高能暂现源 · 每日文献综述

X-ray flaring activity from active galactic nuclei (AGNs) may mimic the expected emission from tidal disruption events (TDEs), thus contaminating TDE searches. To compare X-ray emission properties between TDEs and AGNs, we crossmatch publicly available XMM-Newton and Swift-XRT point-source catalogs with the Million Quasars Catalog and optically selected TDEs. We find that AGNs tend to become softer with a similar hardness ratio (HR) value to TDEs during their flaring events. The soft HRs during flaring events in these AGNs are driven by the emergence of a blackbody-like component below ∼2 keV, likely associated with the soft X-ray excess, as well as the continuum emission becoming steeper with the increase of accretion rate. We find that 2.5% of AGNs in XMM-Newton (23 out of 920) and 4.4% of AGNs in Swift-XRT (179 out of 4089) display flares with peak count rate &gt;2σ from the median count rate and peak HR &lt; −0.75. The rate of such flares in XMM-Newton and Swift-XRT is (1.1─2.5) × 10<SUP>−3</SUP> galaxy<SUP>−1</SUP> yr<SUP>−1</SUP> and (0.9─2.0) × 10<SUP>−3</SUP> galaxy<SUP>−1</SUP> yr<SUP>−1</SUP>, respectively. We also estimate the rate of flares with a maximum flux change (peak/min) of &gt;20× over on a rest-frame timescale of 2 yr or more with an HR &lt; −0.75 at peak to be (4.7─11.0) × 10<SUP>−5</SUP> galaxy<SUP>−1</SUP> yr<SUP>−1</SUP> and (3.1─7.1) × 10<SUP>−5</SUP> galaxy<SUP>−1</SUP> yr<SUP>−1</SUP> in XMM-Newton and Swift-XRT, respectively. Finding an optical or infrared counterpart may help to identify these flaring AGNs. We confirm 61% and 79% of flaring sources from XMM-Newton and Swift-XRT, respectively, as AGNs through variability in Zwicky Transient Facility light curves or a Wide-field Infrared Survey Explorer W1 − W2 &gt; 0.8 mag color cut.