GRS 1758-258: Galactic Microquasar and Black Hole Binary
- GRS 1758-258 is a Galactic black-hole X-ray binary and microquasar characterized by persistent hard X-ray emission and parsec-scale, winged radio jets that draw analogies to extragalactic radio galaxies.
- High-resolution radio and multiwavelength observations reveal dynamic jet evolution, including hotspot shifts, reformation, and distinctive Z-shaped outflows caused by hydrodynamic instabilities and ISM interactions.
- Accretion-state studies demonstrate dim-soft transitions and a dominant hot flow, establishing GRS 1758-258 as a key laboratory for investigating jet-ISM feedback and complex corona dynamics.
Searching arXiv for recent and foundational papers on GRS 1758-258 to ground the encyclopedia entry. GRS 1758−258 is a Galactic black-hole X-ray binary and microquasar toward the Galactic Center direction, long recognized for combining persistent hard X-ray emission with resolved, parsec-scale radio jets and lobes. It is one of the two original “Galactic Center” microquasars, together with 1E 1740.7−2942, and has become a particularly important comparative system because its large-scale outflow morphology links microquasars to extragalactic radio galaxies [(Muñoz-Arjonilla et al., 2010); (Martí et al., 2018)]. Deep radio imaging identified GRS 1758−258 as the first Galactic microquasar with winged radio structures, specifically a Z-type morphology analogous to winged radio galaxies, while multi-epoch radio monitoring demonstrated large-scale evolution on humanly accessible timescales (Martí et al., 2018, Martí et al., 2015). Optical and near-infrared work has constrained the counterpart and donor-star possibilities under conditions of heavy extinction and crowding, while X-ray spectroscopy has established a predominantly hard-state accretion flow punctuated by dim-soft episodes, relativistic reflection signatures, and evidence for a rapidly spinning black hole [(Muñoz-Arjonilla et al., 2010); (Martí et al., 2016); (Hirsch et al., 2019); (Jana et al., 2022)].
1. Source classification and identification
GRS 1758−258 is described as a microquasar: a Galactic X-ray binary with relativistic radio jets and extended radio lobes [(Muñoz-Arjonilla et al., 2010); (Martí et al., 2018)]. It is also treated as a black-hole X-ray binary or black-hole candidate on the basis of its X-ray spectral and timing behavior, its persistent hard-state phenomenology, and its analogy with systems such as Cygnus X-1 (Martí et al., 2015, Jana et al., 2022). In the literature summarized here, it is repeatedly placed at a distance near the Galactic Center, with adopted values of 8 kpc, 8.0 ± 1.0 kpc, or 8.5 kpc depending on the analysis [(Soria et al., 2011); (Tetarenko et al., 2020); (Martí et al., 2018)].
Historically, the source occupied an ambiguous position because its large-scale radio morphology resembles a scaled-down radio galaxy, and an extragalactic origin had not been definitively excluded until improved astrometry, spectroscopy, and causality arguments strengthened the Galactic interpretation [(Muñoz-Arjonilla et al., 2010); (Martí et al., 2015)]. The refined radio core position derived from archival VLA A-configuration data is R.A. (J2000) = $18^{\rm h}\ 01^{\rm m}\ 12\fs40$, Dec. (J2000) = $-25^\circ\ 44′\ 36\farcs3$, with an adopted conservative uncertainty of $0\farcs1$ in each coordinate (Muñoz-Arjonilla et al., 2010). Re-registration of archival optical and near-infrared images using the 2MASS catalogue reduced the field ambiguity to a single plausible counterpart, offset from the radio position by $\Delta \alpha \cos \delta \simeq 0\farcs16$ and $\Delta \delta \simeq 0\farcs14$, values described as clearly consistent with the astrometric errors (Muñoz-Arjonilla et al., 2010).
The Galactic nature was reinforced independently by large-scale jet evolution. Because the northern jet structure of angular size changed substantially over a timescale yr, the causality argument excludes a distant extragalactic interpretation and confines the source to the Milky Way (Martí et al., 2015). This result is methodologically significant because it does not depend on donor-star spectroscopy.
2. Binary system, counterpart, and donor-star constraints
Optical and near-infrared study of GRS 1758−258 is unusually difficult because the field is crowded and heavily reddened. The counterpart is faint, with reported magnitudes , , and $-25^\circ\ 44′\ 36\farcs3$0, and lies near a contaminating star only $-25^\circ\ 44′\ 36\farcs3$1 away that is brighter by about $-25^\circ\ 44′\ 36\farcs3$2 mag in $-25^\circ\ 44′\ 36\farcs3$3 (Martí et al., 2016). Earlier broad-band interpretation favored an irradiated accretion disc dominating the optical and near-infrared output, but later spectroscopy partially constrained the donor by comparing the de-reddened continuum with irradiated-disc models and stellar templates (Martí et al., 2016).
The most direct optical spectroscopic study used the 10.4 m Gran Telescopio Canarias with OSIRIS in long-slit mode on 1 June 2016, employing the R300R grism, $-25^\circ\ 44′\ 36\farcs3$4, a $-25^\circ\ 44′\ 36\farcs3$5 slit, four exposures of 1910 s, and a total exposure of about 2.1 hr (Martí et al., 2016). The spectrum is a highly absorbed, featureless continuum: no intrinsic emission or absorption lines were detected, including no reliable H$-25^\circ\ 44′\ 36\farcs3$6, and a prominent feature near 7600 Å is telluric (Martí et al., 2016). At $-25^\circ\ 44′\ 36\farcs3$7, the flux density is $-25^\circ\ 44′\ 36\farcs3$8 with signal-to-noise ratio $-25^\circ\ 44′\ 36\farcs3$9, corresponding to $0\farcs1$0 (Martí et al., 2016).
The extinction treatment depends on the adopted $0\farcs1$1. One study used $0\farcs1$2 and derived $0\farcs1$3 from $0\farcs1$4 (Martí et al., 2016). Another used $0\farcs1$5 and obtained $0\farcs1$6 mag, $0\farcs1$7, $0\farcs1$8, and $0\farcs1$9 (Muñoz-Arjonilla et al., 2010). Under that extinction correction and assuming 8.5 kpc, the derived magnitudes are $\Delta \alpha \cos \delta \simeq 0\farcs16$0, $\Delta \alpha \cos \delta \simeq 0\farcs16$1, and $\Delta \alpha \cos \delta \simeq 0\farcs16$2, with colors $\Delta \alpha \cos \delta \simeq 0\farcs16$3 and $\Delta \alpha \cos \delta \simeq 0\farcs16$4, inconsistent with any normal stellar spectral type (Muñoz-Arjonilla et al., 2010).
The most explicit donor-star conclusion is tentative rather than definitive: the optical spectroscopy suggests that GRS 1758−258 does not host a giant star companion and that a main-sequence star with mid-A spectral type better agrees with the data (Martí et al., 2016). This suggests that the system may be an intermediate-mass X-ray binary rather than a giant-donor low-mass system. A plausible implication is that any orbital period would be significantly shorter than the previously proposed $\Delta \alpha \cos \delta \simeq 0\farcs16$5 d if Roche-lobe overflow is operating (Martí et al., 2016).
3. Large-scale radio jets, lobes, and winged morphology
GRS 1758−258 has a compact radio core and a bipolar large-scale radio source with northern and southern lobes. Earlier work identified double-sided jets on arcminute scales terminating in radio lobes, with the large-scale appearance resembling a Fanaroff–Riley II radio galaxy (Muñoz-Arjonilla et al., 2010). The full source extent is of order a few arcminutes; one deep study gives the main jet flow as about $\Delta \alpha \cos \delta \simeq 0\farcs16$6, equivalent to a few parsecs at a distance of 8.5 kpc (Martí et al., 2018). Another reports projected angular separations from the core of 1.20 arcmin north and 1.57 arcmin south, corresponding, for distance 8.5 kpc and inclination $\Delta \alpha \cos \delta \simeq 0\farcs16$7, to deprojected lengths of 3.36 pc and 4.41 pc (Mariani et al., 12 Sep 2025).
The decisive morphological advance came from deep 6 cm VLA imaging. New Jansky VLA C-configuration observations obtained during 2016 March 04–22 at about 5 GHz were combined with archival data from 1992, 1993, 1997, and 2008 to produce the deepest radio image of the microquasar to that date, with noise level 4.3 μJy and angular resolution about 10 arc-second (Martí et al., 2018). That map revealed extended low-surface-brightness radio structures attached to the terminal lobes, described as winged features with a Z-type morphology strongly resembling the secondary lobes of winged radio galaxies (Martí et al., 2018).
The wings are secondary, off-axis structures extending away from the principal jet axis, and their lengths are comparable to the full extension of the main jet flow, $\Delta \alpha \cos \delta \simeq 0\farcs16$8 (Martí et al., 2018). The source is therefore presented as the first known winged microquasar and the first Galactic microquasar in which winged radio structures have been detected (Martí et al., 2018). This matters because winged or X/Z-shaped morphologies had previously been associated mainly with extragalactic radio galaxies, especially winged radio galaxies. The detection extends the established microquasar–radio-galaxy analogy from the central jet-launching region to the large-scale jet/ambient-medium interaction zone (Martí et al., 2018).
Later MeerKAT imaging in L-band resolved the source into a core plus five extended regions: region A, a bright spot in the northern lobe; region B, a curved tail in the northern lobe; region C, a bow-shock structure at the end of the northern interaction zone; region D, a brighter compact spot in the southern lobe; and region E, a bow-shock structure at the end of the southern interaction zone (Mariani et al., 12 Sep 2025). The bipolar axis is oriented at about $\Delta \alpha \cos \delta \simeq 0\farcs16$9 with respect to the north–south direction on the sky, and the source shows bow-shaped terminal structures on both sides (Mariani et al., 12 Sep 2025). The northern bow-shaped structure extends more to the right and the southern more to the left, producing the global Z shape (Mariani et al., 12 Sep 2025).
Spectral-index measurements distinguish the emission mechanisms within this morphology. At 1.28 GHz, the core has $\Delta \delta \simeq 0\farcs14$0 and $\Delta \delta \simeq 0\farcs14$1, consistent with a flat spectrum (Mariani et al., 12 Sep 2025). In the northern lobe, regions A and B are steep-spectrum, with $\Delta \delta \simeq 0\farcs14$2 and $\Delta \delta \simeq 0\farcs14$3, consistent with synchrotron emission, while the southern lobe regions D and E are consistent within uncertainties with flat-spectrum thermal bremsstrahlung (Mariani et al., 12 Sep 2025). This north–south difference is central to later jet–ISM interpretations.
4. Real-time jet evolution and dynamical behavior
GRS 1758−258 is among the few jet sources for which large-scale radio evolution can be followed over years rather than geological timescales. A systematic reanalysis of VLA 6 cm C-configuration observations from 1992, 1997, 2001, and 2008, recalibrated in AIPS and imaged with a common restoring beam of $\Delta \delta \simeq 0\farcs14$4, showed that the extended source is not static (Martí et al., 2015). The most striking changes occur in the northern terminal hotspot.
In 1992 and 1997 the northern lobe terminates in a bow-shaped working surface with a conspicuous hotspot at the apex (Martí et al., 2015). The 1992 hotspot has flux density $\Delta \delta \simeq 0\farcs14$5 mJy and deconvolved size $\Delta \delta \simeq 0\farcs14$6 arcsec; the 1997 hotspot has $\Delta \delta \simeq 0\farcs14$7 mJy and size $\Delta \delta \simeq 0\farcs14$8 arcsec (Martí et al., 2015). From 1992 to 1997, the hotspot position shifted by $\Delta \delta \simeq 0\farcs14$9 over 5.4 yr, corresponding to a proper motion of 0 and a projected jet-head speed of 1 at 8.5 kpc (Martí et al., 2015).
By 2001 the previous hotspot had disappeared as a compact apex structure and the northern terminus had fragmented into two elongated components nearly parallel to the jet direction, with fluxes 2 mJy and 3 mJy (Martí et al., 2015). In 2008 a new hotspot was again visible, with flux density 4 mJy and size 5 arcsec (Martí et al., 2015). Because the 2008 hotspot should have been detected in 2001 if already present, the terminal structure must have been destroyed and later re-formed (Martí et al., 2015).
Difference imaging provided an explicit control against imaging artifacts. Subtraction of the 1997 clean-component model from the 2008 visibilities leaves residuals at the 6–7 level aligned with the jet axis, supporting genuine structural change (Martí et al., 2015). The same deep stacked map, assembled from all VLA datasets with total on-source time about 19 hr, rms about 8, and beam about 9, revealed bridges of diffuse emission almost surrounding the bipolar jet complex in an elliptical pattern interpreted as a cocoon-like structure (Martí et al., 2015).
The physical interpretation advanced for the morphological disruption invokes hydrodynamic instabilities. Defining the jet-to-ambient density contrast as 0, the analysis estimates 1, 2, and hence 3, low enough that Kelvin–Helmholtz or Rayleigh–Taylor modes could grow (Martí et al., 2015). With a jet radius estimate 4 pc, the characteristic growth times 5 and 6 are of order several months, broadly compatible with large-scale reorganization on yearly timescales (Martí et al., 2015). The authors reject simple radiative fading, since with 7 and 8, the synchrotron cooling time is 9 Myr, far longer than the observed changes (Martí et al., 2015).
A later analysis explicitly asked whether the changing large-scale radio structure could instead be due to jet precession. Fitting a Hjellming–Johnston kinematic precession model simultaneously to five epochs yielded a formally acceptable fit with reduced 0 for 301 degrees of freedom and parameters including 1, 2, 3, 4, cone half-angle 5, and precession period 6 d (Luque-Escamilla et al., 2020). However, the same study concluded that the dramatic structural changes are not easily attributed to jet precession and that instabilities are a more realistic explanation (Luque-Escamilla et al., 2020). It also argued that the familiar 18.48 d modulation may plausibly be a precession period rather than the orbital period, but not the cause of the large-scale radio morphology (Luque-Escamilla et al., 2020).
5. Jet–interstellar medium interaction and feedback
A consistent theme across radio and millimeter work is that the large-scale source is shaped by interaction with an inhomogeneous interstellar medium. The original winged-morphology study emphasized nearby molecular gas from the Dame et al. CO survey and identified a molecular cloud with kinematic distance similar to that of GRS 1758−258 as a likely collision target of the jets (Martí et al., 2018). In that interpretation, terminal shocks form where the jets impact dense ambient material, shocked plasma flows backward, and the backflow escapes laterally to produce the observed wing-like structures (Martí et al., 2018).
ALMA observations substantially strengthened the environmental picture. In the GRS 1758−258 field, ACA Band 3 mosaics acquired on 2017 October 31 and 2017 December 27 covered 7, while 12 m array follow-up on 2019 November 24 and 2019 December 10 targeted a 8 region near the southern lobe (Tetarenko et al., 2020). Molecular emission was detected in the velocity range 9–0 from 1CO 2, HCN 3, CS 4, and SiO 5, with SiO detected only in the combined ACA+12 m data (Tetarenko et al., 2020). No significant sub-mm continuum was detected, with a 6 limit of about 7 (Tetarenko et al., 2020).
The key empirical result is asymmetric: a bright molecular structure is seen only at the southern lobe, where it lies on the eastern edge of the southern radio lobe and aligns with the candidate jet-blown cavity or cocoon (Tetarenko et al., 2020). The northern lobe lacks a comparable ALMA molecular counterpart within the velocity coverage used (Tetarenko et al., 2020). Southern-lobe 8CO spectra are multi-peaked, with components at approximately 72 and 84 km s9, while CS and SiO show a single component near 0 and HCN, after hyperfine fitting, yields 1 (Tetarenko et al., 2020). The authors infer that dense gas has been compressed and displaced by about 2 relative to the ambient medium (Tetarenko et al., 2020).
The chemistry strengthens the case for shock processing. HCN is a dense-gas tracer; CS traces dense gas and may be enhanced in shocks; SiO is a classic shock diagnostic linked to dust-grain destruction (Tetarenko et al., 2020). The unusually high HCN/3CO intensity ratio of roughly 4 is described as atypically high for normal Galactic molecular clouds and suggestive of an external process enhancing HCN abundance or excitation, although the authors caution that spatial filtering and self-absorption complicate interpretation (Tetarenko et al., 2020). Position–velocity diagrams along the eastern edge of the southern lobe show 5CO emission spectrally displaced to lower velocities at offsets 6–7, while HCN and CS span a broad velocity range (Tetarenko et al., 2020). This is interpreted as consistent with a backflow scenario in which plasma reflects from the cavity end and moves along the cavity wall.
Using the molecular structures as a calorimeter, the ALMA study estimated a time-averaged jet power of 8 sustained over 9 Myr (Tetarenko et al., 2020). The same work argued from the size–line width relation that the interacting gas lies in the Central Molecular Zone and hence constrained the distance to GRS 1758−258 to $-25^\circ\ 44′\ 36\farcs3$00 kpc (Tetarenko et al., 2020). A plausible implication is that the source is dynamically affecting dense molecular material in the CMZ over $-25^\circ\ 44′\ 36\farcs3$01-year timescales.
MeerKAT later provided an alternative calorimetric view. Observed in L-band during 2024 April 4, July 5, and August 15, with total on-source integration 6 h 48 min, GRS 1758−258 was imaged at $-25^\circ\ 44′\ 36\farcs3$02 resolution (Mariani et al., 12 Sep 2025). The study measured ISM densities between 10 and 40 cm$-25^\circ\ 44′\ 36\farcs3$03 across both jets, slightly lower in the northern region; lobe ages from 6 to 51 kyr; and time-averaged jet power between $-25^\circ\ 44′\ 36\farcs3$04 and $-25^\circ\ 44′\ 36\farcs3$05 (Mariani et al., 12 Sep 2025). It also measured a proper motion of 130 mas yr$-25^\circ\ 44′\ 36\farcs3$06 in a portion of the northern jet (Mariani et al., 12 Sep 2025). Because this paper postdates the 2018 winged-microquasar discovery by several years, it is best read as extending rather than replacing the earlier hydrodynamic-backflow interpretation.
6. Accretion states, spectral timing, and high-energy properties
GRS 1758−258 is a persistent black-hole X-ray binary that spends most of its time in the hard state but undergoes occasional softer episodes. Detailed radio/X-ray study of the 2000–2002 and 2008–2009 intervals identified an intermediate state on 2000 Sep 19, a soft state on 2001 Mar 22, and hard states on 2002 Sep 28 and during 2008–2009 (Soria et al., 2011). In the 2000 intermediate state, an absorbed disk-blackbody plus power law was required, with $-25^\circ\ 44′\ 36\farcs3$07, $-25^\circ\ 44′\ 36\farcs3$08, and disk fraction about 27% in the 0.3–12 keV band (Soria et al., 2011). In the 2001 soft state, preferred MOS1 values are $-25^\circ\ 44′\ 36\farcs3$09, $-25^\circ\ 44′\ 36\farcs3$10, and disk fraction about 83% (Soria et al., 2011). In the 2002 hard state, a simple absorbed power law gave $-25^\circ\ 44′\ 36\farcs3$11, while a broken power law improved the fit with $-25^\circ\ 44′\ 36\farcs3$12, $-25^\circ\ 44′\ 36\farcs3$13, and $-25^\circ\ 44′\ 36\farcs3$14 (Soria et al., 2011).
Timing behavior independently supports these classifications. The 2000 intermediate state shows rms variability $-25^\circ\ 44′\ 36\farcs3$15 over $-25^\circ\ 44′\ 36\farcs3$16–50 Hz, with red noise below $-25^\circ\ 44′\ 36\farcs3$17 Hz, a flat-top from $-25^\circ\ 44′\ 36\farcs3$18 to 1 Hz, and steepening above 1 Hz (Soria et al., 2011). The 2001 soft state has rms consistent with zero (Soria et al., 2011). The 2002 hard state has rms $-25^\circ\ 44′\ 36\farcs3$19 with strong band-limited noise and a break near 0.4 Hz (Soria et al., 2011). The radio core follows the standard disc–jet coupling pattern: it is quenched in the soft state, with $-25^\circ\ 44′\ 36\farcs3$20 on 2001 Mar 22, and present in hard states, for example 236 $-25^\circ\ 44′\ 36\farcs3$21Jy ten days before the 2002 Sep 28 hard-state observation (Soria et al., 2011).
Longer-term X-ray monitoring with RXTE/PCA, ASM, and Swift/BAT showed that the source was predominantly hard during nearly 12 years of pointed PCA observations but entered the thermally dominated soft state seven times between 1997 and 2008 (Hirsch et al., 2019). A central result is that these are dim soft states: instead of the more common spectral pivoting around $-25^\circ\ 44′\ 36\farcs3$22 keV, the flux above 3 keV declines strongly (Hirsch et al., 2019). During the 2001 event, the $-25^\circ\ 44′\ 36\farcs3$23–$-25^\circ\ 44′\ 36\farcs3$24 keV flux fell to only $-25^\circ\ 44′\ 36\farcs3$25 (Hirsch et al., 2019). In the hardness–intensity diagram, with hardness defined as $-25^\circ\ 44′\ 36\farcs3$26, the source shows hysteresis typical of transients even though it is persistent (Hirsch et al., 2019).
The same long-term timing study revisited the longstanding $-25^\circ\ 44′\ 36\farcs3$27 d modulation. It found no stable orbital modulation in the range 2–30 d in the PCA or ASM long-term light curves (Hirsch et al., 2019). Instead, after detrending the hard-state PCA data, Lomb–Scargle analysis recovered a peak at $-25^\circ\ 44′\ 36\farcs3$28 in the 1997–2001 interval, while the dynamic power spectrum over the full dataset showed the dominant period drifting between $-25^\circ\ 44′\ 36\farcs3$29 and $-25^\circ\ 44′\ 36\farcs3$30, with significance between 98.15% and 99.98% (Hirsch et al., 2019). This behavior is difficult to reconcile with a coherent orbital clock and was interpreted instead as superorbital modulation, plausibly associated with a warped accretion disk (Hirsch et al., 2019).
Broadband hard-state reflection spectroscopy has added further constraints. Simultaneous Swift/XRT and NuSTAR on 2018-09-28, spanning 0.5–78 keV, showed a broad Fe line in the 5–8 keV range and a Compton reflection hump in the 15–40 keV range (Jana et al., 2022). Fits with several members of the relxill model family found $-25^\circ\ 44′\ 36\farcs3$31–1.57, coronal temperature $-25^\circ\ 44′\ 36\farcs3$32 keV from Comptonized models, ionization $-25^\circ\ 44′\ 36\farcs3$33–3.9, iron abundance $-25^\circ\ 44′\ 36\farcs3$34–3.28 $-25^\circ\ 44′\ 36\farcs3$35, inclination $-25^\circ\ 44′\ 36\farcs3$36–$-25^\circ\ 44′\ 36\farcs3$37, and inner radius very close to the ISCO (Jana et al., 2022). All models indicated a high prograde spin, conservatively summarized as $-25^\circ\ 44′\ 36\farcs3$38, with preferred values near 0.95–0.98 (Jana et al., 2022). The inferred bolometric luminosity was $-25^\circ\ 44′\ 36\farcs3$39, about 1.5% of Eddington for an assumed $-25^\circ\ 44′\ 36\farcs3$40 black hole at 8 kpc (Jana et al., 2022).
A broader multi-mission study over 2016–2022 with AstroSat, NuSTAR, and Swift-XRT confirmed that GRS 1758−258 occupies both a dim-soft state and several hard-state realizations (R. et al., 2023). The 2016 dim-soft state had $-25^\circ\ 44′\ 36\farcs3$41 keV, $-25^\circ\ 44′\ 36\farcs3$42, and $-25^\circ\ 44′\ 36\farcs3$43, about 1% of Eddington (R. et al., 2023). Hard states showed $-25^\circ\ 44′\ 36\farcs3$44–2.22, $-25^\circ\ 44′\ 36\farcs3$45–45 keV, and $-25^\circ\ 44′\ 36\farcs3$46 of about 1–5% $-25^\circ\ 44′\ 36\farcs3$47, with one 2022 hard-state epoch additionally requiring a cool disk component with $-25^\circ\ 44′\ 36\farcs3$48 keV (R. et al., 2023). Relativistic reflection fits tied across hard-state epochs yielded $-25^\circ\ 44′\ 36\farcs3$49, $-25^\circ\ 44′\ 36\farcs3$50, $-25^\circ\ 44′\ 36\farcs3$51, and $-25^\circ\ 44′\ 36\farcs3$52–3.82 (R. et al., 2023). The paper argued that the state changes of GRS 1758−258 are governed importantly by the hot coronal or sub-Keplerian flow rather than by thin-disc luminosity alone (R. et al., 2023).
High-energy monitoring with INTEGRAL/IBIS–ISGRI from 2003 to 2022 showed that GRS 1758−258 is usually described by Comptonized spectra but can display deviations above $-25^\circ\ 44′\ 36\farcs3$53 keV, with a hard tail extending to at least 600 keV in spectral states with more than 8 Ms of exposure (Rodi et al., 24 Feb 2025). The 30–50, 50–100, and 100–300 keV count rates are strongly correlated, but above 300 keV the 300–600 keV count rates are either anti-correlated or not correlated with the 30–50 keV rate, suggesting a different physical origin (Rodi et al., 24 Feb 2025). CompTT+po and Eqpair both fit the spectra acceptably, but the correlation behavior disfavors the hybrid Comptonization scenario in that study (Rodi et al., 24 Feb 2025).
7. Comparative significance and broader implications
GRS 1758−258 is important because it provides a compact, nearby, and time-resolved analog of processes otherwise studied in radio galaxies and other extragalactic jet systems. The source is not simply a microquasar with resolved lobes; it is presented explicitly as the first winged microquasar, with Z-shaped secondary structures comparable to those in winged radio galaxies (Martí et al., 2018). Because the system is an X-ray binary with a non-degenerate companion star and, in evolutionary terms, only one compact object ever hosted by the system, black-hole coalescence can be ruled out as the cause of the winged morphology (Martí et al., 2018). This is diagnostically important because some winged radio galaxy models invoke spin flips after supermassive black-hole mergers. GRS 1758−258 shows that winged morphology can arise without any black-hole merger history (Martí et al., 2018).
The object is also a laboratory for large-scale jet evolution. In radio galaxies, hotspot motion, cocoon inflation, backflow, and instability-driven disruption unfold over $-25^\circ\ 44′\ 36\farcs3$54 yr or more; in GRS 1758−258, analogous processes occur on parsec scales and can be monitored over years to decades (Martí et al., 2015). The combination of hotspot disruption and re-formation, cocoon-like diffuse emission, winged morphology, molecular shock tracers, and asymmetric thermal versus synchrotron lobe structure makes the source unusually rich as a jet–ISM interaction case (Martí et al., 2015, Martí et al., 2018, Tetarenko et al., 2020, Mariani et al., 12 Sep 2025).
A second broader significance concerns accretion-state phenomenology. GRS 1758−258 displays canonical black-hole binary states in their spectral and timing properties, yet the source violates simple expectations that the soft state should be the more luminous state. Hard and soft states overlap strongly in luminosity, and the hard state is often at least as luminous as, or more luminous than, the soft state (Soria et al., 2011). This suggests that accretion rate alone does not control the state transitions. The persistence of dim-soft states, the drifting $-25^\circ\ 44′\ 36\farcs3$55–20 d superorbital-like modulation, and the recurrence of a strong hard-state corona all point toward models in which the hot flow, corona, and disk geometry play essential roles (Hirsch et al., 2019, R. et al., 2023).
A third significance lies in jet efficiency and radio loudness. GRS 1758−258 is extremely faint in core radio emission for its X-ray luminosity, with 5 GHz core flux density varying between $-25^\circ\ 44′\ 36\farcs3$56 and 0.5 mJy over two decades, while typical X-ray luminosities are around $-25^\circ\ 44′\ 36\farcs3$57 (Soria et al., 2011). It therefore lies on the radio-quiet branch of Galactic black holes, even though it possesses long-lived, parsec-scale lobes (Soria et al., 2011). This undercuts any simple equation of radio-quietness with an absence of jets.
In sum, GRS 1758−258 occupies a singular position at the intersection of microquasar accretion physics, jet stability, jet–ISM feedback, and radio-galaxy analogies. It is simultaneously a persistent hard-state black-hole binary, a source of dim-soft transitions and drifting superorbital modulation, a jet system with measurable large-scale structural evolution, and the first known winged microquasar (Hirsch et al., 2019, Martí et al., 2015, Martí et al., 2018). The accumulated evidence suggests that its large-scale radio morphology is shaped primarily by hydrodynamical interaction with an inhomogeneous ISM rather than by merger-induced spin flips or precession alone, while its accretion-state behavior indicates that the coronal or hot-flow component is dynamically central (Martí et al., 2018, Luque-Escamilla et al., 2020, R. et al., 2023).
