Galactic Microquasar Research Articles

Minimalist model of the W50/SS433 extended X-ray jet: Anisotropic wind with recollimation shocks

W50 is a radio nebula around the hyperaccreting Galactic microquasar SS433. We focus on one peculiar feature of W50, that is, on a pair of so-called extended X-ray jets (EXJs). These jets have a size of ∼20 pc and a sharp inner boundary, and their spectra are well represented by a featureless X-ray continuum. We argue that EXJs could be an outcome of a powerful anisotropic wind produced by a supercritical accretion disk. In the simplest version of this model, the wind itself consists of two components. The first component is a nearly isotropic outflow that subtends most of the solid angle as seen from the compact source and creates the quasi-spherical part of the W50 nebula. The second component is a more collimated wind that is aligned with the rotation axis of the binary system (polar wind). The isotropic outflow passes through the termination shock, and its increased thermal pressure creates a sequence of recollimation shocks in the polar wind, giving it the appearance of an extended X-ray structure. In this model, the EXJ continuum spectrum is due to synchrotron emission of electrons that are accelerated at the shocks that arise in the polar wind. At variance with many other studies, the EXJ structures in this model are not directly related to the highly collimated and precessing 0.26 c baryonic jets. Instead, the EXJ and the ears of W50 are produced by the part of the wind whose Eddington-level kinetic luminosity is confined to a half-opening angle of 5–10 degrees. This is not necessarily a recollimated version of the 0.26 c jets.

The radio monitoring of the microquasar GRS 1915+105 with FAST

We report radio observations of the Galactic microquasar GRS 1915+105 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in the band of 1.0−1.5 GHz. The observations were performed in 2020 with a total duration of ∼ 3.5 hours. In this work, we developed the pipeline to perform the calibration of the radio flux density and polarization for the persistent emitting radio source GRS 1915+105. The flux densities of the source during each observation are relatively stable, with the maximum fluctuation in flux density less than 40 mJy in one observation. And the linear polarization is between 20-30%, the polarization position angle is around 89.5∘, and the circular polarization is small and likely to be slightly left polarized. With the linear fitting of the polarization angles, we get a flat rotation measure of ∼ -0.5 rad⋅m−2. Meanwhile, the derived spectral index is near a constant for each observation of α∼−(0.4−0.3), indicating an optically thick emission source. The small variations of flux densities and flat rotation measure imply that GRS 1915+105 is likely in a plateau state during the FAST observations.

A NuSTAR view of SS433

Context. SS433 is a Galactic microquasar with powerful outflows (double jet, accretion disk and winds) with a well-known orbital, precessional, and nutational period. Aims. In this work we characterise different outflow parameters throughout the precessional cycle of the system. Methods. We analysed ten NuSTAR (3–70 keV) observations of ∼30 ks that span ∼1.5 precessional cycles. We extracted averaged spectra and modelled them using a combination of a double thermal jet model (bjet) and pure neutral and relativistic reflection (xillverCp and relxilllpCp) over an accretion disk. Results. We find an average jet bulk velocity of β = v/c ∼ 0.29 with an opening angle of ≲6 deg. Eastern jet kinetic power ranges from 1 to 1039 erg s−1, with base ‘coronal’ temperatures To ranging from between 14 and 18 keV. Nickel-to-iron abundances remain constant at ∼9 (within 1σ). The western to eastern jet-flux ratio becomes ∼1 on intermediate phases, which is about 35% of the total precessional orbit. The 3–70 keV total unabsorbed luminosity of the jet and disk ranges from 2 to 20 × 1037 erg s−1, with the disk reflection component mainly contributing to the hard 20–30 keV excess and the stationary 6.7 keV ionised Fe line complex. Conclusions. At low opening angles Θ, we find that the jet expands sideways following an adiabatic expansion of a gas with temperature To. Finally, the central source and lower parts of the jet could be hidden by an optically thick region of τ > 0.1 and size R ∼ NH/ne0 ∼ 1.5 × 109 cm ∼ 1700 rg for MBH = 3 M⊙.

Highly collimated microquasar jets as efficient cosmic-ray sources

Context. Supernova remnants (SNRs) are currently believed to be the main sites of origin for Galactic cosmic rays. This scenario, however, fails to explain some of the features observed in the cosmic-ray spectrum. Microquasars have been proposed as additional candidates, as their non-thermal emission indicates the existence of efficient particle acceleration mechanisms in their jets. Only a few initial attempts have been made so far to quantify the contribution of microquasars to the Galactic cosmic-ray population. A promising scenario envisages the production of relativistic neutrons in the jets that decay outside the system, injecting relativistic protons to the surroundings. The first investigations of this scenario suggest that microquasars might stand as a fair alternative to cosmic-ray sources. Aims. We aim to assess the role played by the degree of collimation of the jet on the cosmic-ray energetics in the neutron-carrier scenario, as well as the location and size of the emission region and the interactions of protons with photon fields. Our goal is to explain the Galactic component of the observed proton cosmic-ray spectrum at energies higher than ∼10 GeV and to relate the aforementioned jet properties with the power and spectral index of the produced cosmic rays. Methods. Here, we improve upon previous analytical models of relativistic particle transport in microquasar jets by including prescriptions for the jet geometry and convection within it. We introduced the neutron component through catastrophic terms that couple the proton and neutron transport equations, then we computed the escape and decay of these neutrons. Finally, we followed the propagation of the decay products and obtained the proton cosmic-ray spectrum once the particles reached the interstellar medium. Results. We find that collimated jets, with compact acceleration regions close to the jet base, are very efficient sources that could deliver a fraction of up to ∼0.01 of their relativistic proton luminosity into cosmic rays. Collimation is the most significant feature regarding efficiency: a well-collimated jet might be four orders of magnitude more efficient than a poorly collimated one. These sources produce a steep spectral index of ∼2.3 at energies up to ∼10 TeV. Conclusions. Single microquasars may rival SNRs with respect to the power injected to the interstellar medium through cosmic rays. The main advantage of the former is the production of a steeper spectrum than the latter that is closer to what has been observed. The predictions of our model may be used to infer the total contribution of the population of Galactic microquasars to the cosmic ray population and, therefore, to quantitatively assess their significance as cosmic-ray sources.

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.

Modeling Particle Transport in Astrophysical Outflows and Simulations of Associated Emissions from Hadronic Microquasar Jets

In this work, after improving the formulation of the model on particle transport within astrophysical plasma outflows and constructing the appropriate algorithms, we test the reliability and effectiveness of our method through numerical simulations on well-studied galactic microquasars as the SS 433 and the Cyg X-1 systems. Then, we concentrate on predictions of the associated emissions, focusing on detectable high-energy neutrinos and γ -rays originated from the extragalactic M33 X-7 system, which is an X-ray binary discovered in 2006, located in the neighboring galaxy Messier 33, and has not yet been modeled in detail. The particle and radiation energy distributions, produced from magnetized hadronic astrophysical jets in the context of our method, are assumed to originate from decay and scattering processes taking place among the secondary particles created when hot (relativistic) protons of the jet scatter on thermal (cold) ones (p-p interaction mechanism inside the jet). These distributions are computed by solving the system of coupled integrodifferential transport equations of multiparticle processes (reactions chain) following the inelastic proton-proton (p-p) collisions. For the detection of such high-energy neutrinos as well as multiwavelength (radio, X-ray, and gamma-ray) emissions, extremely sensitive space telescopes and other γ -ray and neutrino detection instruments are in operation or have been designed like the CTA, IceCube, ANTARES, KM3NeT, and IceCube-Gen-2.

Accretion Flow Properties of GRS 1915+105 During Its θ Class Using AstroSat Data

Abstract The Galactic microquasar GRS 1915+105 shows rich variability that is categorized into different classes. In this paper, we report the temporal and spectral analysis of GRS 1915+105 to study the properties of the accretion flow when the light curve shows θ class variability. For this purpose, we use the Large Area X-ray Proportional Counter data from the Target of Opportunity observations of India’s first multiwavelength astronomy satellite AstroSat. The θ class is marked by the recurrent appearance of U-shaped regions in the light curve, where the photon count rate first decreases rapidly and then increases slowly. For our analysis, we use U-shaped regions of the first two orbits (02345 and 02346) on 2016 March 4. In both cases, the dynamic power-density spectra (PDS) showed significant power at around 4–5 Hz, suggesting the presence of a low-frequency quasi-periodic oscillation (QPO) around that frequency interval. The QPO frequency is found to increase with time when the energy flux is also enhanced. From the evolution of the spectra, we determine the evolution of the accretion flow parameters in these two observations. Fitting the spectra with the transonic flow solution-based two-component advective flow (TCAF) model in the 4–25 keV energy band shows that the Keplerian disk accretion rate increases with the increase in radiation intensity, while the location of the centrifugal pressure-driven shock front decreases. In both these data, a gradual increment of power-law photon index with intensity is observed, suggesting the progressive softening of the source.

High energy neutrino and gamma-ray emissions from the jets of M33 X-7 microquasar

In this work, after testing the reliability of our algorithms through numerical simulations on the well-studied SS 433 Galactic microquasar, we focus on neutrino and γ-ray emissions from the extragalactic M33 X-7 system. This is a recently discovered X-ray binary system located in the neighbouring galaxy Messier 33 which has not yet been modelled in detail. The neutrino and γ-ray energy spectra, produced from the magnetized astrophysical jet of M33 X-7, in the context of our method are assumed to originate from the decay (and scattering) processes taking place among the secondary particles produced assuming that, first, hot (relativistic) protons of the jet scatter on thermal ones (p-p interaction mechanism).

Models of high-frequency quasi-periodic oscillations and black hole spin estimates in Galactic microquasars

We explore the influence of nongeodesic pressure forces present in an accretion disc on the frequencies of its axisymmetric and nonaxisymmetric epicyclic oscillation modes. We discuss its implications for models of high-frequency quasi-periodic oscillations (QPOs), which have been observed in the X-ray flux of accreting black holes (BHs) in the three Galactic microquasars, GRS 1915+105, GRO J1655−40, and XTE J1550−564. We focus on previously considered QPO models that deal with low-azimuthal-number epicyclic modes, |m| ≤ 2, and outline the consequences for the estimations of BH spin, a ∈ [0, 1]. For four out of six examined models, we find only small, rather insignificant changes compared to the geodesic case. For the other two models, on the other hand, there is a significant increase of the estimated upper limit on the spin. Regarding the falsifiability of the QPO models, we find that one particular model from the examined set is incompatible with the data. If the spectral spin estimates for the microquasars that point to a > 0.65 were fully confirmed, two more QPO models would be ruled out. Moreover, if two very different values of the spin, such as a ≈ 0.65 in GRO J1655−40 and a ≈ 1 in GRS 1915+105, were confirmed, all the models except one would remain unsupported by our results. Finally, we discuss the implications for a model that was recently proposed in the context of neutron star (NS) QPOs as a disc-oscillation-based modification of the relativistic precession model. This model provides overall better fits of the NS data and predicts more realistic values of the NS mass compared to the relativistic precession model. We conclude that it also implies a significantly higher upper limit on the microquasar’s BH spin (a ∼ 0.75 vs. a ∼ 0.55).

Multiwavelength Emission from Galactic Jets: The Case of the Microquasar SS433

Abstract SS433 is a Galactic microquasar with powerful jets, where very-high-energy particles are produced. We study particle acceleration in the jets of SS433 through the use of recent multiwavelength data from radio to TeV gamma-ray. We first present a general framework for the particle acceleration, cooling, and transport in relativistic jets. We then apply this to two X-ray knots in the jets of SS433, focusing on leptonic emission. Our detailed treatment of particle transport and evolution produces substantially different predictions from previous papers. For both regions, our model can account for the multiwavelength data except for the GeV data. This suggests that GeV emission is mostly from different regions and/or mechanisms. We find that the acceleration process should be efficient, which could be realized by diffusive shock acceleration close to the Bohm limit. Provided that protons are accelerated at the same efficiency as electrons, our results imply that SS433 is a PeVatron, i.e., can accelerate protons beyond a PeV. Future hard X-ray and MeV gamma-ray observations can critically test our models by detecting the spectral turnover or cutoff.

Identification of QPO Frequency of GRS 1915+105 as the Relativistic Dynamic Frequency of a Truncated Accretion Disk

Abstract We have analyzed AstroSat observations of the galactic microquasar system GRS 1915+105, when the system exhibited C-type quasi-periodic oscillations (QPOs) in the frequency range of 3.4–5.4 Hz. The broadband spectra (1–50 keV) obtained simultaneously from the Large Area X-ray Proportional Counter and Soft X-ray Telescope can be well described by a dominant relativistic truncated accretion disk along with thermal Comptonization and reflection. We find that while the QPO frequency depends on the inner radii with a large scatter, a much tighter correlation is obtained when both the inner radii and accretion rate of the disk are taken into account. In fact, the frequency varies just as the dynamic frequency (i.e., the inverse of the sound crossing time) does as predicted decades ago by the relativistic standard accretion disk theory for a black hole with a spin parameter of ∼0.9. We show that this identification has been possible due to the simultaneous broadband spectral coverage with temporal information as obtained from AstroSat.

Diagnostics of Parameters for the X-ray Jets of SS 433 from High-Resolution Chandra Spectroscopy

The X-ray spectrum of the Galactic microquasar SS 433 contains a rich set of emission lines of highly ionized atoms of heavy elements whose significant Doppler shift leaves no doubt that they are produced in collimated relativistic jets of outflowing material. We have performed a systematic analysis of the high-resolution X-ray spectra obtained by the Chandra observatory to determine the parameters of the jets within the multitemperature model of their emission that self-consistently predicts the source's line and continuum spectrum. The spectrum of SS 433 at energies below 3 keV is shown to be statistically satisfactorily described by the jet emission model, while the introduction of an additional hard component is required above 3 keV. We summarize the jet parameters (bulk velocity, opening angle, kinetic luminosity, base temperature, and relative elemental abundances) derived by fitting the data below 3 keV and describe the revealed degeneracies and systematic effects due to the presence of an additional component. Using the derived parameters, we show that the hard component is compatible with the emission from the hot (up to 40 keV) extension of the visible part of the jets moderately absorbed ($N_H \sim 2 \times 10^{23}$ cm$^{-2}$) in the cold-wind material. The combined X-ray emission model constructed in this way allows the broadband spectrum of SS 433 to be described self-consistently.

Mass ratio in SS433 revisited

We revisit the determination of binary mass ratio in the Galactic microquasar SS433 based on recent GRAVITY VLTI measurements of mass and angular momentum outflow through a circumbinary disc. The new observations combined with the constancy of the binary orbital period over $\sim 30$ yrs confirm that the mass ratio in SS433 is $q=M_\mathrm{x}/M_\mathrm{v}\gtrsim 0.6$. For the assumed optical star mass $M_\mathrm{v}$ ranging from $\sim 8$ to 15 $M_\odot$ such a mass ratio suggests a low limit of the compact object mass of $M_\mathrm{x}\sim 5-9 M_\odot$, placing the compact object in SS433 as a stellar-mass black hole.

Non‐geodesic corrections to mass‐spin estimates for Galactic microquasars implied by quasiperiodic oscillation models

We study frequencies of axisymmetric and non‐axisymmetric epicyclic modes of accretion disk oscillations and explore the influence of pressure forces present in the disk. We utilize both approximate analytic methods and methods dealing with exact numerical solutions. Our investigation implies consequences for estimations of black hole spin in the three Galactic microquasars, namely GRS 1915+105, GRO J1655‐40, and XTE J1550‐564, which have been carried out based on several models of 3:2 high‐frequency quasiperiodic oscillations (QPOs). In the particular case of 3:2 epicyclic resonance model, the presence of pressure forces affects the predicted QPO frequencies only slightly when a < 0.9. On the contrary, when a > 0.9, the influence of pressure forces is nonnegligible. Our findings indicate that for several models this influence can be quite significant even for low values of spin.

The future of the high energy cosmic ray detection: HERD

The High Energy cosmic-Radiation Detection (HERD) facility will be one of the space astronomy payloads on board the future Chinese space station. The ambitious aim of HERD is the direct detection of cosmic rays towards the “knee” region (~ 1 PeV), with a detector able to measure electrons, photons and nuclei with an excellent energy resolution (1% for electrons and photons at 200 GeV and 20% for nuclei at 100 GeV - PeV), an acceptance 10 times the one of present generation missions (~ 1 m2 sr), and long life-time (> 10 years). The primary objectives of HERD are the indirect search for dark matter particles and the precise measurement of energy distribution and composition of cosmic rays from 30 GeV up to a few PeV, determining the origin of the “knee” structure of the spectrum. Furthermore, HERD will monitor the high energy gamma-ray sky from 500 MeV, observing gamma-ray bursts, active galactic nuclei, galactic microquasars, etc. HERD will be composed of a homogeneous calorimeter, surrounded by a particle tracker and a plastic scintillator detector. Two possible trackers are under study: a 5-side tracker made of silicon strip detectors and a 4-side scintillating fiber tracker with a silicon strip top tracker. The total volume of HERD will be (2.3 × 2.3 × 2.6) m3 with a weight of about 4 t. The HERD design, perspectives, expected performances in terms of energy sensitivity and acceptance will be presented in this contribution.

Study of Timing Evolution from Nonvariable to Structured Large-amplitude Variability Transition in GRS 1915 + 105 Using AstroSat

Abstract In this work, we present a ∼90 ks continuous monitoring of the Galactic microquasar GRS 1915 + 105 with AstroSat when the source undergoes a major transition from a nonvariable, χ class (similar to radio-quiet χ class) to a structured, large-amplitude, periodic heartbeat state (similar to ρ class). We show that such a transition takes place via an intermediate state when the large-amplitude, irregular variability of the order of hundreds of seconds in the soft X-ray band turned into 100–150 s regular, structured, nearly periodic flares. The properties of strong low-frequency (LF) quasi-periodic oscillation (QPO) in the frequency range 3–5 Hz also evolve marginally during these variability transitions. We also study time-lag and rms spectra at the QPO and harmonic component and the dynamic power spectra. We note a few important differences between the heartbeat state and the ρ class. Interestingly, the time-averaged LF QPO properties in the hard X-ray band are relatively stable in three states when compared to the significant evolution observed in the slow variability properties at millihertz frequencies. Such relative stability of LF QPOs implies that the inner disk-corona coupled accretion flow, which determines the LF QPO properties, may be uninterrupted by the launch of long, large-amplitude flares.

SS433's Jet Trace from ALMA Imaging and Global Jet Watch Spectroscopy: Evidence for Post-launch Particle Acceleration

Abstract We present a comparison of Doppler-shifted Hα line emission observed by the Global Jet Watch from freshly launched jet ejecta at the nucleus of the Galactic microquasar SS433 with subsequent Atacama Large Millimeter/submillimeter Array (ALMA) imaging at mm-wavelengths of the same jet ejecta. There is a remarkable similarity between the transversely resolved synchrotron emission and the prediction of the jet trace from optical spectroscopy: this is an a priori prediction and not an a posteriori fit, confirming the ballistic nature of the jet propagation. The mm-wavelength of the ALMA polarimetry is sufficiently short that the Faraday rotation is negligible and therefore that the observed -vector directions are accurately orthogonal to the projected local magnetic field. Close to the nucleus, the -field vectors are perpendicular to the direction of propagation. Further out from the nucleus, the -field vectors that are coincident with the jet instead become parallel to the ridge line; this occurs at a distance where the jet bolides are expected to expand into one another. X-ray variability has also been observed at this location; this has a natural explanation if shocks from the expanding and colliding bolides cause particle acceleration. In regions distinctly separate from the jet ridge line, the fractional polarization approaches the theoretical maximum for synchrotron emission.

SS 433: Two robust determinations fix the mass ratio

Context. The unique Galactic microquasar SS 433 is an X-ray binary with a 13.08 day orbital period. Over some forty years, estimates of the mass of the compact object have spanned the range 1–30 solar masses; from neutron star to massive stellar black hole. Aims. To fix the mass ratio q and hence the mass of the super-Eddington accretor. Methods. We present a new and robust estimate of q derived from data on the circumbinary disk and compare it with a recent determination from the observed constancy of the binary period. Results. These two robust analyses agree to a remarkable extent. They reinforce each other; q is ∼0.7 and the mass of the compact object 15 ± 2 M⊙. Conclusions. The mass is such as to identify the compact object as a massive stellar black hole.

W 50 and SS 433

Context. The Galactic microquasar SS 433 launches oppositely directed jets at speeds approximately a quarter of the speed of light. These appear to have punched through and beyond the supposed supernova remnant shell W 50. The problems with this interpretation are: (i) the precessing jets have somehow been collimated before reaching the shell; (ii) without deceleration, only recently launched jets would have reached no further; and (iii) certain features in the lobes are moving slowly or are stationary. Aims. Hydrodynamic computations have demonstrated that for at least one set of parameters describing the ambient medium, jets that diverge and precess are both decelerated and collimated; the conformation of W 50 could then have been sculpted by the jets of SS 433. However, the parameters adopted for density and pressure in these computations are not consistent with observations of jets at a few years old; nor do they represent conditions within a supernova remnant. Our aim is to investigate whether the computations already performed can be scaled to a realistic W 50. Methods. We find simple and physically based scaling relations. The distance to collimation varies inversely with the square root of the pressure of the ambient medium and the speed with which the head of a collimated jet propagates scales with the square root of the temperature. We extrapolate the results of the hydrodynamic computations to lower densities and pressures. Results. The jets of SS 433, launched into an ambient medium of pressure ~10−9 erg cm−3 and temperature ~108 K, within a supernova remnant, could be responsible for the characteristics of W 50. The precessing jets are collimated within ~10 pc and the head of the resulting cylindrical jet propagates slowly. Conclusions. The problems of relating W 50 to SS 433 may now be solved.

On masses of the components in SS433

A huge optical luminosity of the supercritical accretion disc and powerful stellar wind in the high-mass X-ray binary SS433 make it difficult to reliably estimate the mass ratio of the binary components from spectroscopic observations. We analyse different indirect methods of the mass ratio estimate. We show that with an account of the possible Roche lobe overflow by the optical star, the analysis of X-ray eclipses in the standard and hard X-ray bands suggests the estimate q = Mx/Mv ≳ 0.3. We argue that the double-peak hydrogen Brackett lines in SS433 should form not in the accretion disc but in a circumbinary envelope, suggesting a total mass of |$M_\mathrm{v}+M_\mathrm{x}\gtrsim 40 \, \mathrm{M}_\odot$|⁠. The observed long-term stability of the orbital period in SS433 |$|\dot{P}_b/P_b|\le 1.793\times 10^{-14}$| s−1 over ∼28 yr period is used to place an independent constraint of q ≳ 0.6 in SS433, confirming its being a Galactic microquasar hosting a superaccreting black hole.