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Long-Term Astrometric Monitoring of the Galactic Center Magnetar PSR J1745--2900

T0 review · 2 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read Five years of VLBA monitoring of the Galactic Center magnetar PSR J1745–2900 constrain its proper motion to about 2% and set acceleration limits consistent with a bound orbit around Sgr A*.

desk verdict Solid, transparent astrometry extension; the proper motion is robust, but the headline acceleration limit leans on an ad hoc systematic that absorbs a correlated residual. read the letter →

arxiv 2506.02348 v1 pith:IPR3RETT submitted 2025-06-03 astro-ph.HE

Geoffrey C. Bower , Adam T. Deller , Paul B. Demorest , Jason Dexter , Andreas Brunthaler , Gregory Desvignes , Ralph P. Eatough , Heino Falcke
show 3 more authors
Ciriaco Goddi Michael Kramer F. Yusef-Zadeh
This is my paper
classification astro-ph.HE
keywords PSRJ1745-2900GalacticCentermagnetarVLBAastrometrypropermotionaccelerationSgrA*interstellarscatteringcoreshift
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reports 25 epochs of VLBA astrometry of PSR J1745–2900, the only known pulsar near the Galactic Center black hole Sgr A*, spanning 1984 days. The measurements tighten the magnetar's proper motion to roughly 2% uncertainty and set upper limits on its acceleration of about 0.4 and 0.2 milliarcseconds per year squared in right ascension and declination, consistent with the maximum acceleration expected if the magnetar orbits Sgr A* at its projected separation. The authors argue that if the magnetar re-brightens, an additional epoch of monitoring about a decade later could directly detect that acceleration, confirming the magnetar is bound to the black hole and measuring their separation. They also investigate a ~1600-day oscillation in the astrometric residuals and find no fully satisfactory explanation: a binary companion model fits the positions but is contradicted by pulse-timing measurements.

What carries the argument

The fundamental observable is the position of the magnetar relative to Sgr A*, obtained by calibrating the VLBA data on Sgr A* and transferring the solutions to the magnetar; the Sgr A* reference frame is fixed by an updated ICRF position and proper motion. The astrometric model fits a reference offset, a parallax term relative to Sgr A*, a linear proper motion, a quadratic acceleration term, and a wavelength-dependent core shift. Bootstrap resampling over $10^4$ synthetic series is used to confirm the error estimates, and Lomb-Scargle periodograms of the residuals are used to search for a binary signal. The angular broadening is tracked by fitting Gaussians to the magnetar's image at 2 cm and deconvolving the synthesized beam.

What would settle it

A simultaneous VLBI campaign that ties Sgr A* directly to distant extragalactic sources over the same 1984-day span would reveal any reference-frame wander; if the fitted magnetar proper motion changes by more than the quoted errors when the reference frame is re-anchored, the claimed acceleration limits are biased. Alternatively, a future epoch after re-brightening that measures a tangential acceleration significantly different from zero—or a proper-motion change exceeding the quoted errors at a ~10-year separation—would either confirm the bound orbit or falsify the current limits.

Watch

Extended reading notes

Core claim

The central claim is that the extended astrometric baseline, 41 independent measurements over 1984 days, constrains the proper motion of PSR J1745–2900 to $\lesssim$2% uncertainty ($\mu_\alpha = 2.01 \pm 0.04$, $\mu_\delta = 6.09 \pm 0.02$ mas yr$^{-1}$) and places upper limits on the absolute tangential acceleration of $\lesssim (0.4, 0.2)$ mas yr$^{-2}$ in the two coordinates. These limits are consistent with the maximum acceleration of $\sim 0.03$ mas yr$^{-2}$ expected for a bound orbit around Sgr A* at the projected separation of about 0.1 pc. The paper interprets the proper-motion direction as consistent with the magnetar originating in the clockwise stellar disk with a modest kick. It further reports that the astrometric residuals contain an apparent sinusoidal ~1600-day variation in right ascension that cannot be fully explained: a stellar companion fits the astrometry but is ruled out by pulse-timing measurements, and no other candidate mechanism (refractive wander, changes in Sgr A* structure) is fully satisfactory. No secular change in the magnetar's angular broadening is detected, and the mean core shift of Sgr A* is consistent with zero and with expectations for a compact jet or symmetric accretion flow.

Load-bearing premise

The measurement is relative to Sgr A*, and the calibration forces Sgr A* to have a fixed apparent position and motion, so any real drift of Sgr A*'s centroid—which the paper's own ~1600-day residual hints at—would be absorbed into the magnetar's fitted proper motion and acceleration.

Editorial extensions

If this is right

  • If PSR J1745–2900 re-brightens, a second astrometric measurement separated by ~10 years from the mean epoch would detect the acceleration of Sgr A* at 5–10$\sigma$, directly confirming a bound orbit and measuring the separation.
  • The proper motion direction matches the clockwise stellar disk, supporting the magnetar's origin in that disk with a modest natal kick.
  • Over four years, no change in the magnetar's apparent size is seen, ruling out a significant time-variable scattering screen near the Galactic Center and implying any obscuring ionized gas is patchy on scales larger than ~200 AU.
  • The mean core shift of Sgr A* is consistent with zero and with the magnitude expected from GRMHD jet models, so the data do not discriminate between jet and symmetric-accretion-flow interpretations.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the unexplained ~1600-day right-ascension residual is real wandering of Sgr A*'s centroid rather than motion of the magnetar, the quoted proper motion and acceleration limits absorb that drift, and the true systematic may exceed the per-epoch errors the paper adopts (0.35 mas RA, 0.15 mas Dec).
  • A future campaign that ties Sgr A* to extragalactic reference sources, rather than only measuring the magnetar relative to Sgr A*, could separate reference-frame wander from the magnetar's acceleration and sharpen the test.
  • If the magnetar stays too faint for another decade, the acceleration detection may need to come from stacking relative astrometry of nearby S-stars (already measured with NIR astrometry) rather than this pulsar alone.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 6 minor

Summary. The paper reports 25 epochs (41 measurements) of VLBA astrometry of PSR J1745–2900 over 1984 days, extending the 2014–2015 data with 15 new epochs. The authors fit a model including proper motion, acceleration, parallax, and core shift relative to Sgr A*, and find μ_α ≈ 2.01–2.04 mas yr^{-1}, μ_δ ≈ 6.07–6.10 mas yr^{-1}; quote an acceleration limit of ≲ (0.4, 0.2) mas yr^{-2}; report no secular change in scattering size; find a mean core shift consistent with zero; and interpret the proper motion as consistent with an origin in the clockwise stellar disk. They also discuss a ~1600-day quasi-periodic residual in RA, test and reject a binary companion using pulse-timing data, and constrain a possible second scattering screen.

Significance. The proper motion measurement is a substantial improvement over Bower et al. (2015) and, if the relative-astrometry calibration is sound, provides the strongest astrometric constraint on a Galactic Center pulsar. The paper is honest about systematics: it explicitly states in §4.1 that no tested mechanism fully explains the residuals, and it uses multiple fitting schemes and bootstrap resampling. The CW-disk origin claim is tested against external stellar kinematics and is robust to the acceleration/core-shift additions. The main scientific payoff—future detection of gravitational acceleration by Sgr A* if the magnetar re-brightens—is clearly articulated. However, the acceleration limit is more fragile than the abstract suggests because it depends on treating a correlated residual as white noise.

major comments (2)
  1. [Section 3, Table 3, Abstract] The acceleration upper limit is not robust to the treatment of correlated residuals. The RA residuals after standard fits show a ~1 mas, ~1600-day quasi-periodic signal (Fig. 4; §4.1). The paper makes χ²_α ≈ 1 by adding an uncorrelated per-epoch systematic error of 0.35 mas in RA (§3), but this changes the fitted a_α from –0.252 ± 0.052 mas yr^{-2} (LSQ, no systematic) to –0.111 ± 0.057 mas yr^{-2} (with systematic; Table 3), and the bootstrap resamples epochs independently, so it does not propagate the time correlation. A ~1 mas, ~1600-day quasi-periodic residual can project substantially onto the quadratic term, and the factor-of-two shift in a_α demonstrates the sensitivity. The abstract's limit of ≲ (0.4, 0.2) mas yr^{-2} and the statement that it is consistent with the ~0.03 mas yr^{-2} expectation therefore rest on treating an unexplained correlated signal as white noise. This should either be modeled explicitly (e.g., as a sinusoid or Gaussian process) or the acceleration claim should be reported as a provisional constraint with a clear caveat. The proper-motion and CW-disk-origin conclusions are not affected by this issue.
  2. [Section 2 and §4.1] The quoted '≲2% proper-motion accuracy' is a precision relative to Sgr A* under the assumption that Sgr A*'s apparent centroid is fixed in time. The manuscript itself finds that none of the candidate mechanisms fully explains the RA residuals, and if part of the ~1 mas, ~1600-day residual is apparent centroid motion of the reference source (refractive wander or Sgr A* structure changes), it would contribute a common systematic of order 0.1 mas yr^{-1}—larger than the reported 0.04 mas yr^{-1} random error in μ_α. I ask the authors to state explicitly that the ≲2% figure is statistical precision relative to Sgr A*, and to provide a bound on the reference-source systematic or cite independent evidence that Sgr A*'s centroid is stable at this level over the full multi-year span at these frequencies.
minor comments (6)
  1. [Section 4.1 vs. Figure 3] The text gives π < 0.4 mas while the Figure 3 caption says 'upper limit at 95% confidence of π < 0.6 mas'; please reconcile the value and state the confidence level.
  2. [Figure 1 caption] Typo: 'indistuinghisable' should be 'indistinguishable'.
  3. [Section 3] The bootstrap description says '10 4 astrometric series'; this should be '10^4', and the resampling unit (individual measurements vs. epochs) should be stated explicitly.
  4. [Abstract and Table 3] The acceleration upper limit should state the confidence level (apparently 3σ) and clarify that it is an upper limit on the absolute value in each coordinate, rather than leaving the confidence implicit.
  5. [Table 1] The ellipsis rows for multi-band epochs make the table hard to parse; use explicit repeated MJD values or a footnote to indicate multiple bands at the same epoch.
  6. [References] The reference list gives Bower et al. 2006a and 2006b with identical journal, volume, and page; please verify that the 2006b entry is correct and distinct.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the proper motion and acceleration constraints are direct VLBI measurements compared against external expectations, and the self-citations are procedural or secondary.

full rationale

The paper's derivation chain is empirically self-contained. The fundamental measurement is the VLBI position of PSR J1745-2900 relative to Sgr A*, modeled with Eq. 1 as proper motion, acceleration, parallax, and core shift. The key claims compare these fitted parameters to externally established quantities: Sgr A* proper motion from Reid & Brunthaler (2020), the clockwise stellar disk kinematics, the black hole mass and distance, and jet-model core-shift predictions. No fitted constant is renamed as a prediction: the acceleration upper limit is an independent fit result that is then compared with a Newtonian expectation computed from the adopted black hole mass and projected separation, so the comparison is not definitionally forced. The paper honestly reports the RA residual problem, including the ~1600-day quasi-periodic signal, the 0.35 mas systematic error added to force chi-square to unity, and fits with and without that term; this is a robustness limitation rather than circularity, because the reported values are not derived from the model they are said to constrain. Self-citations to Bower et al. (2015) concern calibration procedures and earlier data, not the load-bearing derivation, and the in-prep timing citation (Eatough et al. 2025) is used only to reject a secondary binary-companion hypothesis, not to define the astrometric claims. Thus no load-bearing step reduces to its own input, and the paper is self-contained against external benchmarks.

Assumptions & free parameters 7 free parameters · 6 assumptions · 2 invented entities

The main hand-chosen inputs are the post hoc per-epoch systematic errors (0.35 mas RA, 0.15 mas Dec) that calibrate the χ2 and materially change the fitted acceleration. The physical interpretation leans on adopted external values: the GC distance (8.3 kpc), the Sgr A* mass and reference frame, and the single-screen scattering model. No genuinely new physical entity is claimed to exist; the two candidate entities (binary companion, second scattering screen) are explicitly ruled out or unconstrained by the data.

free parameters (7)
  • Per-epoch RA systematic error = 0.35 mas (added in quadrature)
    Added post hoc in §3 to force χ2_nu about 1 for RA fits; the fitted acceleration a_α changes from -0.252 to -0.111 mas/yr^2 depending on its inclusion.
  • Per-epoch Dec systematic error = 0.15 mas (added in quadrature)
    Added post hoc in §3 to force χ2_nu about 1 for declination fits.
  • Acceleration a_α = -0.111 ± 0.057 mas/yr^2 (fit with systematics)
    Fitted quadratic term in Eq. 1; the limiting quantity behind the abstract's (0.4, 0.2) upper bounds.
  • Acceleration a_δ = 0.100 ± 0.030 mas/yr^2 (fit with systematics)
    Fitted quadratic term in Eq. 1.
  • Core shift Φ_α, Φ_δ = -0.09 ± 0.10 and 0.02 ± 0.05 mas/cm (PM+Accel+Core Shift fit)
    Fitted wavelength-dependent offsets in Eq. 1; consistent with zero.
  • Parallax π = upper limit < 0.4 mas (95% c.l.)
    Fitted parallax relative to Sgr A*; consistent with the magnetar being at the GC distance.
  • Binary companion elements P, i, a, M2 = P=1600±200 d, i=95±23 deg, a=3.3±1.0 AU, M2 median 5.5 Msun
    Orbit fit to the astrometric residuals; tested as an explanation of the ~1600-day wobble and rejected by timing data, so not part of the central claim.
assumptions (6)
  • domain assumption Sgr A* distance D = 8.3 kpc and mass 4 × 10^6 Msun
    Adopted from Do et al. 2019 and GRAVITY 2022; converts angular broadening to physical scales (§4.2), sets the expected maximum acceleration of ~0.03 mas/yr^2 (§4.1), and fixes the scattering screen distance.
  • domain assumption Sgr A* ICRF position and proper motion from Reid & Brunthaler (2020) and Gordon et al. (2023)
    Adopted in §2 as the astrometric reference frame; a wrong reference frame shifts every reported ICRF position, but the relative astrometry used for proper motion and acceleration is internally consistent.
  • domain assumption The magnetar is at the Galactic Center distance
    Supported by DM, X-ray column density, and the new parallax upper limit (§4.1); the CW-disk origin and the Sgr A* acceleration comparison both assume this.
  • domain assumption Single thin scattering screen with the Cordes & Lazio (1997) relation (Eq. 2)
    Used in §4.2 to derive the screen distance Δ=5.8±0.3 kpc and to set the second-screen constraint θ1 = sqrt(2 θ δθ).
  • standard math Newtonian point-mass acceleration |a| = GM/r^2 with r the projected separation
    Used in §4.1 to compute the maximum expected acceleration of about 0.029 mas/yr^2.
  • domain assumption Pulsar mass 1.4 Msun
    Used in §4.1 to convert the fitted binary semimajor axis into the companion mass range 0.9-35 Msun.
invented entities (2)
  • Unseen binary companion of PSR J1745-2900
    purpose: Explains the ~1600-day quasi-periodic RA astrometric residuals, which would make it a candidate pulsar-black hole binary (§4.1)
    Fits the astrometry (P=1600 d, a=3.3 AU) but is rejected by pulse-timing data (Eatough et al. 2025), so it is a ruled-out hypothesis, not a claimed discovery.
  • Second scattering screen near Sgr A*
    purpose: Hypothesized to obscure a population of GC pulsars through temporal broadening (§4.2)
    Constrained to be distant (Δs <~ 3 kpc would make the magnetar unobservable) or absent; no evidence found.

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Pith. "Pith review of Long-Term Astrometric Monitoring of the Galactic Center Magnetar PSR J1745--2900." pith.science (2026). https://pith.science/paper/IPR3RETT

@misc{pith2026250602348,
  author       = {Pith},
  title        = {Pith review of: Long-Term Astrometric Monitoring of the Galactic Center Magnetar PSR J1745--2900},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/IPR3RETT}},
  note         = {Machine review of arXiv:2506.02348}
}
abstract

We present new astrometric observations of the Galactic Center magnetar, PSR J1745-2900, with the Very Long Baseline Array (VLBA). Combined with previously published measurements in 10 epochs that spanned 477 days, the complete data set consists of 25 epochs and 41 independent measurements that span 1984 days. These data constrain the proper motion to an accuracy of $\lesssim 2\%$ and set an upper limit on the absolute value of the magnetar's acceleration of $\lesssim (0.4, 0.2)\, {\rm mas\,y^{-2}}$ in the two celestial coordinates, consistent with the maximum value of $\sim 0.03\,{\rm mas\,y^{-2}}$ expected for an orbit around Sgr A*. Future measurements have the potential to detect the acceleration of PSR J1745-2900 due to Sgr A* should PSR J1745-2900 re-brighten. We consider several potential sources of systematic variations in the astrometric residuals after fitting for standard parameters, including refractive wander, changes in the structure of Sgr A*, and the presence of an unseen binary companion. While a stellar companion model can be fit to the astrometric data, pulse period measurements are inconsistent with that model. No changes in the apparent image size of the magnetar were detected over the duration of these observations, indicating a lack of change in the properties of the line-of-sight scattering during this period. We also show that the upper limit to the mean core shift of Sgr A* is consistent with expectations for a compact jet or symmetric accretion flow.

Figures

Figures reproduced from arXiv: 2506.02348 by the authors.

Figure 1
Figure 1. Position as a function of time for PSR J1745–2900 relative to Sgr A*. Different colors are used to identify the wavelength of observations. The solid black line shows the best-fit proper motion in each coordinate with µα = 2.04mas yr−1 and µδ = 6.07mas yr−1 . Fits including acceleration and core shift are indistuinghisable on this scale. remain significantly in excess of 1, however, indicative of the presence of sys… view at source ↗
Figure 2
Figure 2. Astrometric residuals after the fit for proper motion (upper left), and after the fit for proper motion, acceleration, and core shift (lower left). could arise from changes in the apparent centroid position of Sgr A* which we discuss in Section 4.3, refractive image wander, and the influence of a binary companion [PITH_FULL_IMAGE:figures/full_fig_p008_2.png] view at source ↗
Figure 3
Figure 3. Empirical probability density distribution of proper motion solutions from the bootstrap resampling method (upper left). Distribution of the measured parallax π of the magnetar referenced to Sgr A* (upper right). The upper limit at 95% confidence of π < 0.6 mas is consistent with the expectation that the magnetar is located in the GC. Empirical probability density distribution of acceleration from the bootstrap resa… view at source ↗
Figures from the paper (4 more)
Figure 4
Figure 4. Figure 4: Lomb-Scargle periodogram of the astrometric and timing data (Eatough et al. 2025). Periodic signals comparable to the data set length are often indicative of red noise variability. A linear trend has been removed the timing residuals before calculation of the periodogr…
Figure 5
Figure 5. Figure 5: The binary model (solid lines) from fitting astrometric data alone, plotted against the residual period from (Eatough et al. 2025) (top) and the astrometric data in right ascension (middle) and declination (bottom). The plotted binary model has a semimajor axis a = 3.3…
Figure 6
Figure 6. Figure 6: Apparent size of the magnetar at 2 cm wavelength as a function of time for Gaussian deconvolutions of the imaged size in major axis (top), minor axis (middle), and position angle (bottom) parameters. Observations previously reported in Bower et al. (2015) are before MJ…
Figure 7
Figure 7. Figure 7: Core shift for the epochs with two frequency observations (black squares) and for the fitted average over all epochs (red circle). Note that the mean core shift includes data from epochs in which both one and two frequency band observations are included, but that singl…

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