thus consistent with host level.