#alternatively smbh

[mulchbird]

smlh (shaking my little head)

[orbemnews]

Black gap information: Astronomers already know of the existence of supermassive black holes (SMBHs), which might attain as much as 10 billion lots of the Solar. Nonetheless, new analysis has hinted that black holes might develop even greater, with house consultants giving them the tag stupendously massive black holes or SLABS. SMBHs kind within the centre of a galaxy and develop to their monumental sizes by consuming stars and all different matter which crosses their paths. This could imply there may be an higher restrict to the scale they will attain as there may be solely a lot round them which could be consumed. SLABS, alternatively, might have shaped shortly after the Universe got here into existence by the Massive Bang. Black holes which shaped within the early Universe are often known as primordial black holes. Professor Bernard Carr stated: “We already know that black holes exist over an unlimited vary of lots, with a SMBH of 4 million photo voltaic lots residing on the centre of our personal galaxy. “While there isn’t presently proof for the existence of SLABs, it’s conceivable that they may exist and so they may also reside outdoors galaxies in intergalactic house, with attention-grabbing observational penalties. “Nonetheless, surprisingly, the thought of SLABs has largely been uncared for till now. “We’ve proposed choices for the way these SLABs would possibly kind, and hope that our work will start to encourage discussions amongst the group.” READ MORE: Black gap: What would occur in the event you fell right into a black gap? Regardless of not understanding what precisely darkish matter is, consultants do know that it’s important to the Universe. The likes of NASA consider darkish matter is an invisible power which holds galaxies and clusters collectively, and has been key within the formation of the Universe. The substance additionally provides mass to the galaxies, however a mass which can’t be seen or detected with scientific devices. Prof Carr added: “SLABs themselves couldn’t present the darkish matter. “But when they exist in any respect, it will have necessary implications for the early Universe and would make it believable that lighter primordial black holes would possibly achieve this.“ if(typeof utag_data.ads.fb_pixel!=="undefined"&&utag_data.ads.fb_pixel==!0)!function(f,b,e,v,n,t,s)if(f.fbq)return;n=f.fbq=function()n.callMethod?n.callMethod.apply(n,arguments):n.queue.push(arguments);if(!f._fbq)f._fbq=n;n.push=n;n.loaded=!0;n.version='2.0';n.queue=[];t=b.createElement(e);t.async=!0;t.src=v;s=b.getElementsByTagName(e)[0];s.parentNode.insertBefore(t,s)(window,document,'script','https://connect.facebook.net/en_US/fbevents.js');fbq('init','568781449942811');fbq('track','PageView') Supply hyperlink #Black #hole #news

[sciencespies]

A Radical Hypothesis Says Planet Nine Could Be a Black Hole. Here's How To Find Out

https://sciencespies.com/space/a-radical-hypothesis-says-planet-nine-could-be-a-black-hole-heres-how-to-find-out/

A Radical Hypothesis Says Planet Nine Could Be a Black Hole. Here's How To Find Out

A comet-eating black hole the size of a planet? It’s possible. And if there’s one out there in the distant Solar System, a pair of researchers think they know how to find it.

If they do, we might finally put the Planet Nine issue to rest.

The researchers are Avi Loeb, a Professor of Science at Harvard, and Amir Siraj, a Harvard undergraduate student. The paper outlining their thinking is titled “Searching for Black Holes in the Outer Solar System with LSST”. It’s been accepted by The Astrophysical Journal Letters.

Out beyond the orbit of Neptune, a cluster of Kuiper Belt Objects (KBOs) is shepherded together gravitationally by some body with enough mass to do so. One of the explanations for this is the Planet Nine hypothesis. But finding that planet is a near-impossible task, and its existence remains hypothetical.

Some researchers have put forth other explanations for the cluster of Kuiper Belt Objects with their unusual orbits. A disc of icy material was one explanation. Another study suggested the collective mass of the KBOs themselves was responsible.

But another potential explanation is emerging.

In their new paper, the pair of researchers suggest that a primordial black hole could be lurking in the region.

We’re accustomed to hearing about stellar black holes (BH). They’re the end result of a massive star reaching the end of its life and collapsing gravitationally. We’re also used to hearing about the supermassive black holes (SMBH) that live at the center of galaxies like ours.

These behemoths can be billions of times more massive than the Sun, almost inconceivably massive.

But primordial black holes are much smaller. They, hypothetically, formed after the Big Bang, due to density fluctuations. They formed without a progenitor star of any kind.

Loeb and Siraj aren’t the first to propose a primordial black hole as the cause of the KBOs unusual orbits. But they think they know how to find one if there’s one out there.

Just watch until it feeds on something, like a comet.

“In the vicinity of a black hole, small bodies that approach it will melt as a result of heating from the background accretion of gas from the interstellar medium onto the black hole,” said Siraj in a press release.

“Once they melt, the small bodies are subject to tidal disruption by the black hole, followed by accretion from the tidally disrupted body onto the black hole.”

Loeb added, “Because black holes are intrinsically dark, the radiation that matter emits on its way to the mouth of the black hole is our only way to illuminate this dark environment.”

“We find that if Planet Nine is a [black hole], its existence can be discovered by [Legacy Survey of Space and Time] (LSST) due to brief accretion flares powered by small bodies from the Oort cloud, which would be detected at a rate of at least a few per year,” the authors write in their paper.

But how do you watch for it if you don’t know where it is?

That’s the job of the Vera C. Rubin Observatory, and its LSST.

The LSST is a 10-year mission to map the southern sky repeatedly. It’ll survey the entire southern sky every three nights, with its wide field lens. Its observations will address a whole host of astronomical, astrophysical, and cosmological questions.

But it’ll also be good at spotting transients, like supernovae, potentially hazardous objects, and even the flaring from black holes as they consume comets or other matter.

“LSST has a wide field of view, covering the entire sky again and again, and searching for transient flares,” said Loeb. “Other telescopes are good at pointing at a known target but we do not know exactly where to look for Planet Nine. We only know the broad region in which it may reside.”

Siraj added, “LSST’s ability to survey the sky twice per week is extremely valuable. In addition, its unprecedented depth will allow for the detection of flares resulting from relatively small impactors, which are more frequent than large ones.”

The Vera C. Rubin will have the capability to detect these flares automatically, whereas other ‘scopes have to be told where to look. “This method can detect or rule out trapped planet-mass black holes out to the edge of the Oort cloud, or about a hundred thousand astronomical units,” said Siraj.

Scientists have wondered if primordial black holes are candidates for dark matter. LSST observations have the potential to confirm the existence of primordial black holes, or to rule them out, and that has consequences for the dark matter/black hole issue.

“It could be capable of placing new limits on the fraction of dark matter contained in primordial black holes.”

If there is a black hole out there, it’s massive, but tiny. The only way to detect it is through flares.

“There has been a great deal of speculation concerning alternative explanations for the anomalous orbits observed in the outer solar system,” said Siraj. “One of the ideas put forth was the possibility that Planet Nine could be a grapefruit-sized black hole with a mass of five to ten times that of the Earth.”

We don’t know how often this tiny black hole would flare, but the beauty of the LSST is that it doesn’t need to focus on the area to see the flares. It watches the whole southern hemisphere and will notice everything that goes on.

Multiple observations of flaring will give scientists an idea of its flare rate. Even better, they’ll be able to track its orbit.

“If multiple bursts are observed over the course of a year,” the authors write, “the proper motion of the source can be used to identify the orbital parameters of the BH.”

If it turns out that there is a primordial black hole out there, in our own Solar System, it’ll be a shock to our understanding of things. And it’ll immediately lead to a bunch of questions. And hopefully a mission to go study the region.

“The outskirts of the solar system is our backyard. Finding Planet Nine is like discovering a cousin living in the shed behind your home which you never knew about,” said Loeb.

“It immediately raises questions: why is it there? How did it obtain its properties? Did it shape the solar system history? Are there more like it?”

This article was originally published by Universe Today. Read the original article.

#Space

[arxt1]

Are Delayed Radio Flares Common in Tidal Disruption Events? The Case of the TDE iPTF16fnl. (arXiv:2109.10921v2 [astro-ph.HE] UPDATED)

Radio emission from tidal disruption events (TDEs) originates from an interaction of an outflow with the super-massive black hole (SMBH) circum nuclear material (CNM). In turn, this radio emission can be used to probe properties of both the outflow launched at the event and the CNM. Until recently, radio emission was detected only for a relatively small number of events. While the observed radio emission pointed to either relativistic or sub-relativistic outflows of different nature, it also indicated that the outflow has been launched shortly after the stellar disruption. Recently, however, delayed radio flares, several months and years after stellar disruption, were reported in the case of the TDE ASASSN-15oi. These delayed flares suggest a delay in the launching of outflows and thus may provide new insights into SMBH accretion physics. Here, we present a new radio dataset of another TDE, iPTF16fnl, and discuss the possibility that a delayed radio flare has been observed also in this case, ~ 5 months after optical discovery, suggesting that this phenomenon may be common in TDEs. Unlike ASASSN-15oi, the data for iPTF16fnl is sparse and the delayed radio flare can be explained by several alternative models: among them are a complex varying CNM density structure and a delayed outflow ejection.

from astro-ph.HE updates on arXiv.org https://ift.tt/3i1olo7

[arxt1]

Chandra Observations of Candidate Sub-Parsec Binary Supermassive Black Holes. (arXiv:2001.08870v2 [astro-ph.HE] UPDATED)

We present analysis of Chandra X-ray observations of seven quasars that were identified as candidate sub-parsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey (CRTS) based on apparent periodicity in their optical light curves. Simulations predict close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs, including harder or softer X-ray spectra, ripple-like profiles in the Fe K-$\alpha$ line, and distinct peaks in the spectrum due to the separation of the accretion disk into a circumbinary disk and mini-disks around each SMBH. We obtained Chandra observations to test these models and assess whether these quasars could contain binary SMBHs. We instead find that the quasar spectra are all well fit by simple absorbed power law models, with the rest frame 2-10 keV photon indices, $\Gamma$, and the X-ray-to-optical power slopes, $\alpha_{\rm OX}$, indistinguishable from the larger quasar population. This may indicate that these seven quasars are not truly sub-parsec binary SMBH systems, or it may simply reflect that our sample size was too small to robustly detect any differences. Alternatively, the X-ray spectral changes might only be evident at higher energies than probed by Chandra. Given the available models and current data, no firm conclusions are drawn. These observations will help motivate and direct further work on theoretical models of binary SMBH systems, such as modeling systems with thinner accretion disks and larger binary separations.

from astro-ph.HE updates on arXiv.org https://ift.tt/2tTNgDZ

[arxt1]

Has LIGO Detected Primordial Black Hole Dark Matter? -- Tidal Disruption in Binary Black Hole Formation. (arXiv:2007.14559v1 [astro-ph.HE])

The frequent detection of binary mergers of $\sim 30 M_{\odot}$ black holes (BHs) by the Laser Interferometer Gravitational-Wave Observatory (LIGO) rekindled researchers' interest in primordial BHs (PBHs) being dark matter (DM). In this work, we looked at PBHs distributed as DM with a monochromatic mass of $30 M_{\odot}$ and examined the encounter-capture scenario of binary formation, where the densest central region of DM halo dominates. Thus, we paid special attention to the tidal effect by the supermassive black hole (SMBH) present. In doing so, we discovered a necessary tool called loss zone that complements the usage of loss cone. We found that the tidal effect is not prominent in affecting binary formation, which also turned out insufficient in explaining the totality of LIGO's event rate estimation, especially due to a microlensing event constraining the DM fraction in PBH at the mass of interest from near unity to an order smaller. Meanwhile, early-universe binary formation scenario proves so prevailing that the LIGO signal in turn constrains the PBH fraction below one percent. Thus, people should put more faith in alternative PBH windows and other DM candidates.

from astro-ph.HE updates on arXiv.org https://ift.tt/3jR9SKA

[arxt1]

Cool-Core Cycles and Phoenix. (arXiv:1909.12888v2 [astro-ph.GA] UPDATED)

Recent observations show that the star formation rate (SFR) in the {\it Phoenix} cluster's central galaxy is $\sim 500$ M$_\odot$ yr$^{-1}$. Even though {\it Phoenix} is a massive cluster ($M_{200} \approx 2.0\times 10^{15}$ M$_\odot$; $z\approx 0.6$) such a high central SFR is not expected in a scenario in which feedback from an active galactic nucleus (AGN) maintains the intracluster medium (ICM) in a state of rough thermal balance. It has been argued that either AGN feedback saturates in very massive clusters or the central supermassive black hole (SMBH) is too small to produce enough kinetic feedback and hence is unable to quench the catastrophic cooling. In this work, we present an alternate scenario wherein intense short-lived cooling and star formation phases followed by strong AGN outbursts are part of the AGN feedback loop. Using results from a 3D hydrodynamic simulation of a standard cool-core cluster ($M_{200}\sim 7\times10^{14}$ M$_\odot$; $z=0$), scaled to account for differences in mass and redshift, we argue that {\it Phoenix} is at the end of a cooling phase in which an AGN outburst has begun but has not yet arrested core cooling. This state of high cooling rate and star formation is expected to last for $\lesssim$ 100 Myr in {\it Phoenix}.

from astro-ph.HE updates on arXiv.org https://ift.tt/2nYWXxS

[arxt1]

A Multi-wavelength View of OJ 287 Activity in 2015-2017: Implications of Spectral Changes on Central-engine Models and MeV-GeV Emission Mechanism. (arXiv:2002.02213v1 [astro-ph.HE])

A diverse range of peculiar properties across the domains of observation have made OJ 287 one of the best-explored BL Lac objects on the issues of relativistic jets and accretion physics as well as the strong theory of gravity. We here present a brief compilation of observational results from the literature and inferences/insights from the extensive studies but focus on the interpretation of its $\sim$ 12-yr QPOOs and high energy emission mechanisms. The QPOOs in one model are attributed to the disk-impact related to dynamics of the binary SMBHs while alternative models attribute it to the geometrical effect related to the precession of a single jet or double jets. We discuss implications of the new spectral features reported during the 2015--2017 multi-wavelength high activity of the source -- a break in the NIR-optical spectrum and hardening of the MeV-GeV emission accompanied by a shift in the location of its peak, in the context of the two. The reported NIR-optical break nicely fits the description of a standard accretion disk emission from an SMBH of mass $\sim~10^{10}~M_\odot$ while the time of its first appearance in end-May 2013 (MJD 56439) is in close coincidence with the time of impact predicted by the disk-impact binary SMBH model. This spectral and temporal coincidence with the model parameters of the disk-impact binary SMBH model provides independent evidence in favor of the model over the geometrical models which argue a total central-engine mass in the range of $\rm 10^{7-9}~M_\odot$. On the other hand, the MeV-GeV spectral change is naturally reproduced by the inverse Compton scattering of photons from the broad-line region and is consistent with the detection of broad emission lines during the previous cycles of quasi-periodic outbursts. Combining this with previous SED studies suggests that in OJ 287, MeV-GeV emission results from external Comptonization.

from astro-ph.HE updates on arXiv.org https://ift.tt/2vcId1I

[arxt1]

Chandra Observations of Candidate Sub-Parsec Binary Supermassive Black Holes. (arXiv:2001.08870v1 [astro-ph.HE])

We present analysis of Chandra X-ray observations of seven quasars that were identified as candidate sub-parsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey (CRTS) based on apparent periodicity in their optical light curves. Simulations predict close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs, including harder or softer X-ray spectra, ripple-like profiles in the Fe K-$\alpha$ line, and distinct peaks in the spectrum due to the separation of the accretion disk into a circumbinary disk and mini-disks around each SMBH. We obtained Chandra observations to test these models and assess whether these quasars could contain binary SMBHs. We instead find that the quasar spectra are all well fit by simple absorbed power law models, with the rest frame 2-10 keV photon indices, $\Gamma$, and the X-ray-to-optical power slopes, $\alpha_{\rm OX}$, indistinguishable from the larger quasar population. This may indicate that these seven quasars are not truly sub-parsec binary SMBH systems, or it may simply reflect that our sample size was too small to robustly detect any differences. Alternatively, the X-ray spectral changes might only be evident at higher energies than probed by Chandra. Given the available models and current data, no firm conclusions are drawn. These observations will help motivate and direct further work on theoretical models of binary SMBH systems, such as modeling systems with thinner accretion disks and larger binary separations.

from astro-ph.HE updates on arXiv.org https://ift.tt/2tTNgDZ

[arxt1]

Cool-Core Cycles and Phoenix. (arXiv:1909.12888v1 [astro-ph.GA])

Recent observations show that the star formation rate (SFR) in the {\it Phoenix} cluster is $\sim 500$ M$_\odot$ yr$^{-1}$. Even though {\it Phoenix} is a massive cluster, M$_{200} \approx 1.8\times 10^{15}$ M$_\odot$, such a high SFR is not expected in a scenario in which feedback from an active galactic nuclei (AGN) maintains the intra-cluster medium (ICM) in a state of rough thermal balance. It has been argued that either AGN feedback saturates in massive clusters or the central super massive black hole (SMBH) is small compared to what is needed for efficient kinetic feedback and hence unable to quench the catastrophic cooling. In this work, we present an alternate scenario wherein intense short-lived cooling and star formation phases are part of the AGN feedback loop -- the cool core cooling and heating cycles. Using results from our 3D hydrodynamic simulation of a standard cool-core cluster (M$_{200}\sim 7\times10^{14}$ M$_\odot$), we argue that {\it Phoenix} is in a cooling state in which an AGN outburst has just started and has not yet arrested core cooling. This state of high cooling rate and star formation is expected to last only for $\lesssim$ 100 Myr in {\it Phoenix}.

from astro-ph.HE updates on arXiv.org https://ift.tt/2nYWXxS

[arxt1]

Broad Band X-ray Constraints on the Accreting Black Hole in Quasar 4C 74.26. (arXiv:1909.05861v1 [astro-ph.HE])

X-ray data for quasar 4C 74.26 have previously been modeled with a broad Fe K$\alpha$ emission line and reflection continuum originating in the inner part of the accretion disk around the central supermassive black hole (SMBH), i.e. the strong gravity regime. We modeled broadband X-ray spectra from $Suzaku$ and $NuSTAR$ with MYTORUS, self-consistently accounting for Fe K$\alpha$ line emission, as well as direct and reflected continuum emission, from finite column density matter. A narrow Fe K$\alpha$ emission line originating in an X-ray reprocessor with solar Fe abundance far from the central SMBH is sufficient to produce excellent fits for all spectra. For the first time, we are able to measure the global, out of the line-of-sight column density to be in the range $\sim$$1.5$ to $\sim$$2.9\times10^{24}$ cm$^{-2}$, i.e. in the Compton thick regime, while the line-of-sight column density is Compton thin in all observations. The Fe K$\alpha$ emission line is unresolved in all but one observations. The Compton scattered continuum from distant matter removes the need for relativistic broadening of the Fe K$\alpha$ emission line, which is required for SMBH spin measurements. The resolved line observation can alternatively be modeled with a relativistic model but we do not find evidence for a truncated accretion disk model. We conclude that the X-ray emission in these 4C 74.26 data is unlikely to originate in the inner accretion disk region and thus cannot be used to measure SMBH spin.

from astro-ph.HE updates on arXiv.org https://ift.tt/2O1bC6r

[arxt1]

Can Superconducting Cosmic Strings Piercing Seed Black Holes Generate Supermassive Black Holes in the Early Universe?. (arXiv:1505.01584v5 [astro-ph.CO] UPDATED)

The discovery of a large number of supermassive black holes (SMBH) at redshifts $z > 6$, when the Universe was only 900 million years old, raises the question of how such massive compact objects could form in a cosmologically short time interval. Each of the standard scenarios proposed, involving rapid accretion of seed black holes or black hole mergers, faces severe theoretical difficulties in explaining the short-time formation of supermassive objects. In this work we propose an alternative scenario for the formation of SMBH in the early Universe, in which energy transfer from superconducting cosmic strings piercing small seed black holes is the main physical process leading to rapid mass increase. As a toy model, the accretion rate of a seed black hole pierced by two antipodal strings carrying constant current is considered. Using an effective action approach, which phenomenologically incorporates a large class of superconducting string models, we estimate the minimum current required to form SMBH with masses of order $M = 2 \times 10^{9}M_{\odot}$ by $z = 7.085$. This corresponds to the mass of the central black hole powering the quasar ULAS J112001.48+064124.3 and is taken as a test case scenario for early-epoch SMBH formation. For GUT scale strings, the required fractional increase in the string energy density, due to the presence of the current, is of order $10^{-7}$, so that their existence remains consistent with current observational bounds on the string tension. In addition, we consider an "exotic" scenario, in which an SMBH is generated when a small seed black hole is pierced by a higher-dimensional $F-$string, predicted by string theory. We find that both topological defect strings and fundamental strings are able to carry currents large enough to generate early-epoch SMBH via our proposed mechanism.

from gr-qc updates on arXiv.org http://ift.tt/1dR2Pi1

[arxt1]

Spectral features of tidal-disruption candidates and alternative origins for such transient flares. (arXiv:1612.08093v2 [astro-ph.HE] UPDATED)

UV and optically selected candidates for stellar tidal disruption events (TDE) often exhibit broad spectral features (HeII emission, H$\alpha$ emission, or absorption lines) on a blackbody-like continuum (1e4K<T<1e5K). The lines presumably emit from TDE debris or circumnuclear clouds photoionized by the flare. Line velocities however are much lower than expected from a stellar disruption by supermassive black hole (SMBH), and are somewhat faster than expected for the broad line region (BLR) clouds of a persistently active galactic nucleus (AGN). The distinctive spectral states are not strongly related to observed luminosity and velocity, nor to SMBH mass estimates. We use exhaustive photoionization modelling to map the domain of fluxes and cloud properties that yield (e.g.) a He-overbright state where a large HeII(4686A)/H$\alpha$ line-ratio creates an illusion of helium enrichment. Although observed line ratios occur in a plausible minority of cases, AGN-like illumination can not reproduce the observed equivalent widths. We therefore propose to explain these properties by a light-echo photoionization model: the initial flash of a hot blackbody (detonation) excites BLR clouds, which are then seen superimposed on continuum from a later, expanded, cooled stage of the central luminous source. The implied cloud mass is substellar, which may be inconsistent with a TDE. Given these and other inconsistencies with TDE models (e.g. host-galaxies distribution) we suggest to also consider alternative origins for these nuclear flares, which we briefly discuss (e.g. nuclear supernovae and starved/subluminous AGNs).

from astro-ph.HE updates on arXiv.org http://ift.tt/2ifa09Y

[arxt1]

Can Superconducting Cosmic Strings Piercing Seed Black Holes Generate Supermassive Black Holes in the Early Universe?. (arXiv:1505.01584v4 [astro-ph.CO] UPDATED)

The discovery of a large number of supermassive black holes (SMBH) at redshifts $z > 6$, when the Universe was only 900 million years old, raises the question of how such massive compact objects could form in a cosmologically short time interval. Each of the standard scenarios proposed, involving rapid accretion of seed black holes or black hole mergers, faces severe theoretical difficulties in explaining the short-time formation of supermassive objects. In this work we propose an alternative scenario for the formation of SMBH in the early Universe, in which energy transfer from superconducting cosmic strings piercing small seed black holes is the main physical process leading to rapid mass increase. As a toy model, the accretion rate of a seed black hole pierced by two antipodal strings carrying constant current is considered. Using an effective action approach, which phenomenologically incorporates a large class of superconducting string models, we estimate the minimum current required to form SMBH with masses of order $M = 2 \times 10^{9}M_{\odot}$ by $z = 7.085$. This corresponds to the mass of the central black hole powering the quasar ULAS J112001.48+064124.3 and is taken as a test case scenario for early-epoch SMBH formation. For GUT scale strings, the required fractional increase in the string energy density, due to the presence of the current, is of order $10^{-7}$, so that their existence remains consistent with current observational bounds on the string tension. In addition, we consider an "exotic" scenario, in which an SMBH is generated when a small seed black hole is pierced by a higher-dimensional $F-$string, predicted by string theory. We find that both topological defect strings and fundamental strings are able to carry currents large enough to generate early-epoch SMBH via our proposed mechanism.

from gr-qc updates on arXiv.org http://ift.tt/1dR2Pi1

[arxt1]

Testing the binary hypothesis: pulsar timing constrains on supermassive black hole binary candidates. (arXiv:1703.10611v1 [astro-ph.HE])

The advent of time domain astronomy is revolutionising our understanding of the Universe. Programs such as the Catalina Real-time Transient Survey (CRTS) or the Palomar Transient Factory (PTF) surveyed millions of objects for several years, allowing variability studies on large statistical samples. The inspection of $\approx$ 250k quasars in CRTS resulted in a catalogue of 111 potentially periodic sources, put forward as supermassive black hole binary (SMBHB) candidates. A similar investigation on PTF data yielded 33 candidates from a sample of 33k objects. Working under the SMBHB hypothesis, we compute the implied SMBHB merger rate and we use it to construct the expected gravitational wave background (GWB) at nano-Hz frequencies, probed by pulsar timing arrays (PTAs). After correcting for incompleteness, we find that the implied GWB exceeds the current most stringent PTA upper limits by almost an order of magnitude for the CRTS sample, and by at least a factor of 2-3 for the PTF sample. Eccentricity and/or coupling with the environment can only suppress the signal by a factor of $\approx 2$ in the relevant frequency range and cannot account for the inconsistency. The inferred GWB can be reconciled with the PTA upper limits only if nearly all 111 candidates are false positives and AGN hosting a single SMBH exhibit prominent variability without the need of an SMBH companion. Alternatively, alleviating this severe tension would require that the typical black hole mass has been overestimated by a factor of $\gtrsim 4$, very low typical mass ratios $q=M_2/M_1<0.01$, or else that the loudest gravitational wave sources are preferentially false positives.

from astro-ph.HE updates on arXiv.org http://ift.tt/2n2zyKE