The Fast Evolving, Tremendous and Blue Superoutburst in ASASSN-21au Reveals a Dichotomy in the Outbursts of Long-period AM CVns

We have obtained X-ray and UV data of ASASSN-21au during its first superoutburst and post-outburst cooling phase with the XRT and UVOT instruments on board the Neil Gehrels Swift Observatory (Swift). Data were taken from 2021-02-17 to 2021-06-11 with a total exposure time of $12$ ks distributed in 14 observations.

ASASSN-21au was detected in X-rays in 9 of the observations, with upper limits on the remaining measurements, mainly due to the short exposure times. The XRT data analysis was performed using XSPEC (Arnaud, 1996, Version 12.11). Count rates were obtained in the X-ray energy range 0.3-10 keV for each of the individual observations. For the spectral analysis we used an absorbed power-law model (tbabs*powerlaw) fixing $N_{h}$ to the Galactic value of $2.87\times 10^{20}$ cm^-2 toward the position of the source. In order to obtain meaningful values for the spectral fits we divided the observations in 2 groups: those obtained during the plateau phase of the superoutburst and those taken during the post-outburst cooling phase where the binary was detected. For each group we binned the data considering at least 5 counts per bin and used Cash-statistics (Cash, 1979) due to the low number of counts acquired.

For each individual Swift observation, UVOT data were obtained in the 6 available filters: $UVW2,UVM2,UVW1,U,B,V$ which in total cover the wavelength range from $1600$ to $8000$ Å. Data were analysed with the suggested tools in the Swift threads²²2https://www.swift.ac.uk/analysis/uvot/. For the background subtraction we used a circular region of $5\arcsec$ radius around the target, and a circular region with a radius of $25\arcsec$ for the background determination. Photometry was calibrated to the AB system.

Data in the clear and V filters were obtained by members of the AAVSO (Kafka, S., 2021) and VSNET with the first measurement starting on 2021-02-12 and the last one taken on 2021-04-15. Several epochs with cadences of 1 min per observation were obtained. We have aligned the data from the different observers into a common frame by using the first dataset of observations as reference, which is consistent with the results from the Swift $V$ filter.

For comparison purposes we have also made use of public observations from the ATLAS project in the cyan and orange bands (Tonry et al., 2018) and from ZTF (Masci et al., 2019) in the $g$ and $r$ bands. Only data with good quality flags have been used for the analysis. Additionally, in the case of the ZTF data, we excluded points with airmass $>1.8$ since the differential chromatic refraction that produces color biases dominates above that value (Masci et al., 2019). We also used data from ASAS-SN (Shappee et al., 2014; Kochanek et al., 2017) in the $g$ band, data from Pan-STARRS (Chambers et al., 2016) in the $g,r,i$ and $z$ bands, from GALEX (Bianchi et al., 2011) in the $NUV$, from Gaia EDR3 (Gaia Collaboration, 2020) and the Gaia alerts³³3http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia21cbs/ in the $G$ band. Photometry from these surveys is here reported in the AB system.

We also checked for indications of previous outburst activity from the binary in the ATLAS, ASAS-SN, ZTF, CSS (Drake et al., 2009) and DASCH (Grindlay et al., 2009) databases, but no outbursts of similar amplitude to the one reported here were previously recorded.

The multiwavelength light curves of ASASSN-21au are shown in Figure 1 and Figure 2. The first brightness increase of ASASSN-21au was detected by Gaia on 2020-10-30 ($JD*=252.7$, where $JD*=JD-2458900$) when the binary was 1 mag above (marked as increased level 1) its original $G$ quiescence level. On 2020-11-24 ($JD*=278$) ZTF $g$ detected the binary at the same increased level 1 (corresponding to $\sim 0.8$ mags above its ZTF $g$ quiescent level). Constraints due to the Sun prevented a good coverage with ZTF to observe the rise phase, but based on the current ZTF and Gaia data, a limit on the duration of the rise and the increased level 1 ($\tau_{L1}$) can be determined as $67$ days $<\tau_{L1}<172$ days. On $JD*=333$ the binary reached a second increased brightness level (marked with an orange line in Figures 1 and 2), which was 1.5 mags above the original Gaia $G$ quiescent value. The system took approximately 2 additional weeks to reach that level and remained in such state for another 2 weeks. On 2021-02-03 ($JD*=349$) ASASSN-21au showed a sudden brightness increase due to the precursor of the superoutburst (denoted with a blue vertical line in Figures 1 and 2).

The first ASAS-SN solid detections of ASASSN-21au were three points at $g\sim 13$ mags on 2021-02-05 (JD*=351), followed by several other points 2-3 days later at $g\sim 16.4-17.4$ mags, which indicate the fading of the precursor. Afterwards the rise to the plateau phase started. The maximum of the precursor was similar to the maximum of the plateau ($\sim 7.5$ mags in $g$ above its ZTF quiescent level). While precursors are common in DNe of the type SU UMa and they have been recently confirmed in multiple AM CVns (Pichardo Marcano et al., 2021; Duffy et al., 2021), they are relatively difficult to detect using ground-based images due to their short duration. But continuous coverage of AM CVns’ superoutbursts with space telescopes now suggest that precursors are a relatively common characteristic (Pichardo Marcano et al., 2021). In the case of the precursor of ASASSN-21au, it is also remarkable that its luminosity drops by 3 magnitudes right before the superoutburst begins, much more than in any other system (Pichardo Marcano et al., 2021; Duffy et al., 2021).

In the bluest UV band ($UVW2$), there was a decrease of 3 orders of magnitude in flux between the plateau of the superoutburst and the post-outburst cooling phase around $JD*=373.3$, going from $1.60\times 10^{-13}$ erg s^-1 cm^-2 to $3.36\times 10^{-16}$ erg s^-1 cm^-2, while in the $V$ band the flux decreased by only 2 orders of magnitude, showing that the dominant emission components during superoubursts were the accretion disk, the boundary layer and the accreting WD.

The superoutburst lasted for 15 days, excluding the precursor, and ended on 2021-02-23 (or $JD*=369$) as indicated by the data presented in Figure 2. This superoutburst was followed by an echo outburst, occurring more than a week after the superoutburst ended; these are frequent in SU UMa systems that show only superoutbursts, and have been interpreted by Hameury & Lasota (2021) as the manifestation of an increased mass-transfer rate, possibly as a result of the secondary irradiation by the heated accreting WD at the end of a superoutburst. This interpretation is strengthened by the fact that in ASASSN-21au, the system has not returned to full quiescence at the end of the superoutburst.

We now compare the behavior between the X-rays, optical and UV observations obtained by UVOT. As shown in other AM CVns, during the post-outburst cooling phase of ASASSN-21au there is X-ray emission indicating the presence of residual accretion. Interestingly, during the post-outburst cooling phase the spectrum was substantially redder than during the peak of the superoutburst. A possible explanation for such a behavior is that the inner parts of the disk may have evaporated in that cooling phase.

Figure 2 also shows a correlation between the optical, UV and X-rays. During the optical and UV maxima, the X-ray emission also peaked, and during the optical and UV decay to the post-outburst cooling phase the X-ray flux decreased by a factor of 3. This behavior is different from that observed in the AM CVns KL Dra (Ramsay et al., 2012b) and SDSS J141118+481257 (henceforth SDSS 1411; Rivera Sandoval & Maccarone, 2019), where an anticorrelation between the optical/UV and the X-rays during superoutburst was observed. The same anticorrelation has been observed in many CVs (e.g. Wheatley et al., 2003; Byckling et al., 2009; Fertig et al., 2011), where it has been explained as due to changes in the optical depth of the boundary layer.

According to the standard theory (Patterson & Raymond, 1985a, b), increases in the X-ray flux with accretion rate are expected so long as the boundary layer remains optically thin; once the boundary layer becomes optically thick, the majority of the flux shifts to the far-UV and the X-ray flux drops. Three nearby CVs, SS Cyg, U Gem and GW Lib, have shown a clear correlation between X-ray and optical/UV flux. SS Cyg showed this during the early, and late, parts of its outburst, with an anticorrelation at the highest optical fluxes (and, presumably, mass-transfer rates; Wheatley et al. 2003). Fertig et al. (2011) summarize observations of 6 well-studied DN outbursts, in which 4 showed suppression of the X-rays with increased mass-transfer at some point. The peak X-ray luminosities of these CVs (in this paradigm, reflecting the critical accretion rate for the boundary layer to turn optically thick) mostly lie between $L_{X}=1\times 10^{32}$ and $1\times 10^{33}$ erg/s, though VW Hyi showed X-ray suppression from a peak of only $6\times 10^{30}$ erg/s.

However, the critical accretion rate for the boundary layer turning optically thick may be different for He vs. H accretion. The peak $L_{X}$ values for the two AM CVn systems with well-studied outbursts where an anticorrelation has been observed have $5\times 10^{30}$ erg/s (KL Dra, Ramsay et al. 2012a) using the Gaia distance of 948 pc (Bailer-Jones et al., 2021), and $2\times 10^{31}$ erg/s (SDSS 1411, Rivera Sandoval & Maccarone 2019). These are substantially smaller than the typical values for H-accreting CVs, suggesting that the critical accretion rate is systematically lower for He accretion. Assuming that ASASSN-21au reaches a peak $L_{X}$ below the peak $L_{X}$ of the other two AM CVn systems (to account for its X-ray/optical correlation), its inferred distance should be below 400 pc. However, since we do not understand the origin of the large scatter in peak $L_{X}$ values we cannot be too confident of this estimate. It is also possible that the X-ray emitting region is still optically thin if the accretion disk does not extend to the accreting WD surface, but is, even at large accretion rates, truncated by, e.g., a strong magnetic field.

We do not see clear changes in the X-ray spectral index of ASASSN-21au between the plateau and the post-outburst cooling phase, despite the changes in flux. Photon indexes of $\Gamma=2.2\pm 0.28$ and $\Gamma=2.08\pm 0.56$ were determined for each phase, consistent within the errors.

Using archival data obtained during full quiescence we have carried out a spectral energy distribution analysis fitting a blackbody to constrain the temperature of ASASSN-21au. We used the GALEX, Pan-STARRS and Gaia DR3 data, together with a E(g-r) $=0.05$ (from the 3D dust map of Green et al., 2019) and obtained T$=14300\pm 1600$ K, which is roughly consistent with expectations from Bildsten et al. (2006) for an AM CVn of P${}_{\rm orb}\sim 58$ min and $M\sim$1 M_⊙ WD. It is possible that there is an IR excess as the Pan-STARRS-$z$ data point is $7\sigma$ above the blackbody fit line (Figure 5). However, given that we have only one IR data point, a fit with 2 blackbodies (accreting WD and disk/donor) is very poorly constrained leading to practically meaningless results. New, deeper IR observations are needed to confirm this excess. As showed in Bildsten et al. (2006), the contribution of the accretion disk in quiescence should be minimal in optical, and the hot WD should dominate the optical light in quiescence.

Note that a comparison of the WD temperature based on the spectral energy distribution during the post-outburst cooling phase is not really possible considering the small amount of data obtained in this period. Also, during that phase a contribution from the accretion disk is still expected which would be difficult to disentangle from the WD emission, especially considering the limited data points available.

Recently, it has been shown that outbursts in long-period AM CVns have a color evolution that is not compatible with the one expected by the DIM (Rivera Sandoval et al., 2020, 2021). In SDSS 0807 and SDSS 1137 the AM CVns become redder and brighter as they reach the maximum of the outburst. That color evolution together with the duration and amplitude of the outbursts suggest that the outbursts are confined to the outer parts of the disk, perhaps due to enhanced mass-transfer from the donor.

In Figure 3 we present the color evolution of ASASSN-21au using the data from ZTF $g$ and $r$ before $JD*=349$ (when the precursor started). We see that the binary becomes slightly redder compared to quiescence as it increases its brightness, reaching ZTF $g-r>0.5$. Note that measurements have large error bars because the binary is faint and is close to the limiting magnitude of ZTF. However, despite the large scatter, one finds that the color behavior of ASASSN-21au is completely compatible with the color evolution observed in SDSS 0807 and SDSS 1137, suggesting then that such part of the light curve in ASASSN-21au can also be explained under an enhanced mass-transfer scenario. Note that during quiescence the binary is blue because the WD is dominating the emission.

On the other hand, in Figure 6 we present the color evolution of ASASSN-21au using the simultaneous UV and optical data obtained by Swift during superoutburst and post-outburst cooling phase. We chose the bluest ($UVW2$) and reddest ($V$) UVOT filters to trace the behavior. Figure 6 shows that ASASSN-21au follows an opposite pattern to that observed in SDSS 0807 and SDSS 1137. In the case of ASASSN-21au, the binary becomes bluer and brighter as it is closer to the peak, and as it cools down it becomes redder. Note that in Figure 6, there is a second turn towards bluer colors at a $V$ magnitude of $17.1$, which is due to a measurement obtained at the end of the echo outburst, when the accretion disk and accreting WD were still hot. After that, the binary continues becoming redder and fainter. The last blue turn in that diagram corresponds to the accreting WD becoming the dominant emitting source, being much brighter in UV than in optical, where just upper limits were detected in $V$. A ZTF color evolution of ASASSN-21au during outburst in $g$ and $r$ is also displayed in Figure 7 of the Appendix.

The color evolution of ASASSN-21au during superoutburst is similar to that of the binary systems SDSS 1411 (Rivera Sandoval et al., 2021), PTF 0719+4858 and SDSS J1043+5632 (Pichardo Marcano et al., 2021). It is also compatible with expectations from the DIM (Hameury et al., 2020). This strengthens our conclusion that the main mechanism responsible for the superoutbursts (after $JD*=349$) in ASASSN-21au is a disk instability.