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Register a new userDiscovery of a time lag between the soft X-ray and radio emission of the tidal disruption flare ASASSN-14li: Evidence for linear disk-jet coupling
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Dheeraj R. Pasham
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The tidal disruption of a star by a supermassive black hole can result in transient radio emission. The electrons producing these synchrotron radio flares could either be accelerated inside a relativistic jet or externally by shocks resulting from an outflow interacting with the circumnuclear medium. Until now, evidence for the internal emission mechanism has been lacking; nearly all tidal disruption flare studies have adopted the external shock model to explain the observed properties of radio flares. Here we report a result that presents a challenge to external emission models: we discovered a cross-correlation between the soft X-ray (0.3-1 keV) and 16 GHz radio flux of the tidal disruption flare ASASSN-14li. Variability features in the X-ray light curve appear again in the radio light curve, but after a time lag of about 13 days. This demonstrates that soft X-ray emitting accretion disk regulates the radio emission. This coupling appears to be inconsistent with all previous external emission models for this source but is naturally explained if the radio emission originates from a freely expanding jet. We show that emission internal to an adiabatically expanding jet can also reproduce the observed evolution of the radio spectral energy distribution. Furthermore, both the correlation between X-ray and radio luminosity as well as our radio spectral modeling imply an approximately linear coupling between the accretion rate and jet power.
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The tidal disruption of a star by a supermassive black hole leads to a short-lived thermal flare. Despite extensive searches, radio follow-up observations of known thermal stellar tidal disruption flares (TDFs) have not yet produced a conclusive detection. We present a detection of variable radio emission from a thermal TDF, which we interpret as originating from a newly-launched jet. The multi-wavelength properties of the source present a natural analogy with accretion state changes of stellar mass black holes, suggesting all TDFs could be accompanied by a jet. In the rest frame of the TDF, our radio observations are an order of magnitude more sensitive than nearly all previous upper limits, explaining how these jets, if common, could thus far have escaped detection.
We carried out the first multi-wavelength (optical/UV and X-ray) photometric reverberation mapping of a tidal disruption flare (TDF) ASASSN-14li. We find that its X-ray variations are correlated with and lag the optical/UV fluctuations by 32$pm$4 days. Based on the direction and the magnitude of the X-ray time lag, we rule out X-ray reprocessing and direct emission from a standard circular thin disk as the dominant source of its optical/UV emission. The lag magnitude also rules out an AGN disk-driven instability as the origin of ASASSN-14li and thus strongly supports the tidal disruption picture for this event and similar objects. We suggest that the majority of the optical/UV emission likely originates from debris stream self-interactions. Perturbations at the self-interaction sites produce optical/UV variability and travel down to the black hole where they modulate the X-rays. The time lag between the optical/UV and the X-rays variations thus correspond to the time taken by these fluctuations to travel from the self-interaction site to close to the black hole. We further discuss these time lags within the context of the three variants of the self-interaction model. High-cadence monitoring observations of future TDFs will be sensitive enough to detect these echoes and would allow us to establish the origin of optical/UV emission in TDFs in general.
We present a Hubble Space Telescope STIS spectrum of ASASSN-14li, the first rest-frame UV spectrum of a tidal disruption flare (TDF). The underlying continuum is well fit by a blackbody with $T_{mathrm{UV}} = 3.5 times 10^{4}$ K, an order of magnitude smaller than the temperature inferred from X-ray spectra (and significantly more precise than previous efforts based on optical and near-UV photometry). Super-imposed on this blue continuum, we detect three classes of features: narrow absorption from the Milky Way (probably a high-velocity cloud), and narrow absorption and broad (FWHM $approx 2000$-8000 km s$^{-1}$) emission lines at/near the systemic host velocity. The absorption lines are blueshifted with respect to the emission lines by $Delta v = -(250$-400) km s$^{-1}$. Due both to this velocity offset and the lack of common low-ionization features (Mg II, Fe II), we argue these arise from the same absorbing material responsible for the low-velocity outflow discovered at X-ray wavelengths. The broad nuclear emission lines display a remarkable abundance pattern: N III], N IV], He II are quite prominent, while the common quasar emission lines of C III] and Mg II are weak or entirely absent. Detailed modeling of this spectrum will help elucidate fundamental questions regarding the nature of the emission process(es) at work in TDFs, while future UV spectroscopy of ASASSN-14li would help to confirm (or refute) the previously proposed connection between TDFs and N-rich quasars.
We report on late time radio and X-ray observations of the tidal disruption event candidate ASASSN-14li, covering the first 1000 days of the decay phase. For the first $sim200$ days the radio and X-ray emission fade in concert. This phase is better fit by an exponential decay at X-ray wavelengths, while the radio emission is well described by either an exponential or the canonical $t^{-5/3}$ decay assumed for tidal disruption events. The correlation between radio and X-ray emission during this period can be fit as $L_{R}propto L_{X}^{1.9pm0.2}$. After 400 days the radio emission at $15.5,textrm{GHz}$ has reached a plateau level of $244pm8,mutextrm{Jy}$ which it maintains for at least the next 600 days, while the X-ray emission continues to fade exponentially. This steady level of radio emission is likely due to relic radio lobes from the weak AGN-like activity implied by historical radio observations. We note that while most existing models are based upon the evolution of ejecta which are decoupled from the central black hole, the radio : X-ray correlation during the declining phase is also consistent with core jet emission coupled to a radiatively efficient accretion flow.
We present late-time observations by Swift and XMM-Newton of the tidal disruption event (TDE) ASASSN-15oi that reveal that the source brightened in the X-rays by a factor of $sim10$ one year after its discovery, while it faded in the UV/optical by a factor of $sim 100$. The XMM-Newton observations measure a soft X-ray blackbody component with $kT_{rm bb} sim 45$ eV, corresponding to radiation from several gravitational radii of a central $sim 10^6 M_odot$ black hole. The last Swift epoch taken almost 600 days after discovery shows that the X-ray source has faded back to its levels during the UV/optical peak. The timescale of the X-ray brightening suggests that the X-ray emission could be coming from delayed accretion through a newly forming debris disk, and that the prompt UV/optical emission is from the prior circularization of the disk through stream-stream collisions. The lack of spectral evolution during the X-ray brightening disfavors ionization breakout of a TDE veiled by obscuring material. This is the first time a TDE has been shown to have a delayed peak in soft X-rays relative to the UV/optical peak, which may be the first clear signature of the real-time assembly of a nascent accretion disk, and provides strong evidence for the origin of the UV/optical emission from circularization, as opposed to reprocessed emission of accretion radiation.
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