Rapid X-ray Variability Research Articles

Achromatic rapid flares in hard X-rays in the γ-ray binary LS I + 61 303

ABSTRACT We report on the presence of very rapid hard X-ray variability in the γ-ray binary LS I + 61 303. The results were obtained by analysing Nuclear Spectroscopic Telescope Array (NuSTAR) data, which show two achromatic strong flares on ks time-scales before apastron. The Swift/Burst Alert Telescope orbital X-ray light curve is also presented, and the NuSTAR data are put in the context of the system orbit. The spectrum and estimated physical conditions of the emitting region indicate that the radiation is synchrotron emission from relativistic electrons, likely produced in a shocked pulsar wind. The achromaticity suggests that losses are dominated by escape or adiabatic cooling in a relativistic flow, and the overall behaviour in hard X-rays can be explained by abrupt changes in the size of the emitting region and/or its motion relative to the line of sight, with Doppler boosting potentially being a prominent effect. The rapid changes of the emitter could be the result of different situations such as quick changes in the intra-binary shock, variations in the re-accelerated shocked pulsar wind outside the binary, or strong fluctuations in the location and size of the Coriolis shock region. Although future multiwavelength observations are needed to further constrain the physical properties of the high-energy emitter, this work already provides important insight into the complex dynamics and radiation processes in LS I + 61 303.

Rapid X-ray variability of the gamma-ray binary LS I +61°303

ABSTRACT The gamma-ray binary LS I +61°303 has been widely monitored at different wavelengths since its discovery more than 60 yr ago. However, the nature of the compact object and the peculiar behaviour of the system are still largely debated. Aimed at investigating the rapid X-ray variability of LS I +61°303, we have analysed all the archival Rossi X-ray Timing Explorer/Proportional Counter Array (RXTE/PCA) data of the source, taken between 1996 and 2011. The timing analysis yields a periodicity of P ∼ 26.6 ± 0.3 d, which is statistically compatible with several periodicities reported in the literature for LS I +61°303. Using this period, we performed a data phase-resolved analysis to produce a set of phase-bin-averaged energy spectra and power density spectra. These power density spectra are dominated by weak red noise below 0.1 Hz, and show no signal above this frequency. The amplitude of the red noise varies mildly with the phase, and shows a maximum that coincides with a dip of the X-ray flux and a softer photon index. Aside from low-frequency noise, this analysis does not provide any statistically significant periodic or quasi-periodic timing feature in the RXTE/PCA data of LS I +61°303.

Strong and Rapid X-Ray Variability of the Super-Eddington Accreting Quasar SDSS J081456.10+532533.5

We report strong and rapid X-ray variability found from the super-Eddington accreting quasar SDSS J081456.10+532533.5 at z = 0.1197. It has a black hole mass of 2.7 × 107 M ⊙ and a dimensionless accretion rate of ≈4 measured from reverberation-mapping observations. It showed weak X-ray emission in the 2021 February Chandra observation, with the 2 keV flux density being times lower compared to an archival Swift observation. The 2 keV flux density is also times weaker compared to the expectation from its optical/UV emission. In a follow-up XMM-Newton observation 32 days later, the 2 keV flux density increased by a factor of , and the spectra are best described by a power law modified with partial-covering absorption; the absorption-corrected intrinsic continuum is at a nominal flux level. Nearly simultaneous optical spectra reveal no variability, and there is only mild long-term optical/infrared variability from archival data (with a maximum variability amplitude of ≈50%). We interpret the X-ray variability with an obscuration scenario, where the intrinsic X-ray continuum does not vary but the absorber has a variable column density and covering factor along the line of sight. The absorber is likely the small-scale clumpy accretion wind that has been proposed to be responsible for similar X-ray variability in other super-Eddington accreting quasars.

The analysis of the iron-rich spectra of a sample of narrow-line Seyfert 1 galaxies

ABSTRACT We analyse medium resolution optical spectra of 14 narrow-line Seyfert 1 galaxies with average to strong iron emission. The targets were observed several times over a 2-yr period to probe the nature of spectral variations. Parameters determined from fitting the line profiles include relative strength, width, and asymmetry. We explore the applicability of various profile combinations to H β. We achieve satisfactory fits using three components: the narrow and broad components characteristic of all Seyfert 1 galaxies and that are reasonably represented by Gaussian profiles, and an intermediate component with width similar to that of the Fe ii lines. This intermediate component accounted for the largest fraction of the H β flux in most of our sample, especially when fitted with a Lorentzian instead of a Gaussian profile. This Lorentzian component improves the overall H β fits in most cases. In contrast to many other types of active galactic nucleus, no dramatic spectral changes were detected in our sample over the 2-yr period spanning this study, even though some of our targets are known for large and rapid X-ray variability. In particular, we find the variability of the Fe ii lines relative to the H β intermediate component to be minimal. For the [O iii] lines, we identified asymmetries in almost all our targets well represented by two Gaussians: a primary central and a second blueshifted component. These may signify different gas dynamics connected with nuclear outflow. As in several previous studies, we identify a clear correlation between [O iii] asymmetry and its core component width.

Rapid X-ray variability in Mkn 421 during a multiwavelength campaign

ABSTRACT The study of short-term variability properties in AGN jets has the potential to shed light on their particle acceleration and emission mechanisms. We report results from a 4-d coordinated multiwavelength campaign on the highly peaked blazar (HBL) Mkn 421 in 2019 January. We obtained X-ray data from AstroSAT, BVRI photometry with the Whole Earth Blazar Telescope (WEBT), and TeV data from First G-APD Cherenkov Telescope to explore short-term multiwavelength variability in this HBL. The X-ray continuum is rapidly variable on time-scales of tens of ks. Fractional variability amplitude increases with energy across the synchrotron hump, consistent with previous studies; we interpret this observation in the context of a model with multiple cells whose emission spectra contain cutoffs that follow a power-law distribution. We also performed time-averaged and time-resolved (time-scales of 6 ks) spectral fits; a broken power-law model fits all spectra well; time-resolved spectral fitting reveals the usual hardening when brightening behaviour. Intra-X-ray cross-correlations yield evidence for the 0.6–0.8 keV band to likely lead the other bands by an average of 4.6 ± 2.6 ks, but only during the first half of the observation. The source displayed minimal night-to-night variability at all wavebands thus precluding significant interband correlations during our campaign. The broad-band SED is modelled well with a standard one-zone leptonic model, yielding jet parameters consistent with those obtained from previous SEDs of this source.

Simple Analytic Formula Relating the Mass and Spin of Accreting Compact Objects to Their Rapid X-Ray Variability

Abstract Following the previous research on epicyclic oscillations of accretion disks around black holes (BHs) and neutron stars (NSs), a new model of high-frequency quasiperiodic oscillations (QPOs) has been proposed, so-called cusp torus (CT) model, which deals with oscillations of fluid in marginally overflowing accretion tori (i.e., tori terminated by cusps). According to preliminary investigations, the model provides better fits of the NS QPO data compared to the relativistic precession (RP) model. It also implies a significantly higher upper limit on the Galactic microquasar BH spins. A short analytic formula has been noticed to well reproduce the model’s predictions on the QPO frequencies in Schwarzschild spacetimes. Here we derive an extended version of this formula that applies to rotating compact objects. We start with the consideration of Kerr spacetimes and derive a formula that is not restricted to a particular specific angular momentum distribution of the inner accretion flow, such as a Keplerian or constant one. Finally, we consider Hartle–Thorne spacetimes and include corrections implied by the NS oblateness. For a particular choice of a single parameter, our relation provides frequencies predicted by the CT model. For another value, it provides frequencies predicted by the RP model. We conclude that the formula is well applicable to rotating oblate NSs and both models. We briefly illustrate the application of our simple formula on several NS sources and confirm the expectation that the CT model is compatible with realistic values of the NS mass and provides better fits of data than the RP model.

Error formulae for the energy-dependent cross-spectrum

Abstract I present analytic error formulae for the energy-dependent cross-spectrum and rms spectrum, which are Fourier statistics widely used to probe the rapid X-ray variability observed from accreting compact objects. The new formulae cover the modulus, phase, real and imaginary parts of the cross-spectrum, and are valid for any value of intrinsic coherence between variability in different energy bands. I show that existing error formulae (including that for the phase lag), which are valid for a single cross-spectrum or power spectrum, lead to over-fitting when applied to the energy-dependent cross-spectrum - which consists of cross-spectra between individual energy channels and a common reference band. I also introduce an optimal, unbiased way to define the reference band and an accurate way to calculate the intrinsic coherence between energy bands. I find that the traditional use of the old formulae has likely had a rather benign impact on the literature, but recommend the use of the new formulae in future wherever appropriate, since they are more accurate and are no harder to implement than existing error estimates. A code to implement the new error formulae on observational data is available online.

Chandra, MDM, Swift, and NuSTAR Observations Confirming the SFXT Nature of AX J1949.8+2534

AX J1949.8+2534 is a candidate supergiant fast X-ray transient (SFXT) observed in outburst by INTEGRAL (IGR J19498+2534). We report on the results of six Neil Gehrels Swift-XRT, one Chandra, and one NuSTAR observation of the source. We find evidence of rapid X-ray variability on a few ks timescales. Fortunately, Chandra observed the source in a relatively bright state, allowing us to confidently identify the optical/NIR counterpart of the source. We also obtained an optical spectrum of this counterpart, which shows an H$\alpha$ emission line and He I absorption features. The photometry and spectrum of the source allow us to constrain its distance, $\sim7-8$ kpc, and reddening, $A_V=8.5-9.5$. We find that the star is likely an early B-type Ia supergiant, confirming that AX J1949.8+2534 is indeed an SFXT.

Large-amplitude Rapid X-Ray Variability in the Narrow-line Seyfert 1 Galaxy PG 1404+226

Abstract We present the first results from a detailed analysis of a new, long (∼100 ks) XMM-Newton observation of the narrow-line Seyfert 1 galaxy PG 1404+226, which showed a large-amplitude, rapid X-ray variability by a factor of ∼7 in ∼10 ks with an exponential rise and a sharp fall in the count rate. We investigate the origin of the soft X-ray excess emission and rapid X-ray variability in the source through time-resolved spectroscopy and fractional rms spectral modeling. The strong soft X-ray excess below 1 keV observed in both the time-averaged and time-resolved spectra is described by the intrinsic disk Comptonization model as well as the relativistic reflection model where the emission is intense merely in the inner regions ( ) of an ionized accretion disk. We detected no significant UV variability, while the soft X-ray excess flux varies together with the primary power-law emission (as ), although with a smaller amplitude, as expected in the reflection scenario. The observed X-ray fractional rms spectrum is approximately constant with a drop at ∼0.6 keV and is described by a non-variable emission line component with the observed energy of ∼0.6 keV and two variable spectral components: a more variable primary power-law emission and a less variable soft excess emission. Our results suggest the “lamppost geometry” for the primary X-ray-emitting hot corona, which illuminates the innermost accretion disk due to strong gravity and gives rise to the soft X-ray excess emission.

Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries

Here we study the rapid X-ray variability (using XMM-Newton observations) of three neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and IGR J18245-2452) during their recently proposed very hard spectral state (Parikh et al. 2017). All our systems exhibit a strong to very strong noise component in their power density spectra (rms amplitudes ranging from 34% to 102%) with very low characteristic frequencies (as low as 0.01 Hz). These properties are more extreme than what is commonly observed in the canonical hard state of neutron-star low-mass X-ray binaries observed at X-ray luminosities similar to those we observe from our sources. This suggests that indeed the very hard state is a distinct spectral-timing state from the hard state, although we argue that the variability behaviour of IGR J18245-2452 is very extreme and possibly this source was in a very unusual state. We also compare our results with the rapid X-ray variability of the accreting millisecond X-ray pulsars IGR J00291+5934 and Swift J0911.9-6452 (also using XMM-Newton data) for which previously similar variability phenomena were observed. Although their energy spectra (as observed using the Swift X-ray telescope) were not necessarily as hard (i.e., for Swift J0911.9-6452) as for our other three sources, we conclude that likely both sources were also in very similar state during their XMM-Newton observations. This suggest that different sources that are found in this new state might exhibit different spectral hardness and one has to study both the spectral as well as rapid variability to identify this unusual state.

LOW-MASS ACTIVE GALACTIC NUCLEI WITH RAPID X-RAY VARIABILITY

ABSTRACT We present a detailed study of the optical spectroscopic properties of 12 active galactic nuclei (AGNs) with candidate low-mass black holes (BHs) selected by Kamizasa et al. through rapid X-ray variability. The high-quality, echellette Magellan spectra reveal broad Hα emission in all the sources, allowing us to estimate robust virial BH masses and Eddington ratios for this unique sample. We confirm that the sample contains low-mass BHs accreting at high rates: the median M BH = 1.2 × 106 M ⊙ and median L bol/L Edd = 0.44. The sample follows the M BH–σ * relation, within the considerable scatter typical of pseudobulges, the probable hosts of these low-mass AGNs. Various lines of evidence suggest that ongoing star formation is prevalent in these systems. We propose a new strategy to estimate star formation rates in AGNs hosted by low-mass, low-metallicity galaxies, based on modification of an existing method using the strength of [O ii] λ3727, [O iii] λ5007, and X-rays.

A MULTIWAVELENGTH STUDY OF THE RELATIVISTIC TIDAL DISRUPTION CANDIDATE SWIFT J2058.4+0516 AT LATE TIMES

We report a multiwavelength (X-ray, ultraviolet/optical/infrared, radio) analysis of the relativistic tidal disruption event candidate Sw J2058+05 from 3 months to 3 yr post-discovery in order to study its properties and compare its behavior with that of Sw J1644+57. Our main results are as follows. (1) The long-term X-ray light curve of Sw J2058+05 shows a remarkably similar trend to that of Sw J1644+57. After a prolonged power-law decay, the X-ray flux drops off rapidly by a factor of $\gtrsim 160$ within a span of $\Delta$$t$/$t$ $\le$ 0.95. Associating this sudden decline with the transition from super-Eddington to sub-Eddington accretion, we estimate the black hole mass to be in the range of $10^{4-6}$ M$_{\odot}$. (2) We detect rapid ($\lesssim 500$ s) X-ray variability before the dropoff, suggesting that, even at late times, the X-rays originate from close to the black hole (ruling out a forward-shock origin). (3) We confirm using HST and VLBA astrometry that the location of the source coincides with the galaxy's center to within $\lesssim 400$ pc (in projection). (4) We modeled Sw J2058+05's ultraviolet/optical/infrared spectral energy distribution with a single-temperature blackbody and find that while the radius remains more or less constant at a value of $63.4 \pm 4.5$ AU ($\sim 10^{15}$ cm) at all times during the outburst, the blackbody temperature drops significantly from $\sim$ 30,000 K at early times to a value of $\sim$ 15,000 K at late times (before the X-ray dropoff). Our results strengthen Sw J2058+05's interpretation as a tidal disruption event similar to Sw J1644+57.

Searching for Black Holes in Space

Although General Relativity had provided the physical basis of black holes, evidence for their existence had to await the Space Era when X-ray observations first directed the attention of astronomers to the unusual binary stars Cygnus X-1 and A0620-00. Subsequently, a number of faint Ariel 5 and Uhuru X-ray sources, mainly at high Galactic latitude, were found to lie close to bright Seyfert galaxies, suggesting the nuclear activity in AGN might also be driven by accretion in the strong gravity of a black hole. Detection of rapid X-ray variability with EXOSAT later confirmed that the accreting object in an AGN is almost certainly a supermassive black hole.

COSMIC-RAY-INDUCED FILAMENTATION INSTABILITY IN COLLISIONLESS SHOCKS

We used unprecedentedly large 2D and 3D hybrid (kinetic ions - fluid electrons) simulations of non-relativistic collisionless strong shocks in order to investigate the effects of self-consistently accelerated ions on the overall shock dynamics. The current driven by suprathermal particles streaming ahead of the shock excites modes transverse to the background magnetic field. The Lorentz force induced by these self-amplified fields tends to excavate tubular, underdense, magnetic-field-depleted cavities that are advected with the fluid and perturb the shock surface, triggering downstream turbulent motions. These motions further amplify the magnetic field, up to factors of 50-100 in knot-like structures. Once downstream, the cavities tend to be filled by hot plasma plumes that compress and stretch the magnetic fields in elongated filaments; this effect is particularly evident if the shock propagates parallel to the background field. Highly-magnetized knots and filaments may provide explanations for the rapid X-ray variability observed in RX J1713.7-3946 and for the regular pattern of X-ray bright stripes detected in Tycho's supernova remnant.

The enigmatic black hole candidate and X-ray transient IGR J17091–3624 in its quiescent state as seen with XMM—Newton

Abstract We report on two short XMM—Newton observations performed in 2006 August and 2007 February during the quiescence state of the enigmatic black hole candidate system IGR J17091−3624. During both observations the source was clearly detected. Although the errors on the estimated fluxes are large, the source appears to be brighter by several tens of per cent during the 2007 February observation compared to the 2006 August observation. During both observations, the 2–10 keV luminosity of the source was close to ∼1033 erg s−1 for an assumed distance of 10 kpc. However, we note that the distance to this source is not well constrained and it has been suggested it might be as far as 35 kpc which would result in an order of magnitude higher luminosities. If the empirically found relation between the orbital period and the quiescence luminosity of black hole transients is also valid for IGR J17091−3624, then we can estimate an orbital period of >100 h (>4 d) for a distance of 10 kpc but it could be as large as tens of days if the source is truly much further away. Such a large orbital period would be similar to GRS 1915+105 which has an orbital period of ∼34 d. Orbital periods this large could possibly be connected to the fact that both sources exhibit the same very violent and extreme rapid X-ray variability which has so far not yet been seen from any other black hole system. Alternatively the orbital period of IGR J17091−3624 might be more in line with the other systems (<100 h), but we happened to have observed the source in an episode of elevated accretion which was significantly higher than its true quiescent accretion rate. In that case, the absence or presence of extreme short-term variability properties as is seen for IGR J17091−3624 and GRS 1915+105 is not related to the orbital periods of these black hole systems.

Relativistic tidal disruption events

In March 2011 Swift detected an extremely luminous and long-lived outburst from the nucleus of an otherwise quiescent, low luminosity (LMC-like) galaxy. Named Swift J1644+57, its combination of high-energy luminosity (1048 ergs s−1 at peak), rapid X-ray variability (factors of >100 on timescales of 100 seconds) and luminous, rising radio emission suggested that we were witnessing the birth of a moderately relativistic jet (Γ ∼ 2 − 5), created when a star is tidally disrupted by the supermassive black hole in the centre of the galaxy. A second event, Swift J2058+0516, detected two months later, with broadly similar properties lends further weight to this interpretation. Taken together this suggests that a fraction of tidal disruption events do indeed create relativistic outflows, demonstrates their detectability, and also implies that low mass galaxies can host massive black holes. Here, I briefly outline the observational properties of these relativistic tidal flares observed last year, and their evolution over the first year since their discovery.

Modelling variability in black hole binaries: linking simulations to observations

Black hole accretion flows show rapid X-ray variability. The Power Spectral Density (PSD) of this is typically fit by a phenomenological model of multiple Lorentzians for both the broad band noise and Quasi-Periodic Oscillations (QPOs). Our previous paper (Ingram & Done 2011) developed the first physical model for the PSD and fit this to observational data. This was based on the same truncated disc/hot inner flow geometry which can explain the correlated properties of the energy spectra. This assumes that the broad band noise is from propagating fluctuations in mass accretion rate within the hot flow, while the QPO is produced by global Lense-Thirring precession of the same hot flow. Here we develop this model, making some significant improvements. Firstly we specify that the viscous frequency (equivalently, surface density) in the hot flow has the same form as that measured from numerical simulations of precessing, tilted accretion flows. Secondly, we refine the statistical techniques which we use to fit the model to the data. We re-analyse the PSD from the 1998 rise to outburst of XTE J1550-564 with our new model in order to assess the impact of these changes. We find that the derived outer radii of the hot flow (set by the inner radius of the truncated disc) are rather similar, changing from ~68-13 Rg throughout the outburst rise. However, the more physical assumptions of our new model also allow us to constrain the scale height of the flow. This decreases as the outer radius of the flow decreases, as expected from the spectral evolution. The spectrum steepens in response to the increased cooling as the as the truncation radius sweeps in, so gas pressure support for the flow decreases. The new model, propfluc, is publically available within the xspec spectral fitting package.

The rapid X-ray variability of NGC 4051

We present an analysis of the high-frequency X-ray variability of NGC 4051 (MBH∼ 1.7 × 106 M⊙) based on a series of XMM–Newton observations taken in 2009 with a total exposure of ∼570 ks (EPIC pn). These data reveal the form of the power spectrum over frequencies from 10−4 Hz, below the previously detected power spectral break, to ≳10−2 Hz, above the frequency of the innermost stable circular orbit (ISCO) around the black hole (νISCO∼ 10−3–10−2 Hz, depending on the black hole spin parameter j). This is equivalent to probing frequencies of ≳1 kHz in a stellar mass (MBH∼ 10 M⊙) black hole binary system. The power spectrum is a featureless power law over the region of the expected ISCO frequency, suggesting no strong enhancement or change in the variability at the fastest orbital period in the system. Despite the huge amplitude of the flux variations between the observations (peak-to-peak factor of ≳50), the power spectrum appears to be stationary in shape and varies in amplitude at all observed frequencies following the previously established linear rms–flux relation. The rms–flux relation is offset in flux by a small and energy-dependent amount. The simplest interpretation of the offset is in terms of a very soft spectral component that is practically constant (compared to the primary source of variability). One possible origin for this emission is a circumnuclear shock energized by a radiatively driven outflow from the central regions and emitting via inverse-Compton scattering of the central engine’s optical–UV continuum (as inferred from a separate analysis of the energy spectrum). A comparison with the power spectrum of a long XMM–Newton observation taken in 2001 gives only weak evidence for non-stationarity in power spectral shape or amplitude. Despite being among the most precisely estimated power spectra for any active galaxy, we find no strong evidence for quasi-periodic oscillations (QPOs) and determine an upper limit on the strength of a plausible QPO of ≲2 per cent rms in the 3 × 10−3 -0.1 Hz range and ∼5–10 per cent in the 10−4 -3 × 10−3 Hz range. We compare these results to the known properties of accreting stellar mass black holes in X-ray binaries, with the further aim of developing a ‘black hole unification’ scheme.

The physics of disk winds, jets, and X-ray variability in GRS 1915+105

AbstractWe present new insights about accretion and ejection physics based on joint RXTE/Chandra HETGS studies of rapid X-ray variability in GRS 1915+105. For the first time, with fast phase-resolved spectroscopy of the ρ state, we are able to show that changes in the broadband X-ray spectrum (RXTE) on timescales of seconds are associated with measurable changes in absorption lines (Chandra HETGS) from the accretion disk wind. Additionally, we make a direct detection of material evaporating from the radiation-pressure-dominated inner disk. Our X-ray data thus reveal the black hole as it ejects a portion of the inner accretion flow and then drives a wind from the outer disk, all in a bizarre cycle that lasts fewer than 60 seconds but can repeat for weeks. We find that the accretion disk wind may be sufficiently massive to play an active role in GRS 1915+105, not only in quenching the jet on long timescales, but also in possibly producing or facilitating transitions between classes of X-ray variability.

A VARIABLE BLACK HOLE X-RAY SOURCE IN AN NGC 1399 GLOBULAR CLUSTER

We have discovered an accreting black hole (BH) in a spectroscopically confirmed globular cluster (GC) in NGC 1399 through monitoring of its X-ray activity. The source, with a peak luminosity of L_x=2x10^39 ergs/s, reveals an order of magnitude change in the count rate within ~10 ks in a Chandra observation. The BH resides in a metal-rich [Fe/H]~0.2 globular cluster. After RZ2109 in NGC 4472 this is only the second black-hole X-ray source in a GC confirmed via rapid X-ray variability. Unlike RZ2109, the X-ray spectrum of this BH source did not change during the period of rapid variability. In addition to the short-term variability the source also exhibits long-term variability. After being bright for at least a decade since 1993 within a span of 2 years it became progressively fainter, and eventually undetectable, or marginally detectable, in deep Chandra and XMM observations. The source also became harder as it faded. The characteristics of the long term variability in itself provide sufficient evidence to identify the source as a BH. The long term decline in the luminosity of this object was likely not recognized in previous studies because the rapid variability within the bright epoch suppressed the average luminosity in that integration. The hardening of the spectrum accompanying the fading would also make this black hole source indistinguishable from an accreting neutron star in some epochs. Therefore some low mass X-ray binaries identified as NS accretors in snapshot studies of nearby galaxies may also be BHs. Thus the discovery of the second confirmed BH in an extragalactic GC through rapid variability at the very least suggests that accreting BHs in GCs are not exceedingly rare occurences.