# Implications from Late-Time X-ray Detections of Optically Selected Tidal Disruption Events: State Changes, Unification, and Detection Rates

## TL;DR

This study presents late-time X-ray observations of optically-selected TDEs, revealing state changes, proposing a delay between optical and X-ray peaks, and updating detection rate predictions for future surveys.

## Contribution

It provides evidence for state transitions in TDEs years after disruption and refines TDE rate estimates for the eROSITA survey, considering black hole spin and observational biases.

## Key findings

- TDEs transition from soft to hard X-ray states over years.
- X-ray peaks may lag optical emission, affecting observed properties.
- Predicted eROSITA TDE detection rates vary widely based on parameters.

## Abstract

We present Chandra X-ray observations of four optically-selected tidal disruption events (TDEs) obtained 4-9 years after discovery. Three sources were detected with luminosities between 9X10^40 and 3X10^42 erg/s. The spectrum of PTF09axc is consistent with a power law of index 2.5+-0.1, whereas the spectrum of PTF09ge is very soft. The power law spectrum of PTF09axc and prior literature findings, provide evidence that TDEs transition from an early-time soft state to a late-time hard state many years after disruption. We propose that the time to peak luminosity for optical and X-ray emission may differ substantially in TDEs, with X-rays being produced or becoming observable later. This delay helps explain the differences in observed properties such as L_opt/L_ X of optically and X-ray selected TDEs. We update TDE rate predictions for the eROSITA instrument: it ranges from 3 per yr to 990 per yr, depending sensitively on the distribution of black hole spins and the time delay between disruption and peak X-ray brightness. We further predict an asymmetry in the number of retrograde and prograde disks in samples of optically and X-ray selected TDEs. The details of the observational biases can contribute to observed differences between optically and X-ray selected TDEs (with optically selected TDEs being fainter in X-rays for retrograde TDE disks).