Galactic Black Hole Research Articles

The dark mass signature in the orbit of S2

Context.The Schwarzschild precession of star S2, which orbits the massive black hole at the centre of the Milky Way, has recently been detected with the result of ∼12 arcmin per orbit. The same study also improved the 1σupper bound on a possibly present dark continuous extended mass distribution (e.g. faint stars, stellar remnants, stellar mass black holes, or dark matter) within the orbit of S2 to ∼4000 M⊙. The secular (i.e. net) effect of an extended mass onto a stellar orbit is known as mass precession, and it runs counter to the Schwarzschild precession.Aims.We explore a strategy for how the Schwarzschild and mass precessions can be separated from each other despite their secular interference, by pinpointing their signatures within a single orbit. From these insights, we then seek to assess the prospects for improving the dark mass constraints in the coming years.Methods.We analysed the dependence of the osculating orbital elements and of the observables on true anomaly, and we compared these functions for models with and without extended mass. We then translated the maximum astrometric impacts within one orbit to detection thresholds given hypothetical data of different accuracies. These theoretical investigations were then supported and complemented by an extensive mock-data fitting analysis.Results.We have four main results. 1. While the mass precession almost exclusively impacts the orbit in the apocentre half, the Schwarzschild precession almost exclusively impacts it in the pericentre half, allowing for a clear separation of the effects. 2. Data that are limited to the pericentre half are not sensitive to a dark mass, while data limited to the apocentre half are, but only to a limited extent. 3. A full orbit of data is required to substantially constrain a dark mass. 4. For a full orbit of astrometric and spectroscopic data, the astrometric component in the pericentre halff plays the stronger role in constraining the dark mass than the astrometric data in the apocentre half. Furthermore, we determine the 1σdark mass detection thresholds given different datasets on one full orbit. In particular, with a full orbit of data of 50 microarcsec (VLTI/GRAVITY) and 10 km s−1(VLT/SINFONI) precision, the 1σbound would improve to ∼1000 M⊙, for example.Conclusions.The current upper dark mass bound of ∼4000 M⊙has mainly been obtained from a combination of GRAVITY and VLT/NACO astrometric data, as well as from SINFONI spectroscopic data, where the GRAVITY data were limited to the pericentre half. From our results 3 and 4, we know that all components were thereby crucial, but also that the GRAVITY data were dominant in the astrometric components in constraining the dark mass. From results 1 and 2, we deduce that a future population of the apocentre half with GRAVITY data points will substantially further improve the dark mass sensitivity of the dataset, and we note that at the time of publication, we already entered this regime. In the context of the larger picture, our analysis demonstrates how precession effects that interfere on secular timescales can clearly be distinguished from each other based on their distinct astrometric signatures within a single orbit. The extension of our analysis to the Lense-Thirring precession should thus be of value in order to assess future spin detection prospects for the galactic centre massive black hole.

The spins of the Galactic black holes in MAXI J1535–571 and 4U 1630–472 from Insight-HXMT

ABSTRACT Insight-HXMT is the first Chinese X-ray astronomical mission. With a 1–250 keV bandpass, an energy resolution of 150 eV at 6 keV, and without being subject to pile-up distortions, it is suitable to study bright X-ray sources like Galactic black holes. In this paper, we study Insight-HXMT observations of the X-ray binaries MAXI J1535–571 and 4U 1630–472 during their outbursts in 2017 and 2020, respectively. From the analysis of the reflection features, we are able to infer the black hole spin parameter in the two sources. For MAXI J1535–571, we find a spin parameter very close to 1, a* = 0.9916 ± 0.0012 (90 per cent C.L., statistical error), which is consistent with the previous NuSTAR measurement. For 4U 1630–472, we find a moderately high value of the black hole spin parameter, a* = 0.817 ± 0.014 (90 per cent C.L., statistical error), which is lower than the almost extremal value found with NuSTAR data.

Galactic dynamo seeds and black hole singularities driven by Einstein–Cartan QCD walls

Torsion spin polarised wall solution of Einstein–Cartan equations as torsion defects (Garcia de Andrade, Ann Phys 432 (2021)) where spin is distributed orthogonally to the wall has recently been obtained. Moreover, torsional inflating defect [Class and Quantum Gravity, 16 (1999) 2097] in the form of a dilatonic domain wall has also been found. Our goal in this paper is to obtain thin torsion wall solutions as teleparallel and Riemann–Cartan (RC) spacetime walls, with Einstein–Cartan stress–energy tensor, similar to Kalb–Ramond torsion fields. In thin teleparallel wall, we show that torsion is proportional to a time Dirac delta function which possesses a time singularity at the wall like Big Bang and black hole GR ones. This magnetic field has a torsional contribution of BQCD∼1017G in RC walls, which is suitable for dynamo amplification from a seed field of Bseed∼1012G as Big Bang Nucleosynthesis (BBN). From QCD cosmic time scale of tQCD∼10−6s torsion of the order of J0∼10−11cm−1 is obtained. From a 1 MeV torsion and seed field of Bseed∼10−32G, found by Barrow et al [PRD] in the framework of general relativity (GR) which is suitable to seed galactic dynamo field. In this case a domain wall magnetic field of BDW∼10−28G which is stronger, and more suitable as a galactic dynamo seed, is obtained. Magnetic field seed of 10−14G, quite close to orders of magnitude of O(10−15−10−18G) obtained by Atrayed et al. (EPJC (2018) 78: 1027) on a torsionless GR domain wall, generates a wall (DW) magnetic field of order of BDW∼1018G obtained at 1 kpc scales. These similarities between GR results and teleparallel one were expected due to the fact that the teleparallel theory of gravity considered in this paper is the teleparallel equivalent to GR or (TEGR). Chiral dynamos in dilatonic conformally flat metric are also discussed.

Discovery and Long-term Broadband X-Ray Monitoring of Galactic Black Hole Candidate MAXI J1803–298

Abstract We report the results from the broadband X-ray monitoring of the new Galactic black hole candidate MAXI J1803−298 with MAXI/GSC and Swift/BAT during its outburst. After the discovery on 2021 May 1, the soft X-ray flux below 10 keV rapidly increased for ∼10 days, then gradually decreased over five months. In the brightest phase, the source exhibited the state transition from the low/hard state to the high/soft state via the intermediate state. The broadband X-ray spectrum during the outburst is well described with a disk blackbody plus its thermal or nonthermal Comptonization. Before the transition, the source spectrum is described by a thermal Comptonization component with a photon index of ∼1.7 and an electron temperature of ∼30 keV, while a strong disk blackbody component is observed after the transition. The spectral properties in these periods are consistent with the low/hard state and the high/soft state, respectively. A sudden flux drop with a duration of a few days, unassociated with a significant change in the hardness ratio, was found in the intermediate state. A possible cause of this variation is that the mass accretion rate rapidly increased at the disk transition, which induced a strong Compton-thick outflow and scattered out the X-ray flux. Assuming a nonspinning black hole, we estimate the black hole mass of MAXI J1803−298 to be 5.8 ± 0.4 ( cos i / cos 70 ° ) − 1 / 2 ( D / 8 kpc ) M ⊙ (where i and D are the inclination angle and the distance, respectively) from the inner disk radius obtained in the high/soft state.

Quantum mechanism of extremely high energy processes at neutron star collapse and of quasar luminosity

Using the braid group topological approach to quantum statistics we demonstrate that for strong gravitational field occurring in neutron star merger with mass exceeding the Tolman, Oppenheimer and Volkoff limit the quantum statistics of particles beneath the photon sphere decays, which results in an instant relief of quantum degeneracy pressure in the star. This causes a rapid shrink of the matter to a black hole with quite different quantum collective character of particles. The scheme of neutron Fermi sphere collapse is proposed as the possible isotropic source of short giant gamma-ray bursts. The similar quantum mechanism of collapse of Fermi spheres of electrons and protons in plasma of accretion disc in vicinity of the Schwarzschild horizon of galactic black hole is able to elucidate the giant luminosity of remote quasars. The efficiency of the mass to energy conversion via collapse of the Fermi sphere is ca. 30 %, not reachable for any other known physical process except of the matter-antimatter annihilation.

Constraints on Cosmic Ray Acceleration Capabilities of Black Holes in X-ray Binaries and Active Galactic Nuclei

Rotating black holes (BHs) are likely the largest energy reservoirs in the Universe as predicted by BH thermodynamics, while cosmic rays (CRs) are the most energetic among particles detected on Earth. Magnetic fields surrounding BHs combined with strong gravity effects, thanks to the spacetime symmetries, turn the BHs into powerful accelerators of charged particles. At the same time, in the age of multi-wavelength and multi-messenger astronomy, BHs and their environments have not yet been probed with CR messengers, despite being observed across most of the electromagnetic spectrum, and neutrino and gravitational waves. In this paper, we probe the acceleration capabilities of BHs in 8 galactic X-ray binaries and 25 local active galactic nuclei (AGNs) within 100 Mpc, based on the ultra-efficient regime of the magnetic Penrose process of a BH energy extraction combined with observational data. We find that the maximum energy of the galactic BHs can reach only up to the knee of the CR spectrum, including supermassive BH Sgr A* at the Galactic Center. On the other hand, for supermassive BHs in AGNs, we find that the mean energy of primary CRs is of the order of 1019 eV. It is therefore likely that local supermassive BHs give sufficient contribution to the ankle—a sharp change in the slope of the cosmic ray spectrum around 1018.6 eV energy. We also discuss the energy losses of primary CRs close to the acceleration zones. In the galactic BH cases, it is likely dominated by synchrotron radiation losses.

Is It Possible to See the Breaking Point of General Relativity near the Galactic Center Black Hole? Consideration of Scalaron and Higher-dimensional Gravity

Abstract The Galactic center black hole is a putative laboratory to test general relativity (GR) and constrain its alternatives. f(R) scalaron gravity is an interesting alternative to GR and has tremendous prospects for astrophysics and fundamental physics near the black hole. In this work, we search for breaking points of GR through estimation of pericenter shift of stellar orbits with semimajor axis a = (45–1000) au. The black hole spin is taken as the maximum χ = 0.99, and orbital eccentricity is taken as e = 0.9. We work with theoretical scalaron field amplitude and coupling, predicted by Kalita, and also consider the constraints reported by Hees et al. The scalaron mass is taken in the range (10−22–10−17) eV. It is found that GR suppresses scalaron gravity at all orbital radii for the theoretical values of scalaron field coupling predicted by Kalita. Breaking point arises only for higher scalaron coupling resulting from the Hees et al. observations within a few tens of au to a = 1000 au. We also estimate the pericenter shift with a power-law potential V(r) ∼ 1/r 2 arising in five-dimensional gravity and obtain allowed ranges of the five-dimensional Planck mass through existing bounds on the parameterized post-Newtonian parameters coming from the orbits of S-2, S-38, and S-55. The breaking point for GR arises for a five-dimensional Planck mass of about 104 GeV. Constraint on this parameter, expected from the astrometric capabilities of existing and upcoming large telescopes, is also presented.

The Intrinsic Structure of Sagittarius A* at 1.3 cm and 7 mm

Abstract Sagittarius A* (Sgr A*), the Galactic Center supermassive black hole (SMBH), is one of the best targets in which to resolve the innermost region of an SMBH with very long baseline interferometry (VLBI). In this study, we have carried out observations toward Sgr A* at 1.349 cm (22.223 GHz) and 6.950 mm (43.135 GHz) with the East Asian VLBI Network, as a part of the multiwavelength campaign of the Event Horizon Telescope (EHT) in 2017 April. To mitigate scattering effects, the physically motivated scattering kernel model from Psaltis et al. (2018) and the scattering parameters from Johnson et al. (2018) have been applied. As a result, a single, symmetric Gaussian model well describes the intrinsic structure of Sgr A* at both wavelengths. From closure amplitudes, the major-axis sizes are ∼704 ± 102 μas (axial ratio ∼ 1.19 − 0.19 + 0.24 ) and ∼300 ± 25 μas (axial ratio ∼1.28 ± 0.2) at 1.349 cm and 6.95 mm, respectively. Together with a quasi-simultaneous observation at 3.5 mm (86 GHz) by Issaoun et al. (2019), we show that the intrinsic size scales with observing wavelength as a power law, with an index ∼1.2 ± 0.2. Our results also provide estimates of the size and compact flux density at 1.3 mm, which can be incorporated into the analysis of the EHT observations. In terms of the origin of radio emission, we have compared the intrinsic structures with the accretion flow scenario, especially the radiatively inefficient accretion flow based on the Keplerian shell model. With this, we show that a nonthermal electron population is necessary to reproduce the source sizes.

The Origin of Cosmic Structures Part 3 — Supermassive Black Holes and Galaxy Cluster Evolution

In Part 1 of this work, we showed that our new model of cosmology can account for the origin of all cosmic structures ranging in size from stars up to superclusters. In this model, at the time of nucleosynthesis, an imprint embedded in the vacuum regulated the creation of the protons (and electrons) that later made up the structures. Immediately after nucleosynthesis and for a considerable period afterward, the evolution was completely determined by the expansion of the universe. Gradually, however, gravitational influences became more important until finally, the expansion of the structures-to-be ceased at their zero velocity points. Stars, galaxies, and galaxy clusters all reached their zero velocity points more or less simultaneously at the usually accepted time of the beginning of galaxy formation. From that point onward, the evolution gravitation came to dominate the evolution although the expansion still exerted its influence. In this paper, we examine the subsequent cluster evolution in some detail. We establish the conditions required to prevent a free-fall collapse of the clusters and then show that galaxies with quasar-like active nuclei located within the cluster were the sources of the necessary radiation. We also show that the required galactic supermassive black holes were a consequence of the initial free-fall collapse of all galaxies.

A long-lived compact jet in the black hole X-ray binary candidate AT2019wey

AT2019wey is a transient discovered by the Asteroid Terrestrial-impact Last Alert System survey in December of 2019. Follow-up optical, radio, and X-ray observations led to classification of this source as a Galactic black hole X-ray binary candidate. We carried out one-epoch 6.7 GHz European Very Long Baseline Interferometry Network and two-epoch multi-frequency (1.6, 4.5, 6.7 GHz) Very Long Baseline Array observations within a year after its discovery. These observations reveal a fading and flat-spectrum radio source with no discernible motion. These features suggest the detection of a compact jet. The source appears resolved at milliarcsecond scales, and the source angular size versus frequency trend is consistent with scatter broadening. This allows us to constrain the lower limit of the source distance to 6 kpc if the scattering medium is in a Galactic spiral arm. For a source location at greater than 3 kpc, the estimated upper limit of the peculiar velocity suggests the asymmetric natal kick may have occurred during the black hole formation stage.

Anomalous nature of outbursts of black hole candidate 4U 1630−472

ABSTRACT The Galactic black hole candidate (BHC) 4U 1630−472 has gone through several outbursts (13 to be particular) in the last two and a half decades starting from the RXTE era till date. Like the outbursts of other transient BHCs, the outbursts of this source show variations in duration, peak numbers, highest peak flux, etc. However, unlike other soft X-ray transients, this source showed outbursts of two types, such as normal and super. The normal outbursts of duration ∼100−200 d are observed with an average recurrence or quiescence period of ∼500 d. The super outbursts of duration ∼1.5−2.5 yr, consist of more than one normal outburst and one mega outburst. We make an effort to separate the flux contributions of the normal and mega components from the super outbursts, and try to understand the nature of evolution of both types (normal and mega) of outbursts, based on the quiescent period prior to the outbursts. Archival data of RXTE or ASM from 1996 January to 2011 June, and MAXI or GSC from 2009 August to 2020 July are used for our study. Based on the variations of various properties of both the normal and mega outbursts, we think that this BHC is a special source which may contain two companions. Two companions might be responsible for two types of outbursts.

Accurate X-ray timing in the presence of systematic biases with simulation-based inference

ABSTRACT Because many of our X-ray telescopes are optimized towards observing faint sources, observations of bright sources like X-ray binaries in outburst are often affected by instrumental biases. These effects include dead time and photon pile-up, which can dramatically change the statistical inference of physical parameters from these observations. While dead time is difficult to take into account in a statistically consistent manner, simulating dead-time-affected data is often straightforward. This structure makes the issue of inferring physical properties from dead-time-affected observations fall into a class of problems common across many scientific disciplines. There is a growing number of methods to address them under the name of simulation-based inference (SBI), aided by new developments in density estimation and statistical machine learning. In this paper, we introduce SBI as a principled way to infer variability properties from dead-time-affected light curves. We use sequential neural posterior estimation to estimate the posterior probability for variability properties. We show that this method can recover variability parameters on simulated data even when dead time is variable, and present results of an application of this approach to NuSTAR observations of the galactic black hole X-ray binary GRS 1915+105.

MeerKAT radio detection of the Galactic black hole candidate Swift J1842.5−1124 during its 2020 outburst

ABSTRACT Swift J1842.5−1124 is a transient Galactic black hole X-ray binary candidate, which underwent a new outburst in 2020 May. We performed multi-epoch MeerKAT radio observations under the ThunderKAT large survey programme, coordinated with quasi-simultaneous Swift/XRT X-ray observations during the outburst, which lasted nearly a month. We were able to make the first-ever radio detection of this black hole binary with the highest flux density of 229 ± 31 $\mu$Jy when the source was in the hard state, after non-detection in the radio band in the soft state which occurred immediately after its emergence during the new X-ray outburst. Therefore, its radio and X-ray properties are consistent with the disc-jet coupling picture established in other black hole X-ray binaries. We place the source’s quasi-simultaneous X-ray and radio measurements on the radio/X-ray luminosity correlation plane; two quasi-simultaneous radio/X-ray measurements separated by 11 d were obtained, which span ∼2 dex in the X-ray luminosity. If the source follows the black hole track in the radio/X-ray correlation plane during the outburst, it would lie at a distance beyond ∼5 kpc.

A Reflection Model with a Radial Disk Density Profile

Abstract In this paper we present relxilldgrad_nk, a relativistic reflection model in which the electron density of the accretion disk is allowed to have a radial power-law profile. The ionization parameter also has a nonconstant radial profile and is calculated self-consistently from the electron density and the emissivity. We show the impact of the implementation of the electron density gradient in our model by analyzing a NuSTAR spectrum of the Galactic black hole in EXO 1846–031 during its last outburst in 2019 and a putative future observation of the same source with Athena and eXTP. For the NuSTAR spectrum, we find that the new model provides a better fit, but there is no significant difference in the estimation of the model parameters. For the Athena+eXTP simulation, we find that a model without a disk density profile is unsuitable to test the spacetime metric around the compact object in the sense that modeling uncertainties can incorrectly lead to finding a nonvanishing deformation from the Kerr solution.

First Space-VLBI Observations of Sagittarius A*

Abstract We report results from the first Earth-space VLBI observations of the Galactic Center supermassive black hole, Sgr A*. These observations used the space telescope Spektr-R of the RadioAstron project together with a global network of 20 ground telescopes, observing at a wavelength of 1.35 cm. Spektr-R provided baselines up to 3.9 times the diameter of the Earth, corresponding to an angular resolution of approximately 55 μas and a spatial resolution of 5.5R Sch at the source, where R Sch ≡ 2GM/c 2 is the Schwarzschild radius of Sgr A*. Our short ground baseline measurements ( ≲ 80 Mλ) are consistent with an anisotropic Gaussian image, while our intermediate ground baseline measurements (100–250 Mλ) confirm the presence of persistent image substructure in Sgr A*. Both features are consistent with theoretical expectations for strong scattering in the ionized interstellar medium, which produces Gaussian scatter-broadening on short baselines and refractive substructure on long baselines. We do not detect interferometric fringes on any of the longer ground baselines or on any ground–space baselines. While space-VLBI offers a promising pathway to sharper angular resolution and the measurement of key gravitational signatures in black holes, such as their photon rings, our results demonstrate that space-VLBI studies of Sgr A* will require sensitive observations at submillimeter wavelengths.

F(R)gravity after the detection of the orbital precession of the S2 star around the Galactic Center massive black hole

The GRAVITY Collaboration achieved the remarkable detection of the orbital precession of the S2 star around the Galactic centre supermassive black hole, providing yet another proof of the validity of the General Relativity. The departure from the Schwarzschild precession is encoded in the parameter $f_{\rm SP}$ which multiplies the predicted general relativistic precession. Such a parameter results to be $f_{\rm SP}=1.10\pm0.19$, which is consistent with General Relativity ($f_{\rm SP}=1$) at 1$\sigma$ level. Nevertheless, this parameter may also hide an effect of modified theories of gravity. Thus, we consider the orbital precession due to the Yukawa-like gravitational potential arising in the weak field limit of $f(R)$-gravity, and we use the current bound on the $f_{\rm SP}$ to constrain the strength and the scale length of the Yukawa-like potential. No deviation from GR are revealed at scale of $\lambda<6300$ AU with the strength of the Yukawa potential restricted to $\delta = -0.01_{-0.14}^{+0.61}$.

Gravitational time delay effects by Kerr and Kerr-Newman black holes in strong field limits

We study the time delay between two relativistic images due to strong gravitational lensing of the light rays caused by the Kerr and Kerr-Newman black holes. The trajectories of the light rays are restricted on the equatorial plane. Using the known form of the deflection angle in the strong deflection limit (SDL) allows us to analytically develop the formalism for the travel time of the light from the distant source winding around the black hole several times and reaching the observer. We find that the black hole with higher mass or with spin of the extreme black hole potentially have higher time delay. The effect of the charge of the black hole enhances the time delay between the images lying on the opposite side of the optical axis resulting from the light rays when one light ray is in the direct orbit and the other is in the retrograde orbit. In contrary, when both light rays travel along either direct or retrograde orbits giving the images on the same side of the optical axis, the charge effect reduces the time delay between them. We then examine the time delay observations due to the galactic and supermassive black holes respectively

A Classical Physics Approach for Space Expansion: The Hubble Drift

Accelerating expansion of metric space AEMS is investigated with classical Newtonian mechanics. Relying on earlier positions, the results are analyzed to reveal what could be a new understanding of the theoretical framework of the subject. Notably, it is shown that space is physical; it comprises aggregated waveforms of the chemical elements and shares identical quantization, periodicity and mass-evolution with matter. Three plausible methods are identified for classical investigation of the Hubble effect, all three give same result, Ho= 49.5 km s-1Mpc.&amp;nbsp;AEMS results from coupling of light&amp;rsquo;s 36.9o&amp;nbsp;tangential component (vr=0.75r&amp;omega;)&amp;nbsp;to periodic space, i.e., a component of the vacuum field&amp;rsquo;s e-m radiation couples to logarithmically decreasing distance scales, vr(E)/drE,&amp;nbsp;to create an acceleration relative to space not time. Multiplicity of the Hubble constant aHo&amp;nbsp;is traceable to corresponding multiplicity of universes nested within our universe. Mass ejection from a cosmic quantum envelope is the cosmic equivalence of radioactivity, it signals ageing and eventual disappearance of the host periodic envelope from visibility. Reality is an imperturbable (ideal) Steady-State, observations thought to invalidate this view are hugely misinterpreted, an explosion in or of spacetime marking the beginning of time could not conceivably sustain, over the aeons, an accelerating expansion of metric space; furthermore, the cosmic microwave background is the zero-point energy or vacuum radiation. The active galactic nucleus or black hole is not a singularity, it is a two-way valve that facilitates circulation of mass-energy matrices across the four phases or ref. frames of reality. There is no new creation of space or matter, only continuous recycling in line with NASA&amp;rsquo;s recent observation.

The Observed Mass Distribution of Galactic Black Hole LMXBs Is Biased against Massive Black Holes

Abstract The discovery of gravitational-wave radiation from merging black holes (BHs) also uncovered BHs with masses in the range of ≈20–160 M ⊙. In contrast, the most massive Galactic stellar-mass BH currently known has a mass of ≈21 M ⊙. While low-mass X-ray binaries (LMXBs) will never independently evolve into a binary BH system, and binary evolution effects can play an important role in explaining the different BH masses found through studies of X-ray binaries and gravitational-wave events, (electromagnetic) selection effects may also play a role in this discrepancy. Assuming BH LMXBs originate in the Galactic plane, we show that the spatial distributions of the current samples of confirmed and candidate BH LMXBs are both biased to sources that lie at a large distance from the plane. Specifically, most of the confirmed and candidate BH LMXBs are found at a Galactic height larger than three times the scale height for massive star formation. In addition, the confirmed BH LMXBs are found at larger distances to the Galactic center than the candidate BH LMXBs. Interstellar absorption makes candidate BH LMXBs in the plane and bulge too faint for a dynamical mass measurement using current instrumentation. Given the observed and theoretical evidence for BH natal and/or Blaauw kicks, their relation with BH mass and binary orbital period, and the relation between outburst recurrence time and BH mass, the observational selection effects imply that the current sample of confirmed BH LMXBs is biased against the most massive BHs.

Electric Penrose process: High-energy acceleration of ionized particles by nonrotating weakly charged black hole

In many astrophysical scenarios the charge of the black hole is often neglected due to unrealistically large values of the charge required for the Reissner-Nordstr\"om spacetime metric. However, black holes may possess small electric charge due to various selective accretion mechanisms. In this paper we investigate the effect of a small hypothetical electric charge of a Schwarzschild black hole on the ionization of a freely falling neutral particle and subsequent escape of the ionized particle from the black hole. We show that the energy of ionized particle can grow ultra-high and discuss distinguishing signatures of particle acceleration by weakly charged black holes. We also discuss a possible application of the proposed mechanism as an alternative cosmic ray acceleration scenario. In particular we show that the Galactic centre supermassive black hole is capable to act as a PeVatron of protons. The presented mechanism can serve as a simple toy model of a non-rotating compact object acting as a particle accelerator with a potential astrophysical implementations related to the cosmic ray physics and beyond.