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Register a new userNine-hour X-ray quasi-periodic eruptions from a low-mass black hole galactic nucleus
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In the past two decades, high amplitude electromagnetic outbursts have been detected from dormant galaxies and often attributed to the tidal disruption of a star by the central black hole. X-ray emission from the Seyfert 2 galaxy GSN 069 (2MASX J01190869-3411305) at redshift z = 0.018 was first detected in 2010 July and implies an X-ray brightening of more than a factor of 240 over ROSAT observations performed 16 years earlier. The emission has smoothly decayed over time since 2010, possibly indicating a long-lived tidal disruption event. The X-ray spectrum is ultra-soft and can be described by accretion disc emission with luminosity proportional to the fourth power of the disc temperature during long-term evolution. Here we report observations of X-ray quasi-periodic eruptions from the nucleus of GSN 069 over the course of 54 days, 2018 December onwards. During these eruptions, the X-ray count rate increases by up to two orders of magnitude with event duration of just over 1 hour and recurrence time of about 9 hours. These eruptions are associated with fast spectral transitions between a cold and a warm phase in the accretion flow around a low-mass black hole (of approximately 4x10$^5$ solar masses) with peak X-ray luminosity of ~ 5x10$^{42}$ ergs per second. The warm phase has a temperature of about 120 eV, reminiscent of the typical soft X-ray excess, an almost universal thermal-like feature in the X-ray spectra of luminous active nuclei. If the observed properties are not unique to GSN 069, and assuming standard scaling of timescales with black hole mass and accretion properties, typical active galactic nuclei with more massive black holes can be expected to exhibit high-amplitude optical to X-ray variability on timescales as short as months or years.
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RX J1301.9+2747 is an ultrasoft active galactic nucleus (AGN) with unusual X-ray variability that is characterized by a long quiescent state and a short-lived flare state. The X-ray flares are found to recur quasi-periodically on a timescale of 13-20 ks. Here, we report the analysis of the light curve in the quiescent state from two XMM observations spanning 18.5 years, along with the discovery of a possible quasi-periodic X-ray oscillation (QPO) with a period of ~1500s. The QPO is detected at the same frequency in the two independent observations, with a combined significance of >99.89%. The QPO is in agreement with the relation between frequency and black hole mass (M_BH) that has been reported in previous works for AGNs and Galactic black hole X-ray binaries (XRBs). The QPO frequency is stable over almost two decades, suggesting that it may correspond to the high-frequency type found in XRBs and originates, perhaps, from a certain disk resonance mode. In the 3:2 twin-frequency resonance model, our best estimate on the M_BH range implies that a maximal black hole spin can be ruled out. We find that all ultrasoft AGNs reported so far display quasi-periodicities in the X-ray emission, suggesting a possible link on the part of the extreme variability phenomenon to the ultrasoft X-ray component. This indicates that ultrasoft AGNs could be the most promising candidates in future searches for X-ray periodicities.
Quasi-Periodic Eruptions (QPEs) are extreme high-amplitude bursts of X-ray radiation recurring every few hours and originating near the central supermassive black holes in galactic nuclei. It is currently unknown what triggers these events, how long they last and how they are connected to the physical properties of the inner accretion flows. Previously, only two such sources were known, found either serendipitously or in archival data, with emission lines in their optical spectra classifying their nuclei as hosting an actively accreting supermassive black hole. Here we present the detection of QPEs in two further galaxies, obtained with a blind and systematic search over half of the X-ray sky. The optical spectra of these galaxies show no signature of black hole activity, indicating that a pre-existing accretion flow typical of active nuclei is not required to trigger these events. Indeed, the periods, amplitudes and profiles of the newly discovered QPEs are inconsistent with current models that invoke radiation-pressure driven accretion disk instabilities. Instead, QPEs might be driven by an orbiting compact object. Furthermore, their observed properties require the mass of the secondary object to be much smaller than the main body and future X-ray observations may constrain possible changes in the period due to orbital evolution. This scenario could make QPEs a viable candidate for the electromagnetic counterparts of the so-called extreme mass ratio inspirals, with considerable implications for multi-messenger astrophysics and cosmology.
Very few galactic nuclei are found to show significant X-ray quasi-periodic oscillations (QPOs). After carefully modeling the noise continuum, we find that the ~3.8 hr QPO in the ultrasoft active galactic nucleus (AGN) candidate 2XMM J123103.2+110648 was significantly detected (~5sigma) in two XMM-Newton observations in 2005, but not in the one in 2003. The QPO rms is very high and increases from ~25% in 0.2-0.5 keV to ~50% in 1-2 keV. The QPO probably corresponds to the low-frequency type in Galactic black hole X-ray binaries, considering its large rms and the probably low mass (~10^5 msun) of the black hole in the nucleus. We also fit the soft X-ray spectra from the three XMM-Newton observations and find that they can be described with either pure thermal disk emission or optically thick low-temperature Comptonization. We see no clear X-ray emission from the two Swift observations in 2013, indicating lower source fluxes than those in XMM-Newton observations.
Spectroscopy of the low mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ~2 hr periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main sequence K-dwarf together with a canonical 1.4Msun neutron star gives a good description of the system. In this scenario the most likely place for the emission lines to arise is the accretion disk, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (<10 degrees) of Ser X-1.
A calibration is made for the correlation between the X-ray Variability Amplitude (XVA) and Black Hole (BH) mass. The correlation for 21 reverberation-mapped Active Galactic Nuclei (AGN) appears very tight, with an intrinsic dispersion of 0.20 dex. The intrinsic dispersion of 0.27 dex can be obtained if BH masses are estimated from the stellar velocity dispersions. We further test the uncertainties of mass estimates from XVAs for objects which have been observed multiple times with good enough data quality. The results show that the XVAs derived from multiple observations change by a factor of 3. This means that BH mass uncertainty from a single observation is slightly worse than either reverberation-mapping or stellar velocity dispersion measurements; however BH mass estimates with X-ray data only can be more accurate if the mean XVA value from more observations is used. Applying this relation, the BH mass of RE J1034+396 is found to be $4^{+3}_{-2} times 10^6$ $M_{odot}$. The high end of the mass range follows the relationship between the 2$f_0$ frequencies of high-frequency QPO and the BH masses derived from the Galactic X-ray binaries. We also calculate the high-frequency constant $C= 2.37 M_odot$ Hz$^{-1}$ from 21 reverberation-mapped AGN. As suggested by Gierlinski et al., $M_{rm BH}=C/C_{rm M}$, where $C_{rm M}$ is the high-frequency variability derived from XVA. Given the similar shape of power-law dominated X-ray spectra in ULXs and AGN, this can be applied to BH mass estimates of ULXs. We discuss the observed QPO frequencies and BH mass estimates in the Ultra-Luminous X-ray source M82 X-1 and NGC 5408 X-1 and favor ULXs as intermediate mass BH systems (abridged).
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