INTEGRAL/ISGRI Post 2024-Periastron View of PSR B1259-63

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper primarily focuses on investigating the hard X-ray emission properties of the gamma-ray binary system PSR B1259-63/LS 2883 following its 2024 periastron passage. Specifically, it aims to study the spectral evolution of the system in the 0.3–300 keV energy range using observations from the INTEGRAL/ISGRI instrument, combined with quasi-simultaneous data from the Swift/XRT telescope. The research seeks to constrain key spectral parameters, explore potential spectral curvature or breaks, and link the X-ray emission to underlying non-thermal radiation mechanisms involving high-energy electrons.  I noticed that this manuscript  possesses significant scientific value, with a fluent and logical structure, and generally clear content presentation. However, several issues should be addressed before it is accepted for publication:

 1. On the model  assumption: The one-zone leptonic model’s assumptions (e.g., isotropic inverse Compton) are not rigorously tested, which should be explained. 

2.  Limited discussion connects results to other gamma-ray binaries  or broader trends in pulsar-Be star interactions, reducing the work’s contextual impact, which should be 
discussed or mentioned.

3.  The "prolonged second hump" in 2024’s light curve is mentioned but not fully analyzed. A detailed  discussions  would clarify variability patterns.

4.  The paper derives a magnetic field strength of ~2 G, but pulsars in Be binary systems with a little accretions typically have magnetic fields on the order of 10¹² G 
(see, e.g., https://ui.adsabs.harvard.edu/abs/2006MNRAS.366..137Z/abstract).  It is crucial to clarify whether this magnetic field is associated with the pulsar, the companion star, or the emission region, and if applicable, specify the distance from the pulsar where this magnetic  field is measured. A detailed discussion of the field’s origin and spatial context is necessary to avoid confusion.

5. The pulsar is known to be spinning down, yet the paper does not address its spin-down rate issue. It may  provide constraints or discussions on it, comparing it with typical isolated pulsars (e.g., the Crab pulsar) and pulsars in other accretion  neutron star binary systems. Such a comparison would help contextualize whether the spin evolution of PSR B1259-63 is consistent with expectations or exhibits unique characteristics.

6. While the paper focuses on spectral properties, it lacks analysis of temporal variability of emission, such as time-dependent features in light curves across energy bands. Additionally, the pulsation profiles (e.g., pulse shape, phase, and intensity) in multi-wavelength observations (radio to gamma-ray) are not discussed. Investigating these temporal and pulse profile differences would provide a more holistic understanding of the system’s emission mechanisms. So, some discussions may be provided, if possible.  Addressing these points would  significantly enhance the paper’s completeness. 

In conclusion, this paper provides valuable constraints on the post-periastron emission of PSR B1259-63/LS 2883, by using high-quality INTEGRAL and Swift data. Addressing the above suggestions would enhance its rigor, contextual relevance, and impact on the field of high-energy binary systems. With revisions, the work will be a strong contribution to understanding gamma-ray binaries and non-thermal radiation mechanisms.

 

Comments on the Quality of English Language

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Author Response

We thank our anonymous referee for the comments, all of which we took into account, please see below. In the manuscript, we marked all changes in red as requested. 

 

Comments 1: On the model  assumption: The one-zone leptonic model’s assumptions (e.g., isotropic inverse Compton) are not rigorously tested, which should be explained. 

Response 1: We leave the discussion of the applicability of such a model to a future paper, pointing out the potential significance of the IC emission anisotropy and the spatial extension of the emission zone along the intrabinary shock. We clarified that there are possible limits to the one-zone model we use.

 

Comments 2: Limited discussion connects results to other gamma-ray binaries  or broader trends in pulsar-Be star interactions, reducing the work’s contextual impact, which should be 

discussed or mentioned. 

Response 2: Redback pulsar systems, in which the X-ray emission is modeled as synchrotron radiation from ultra-relativistic electrons in the intrabinary shock, are known to exhibit a spectral break in the keV range \citep{SR25}, interpreted as evidence of synchrotron cooling in the electron population. Note, however, that electron evolution in redbacks differs from that in gamma-ray binaries, as the inferred magnetic field is higher—$B \sim 50$ G in redbacks compared to $B \sim 1$ G in gamma-ray binaries (see further). We agree with the comment. We added several paragraphs in Discussion, connecting the studied system with other objects of similar nature. 

 

Comments 3: The "prolonged second hump" in 2024’s light curve is mentioned but not fully analyzed. A detailed  discussion  would clarify variability patterns.

Response 3: The Figure shows that the light curve in the hard X-ray band traces the same secondary hump at $\sim 25-40$ days after the periastron as previously reported in the soft X-ray band, the nature of which remains unclear. For example, it could be attributed to a longer electron runaway time caused by the disk's disruption \citep[see, e.g.][]{Che21}.} We have added a short discussion on this point to Sec.3

 

Comments 4: The paper derives a magnetic field strength of ~2 G, but pulsars in Be binary systems with a little accretions typically have magnetic fields on the order of 10¹² G 

(see, e.g., https://ui.adsabs.harvard.edu/abs/2006MNRAS.366..137Z/abstract).  It is crucial to clarify whether this magnetic field is associated with the pulsar, the companion star, or the emission region, and if applicable, specify the distance from the pulsar where this magnetic  field is measured. A detailed discussion of the field’s origin and spatial context is necessary to avoid confusion.

Response 4: The field of 2 G is the fitted value of the field in the emission zone (without any assumptions made about the origin of it). To avoid any confusion in the following discussion, we clarified this in Sec~3. We intended to make the following discussion as general as possible, so we only give an estimate of distance from the pulsar/optical star to the emission zone. 

 

Comments 5:  The pulsar is known to be spinning down, yet the paper does not address its spin-down rate issue. It may  provide constraints or discussions on it, comparing it with typical isolated pulsars (e.g., the Crab pulsar) and pulsars in other accretion  neutron star binary systems. Such a comparison would help contextualize whether the spin evolution of PSR B1259-63 is consistent with expectations or exhibits unique characteristics.

Response 5: We believe the question of the pulsar spin-down is beyond the scope of the present paper. We note also that the spin-down of the pulsar as well as a pulsed emission from the system are not currently detected in X-rays, please see answer to the next comment.

Comments 6: While the paper focuses on spectral properties, it lacks analysis of temporal variability of emission, such as time-dependent features in light curves across energy bands. Additionally, the pulsation profiles (e.g., pulse shape, phase, and intensity) in multi-wavelength observations (radio to gamma-ray) are not discussed. Investigating these temporal and pulse profile differences would provide a more holistic understanding of the system’s emission mechanisms. So, some discussions may be provided, if possible.  Addressing these points would  significantly enhance the paper’s completeness.  

Response 6: We agree that the question of temporal evolution deserves its own discussion, and since the exact nature of some temporal features are currently unclear, or at least there exists no consensus model of them, this question is left for the future paper.  Concerning the pulse profiles, we would like to point out that the pulsations for PSR B1259-63 are being detected only in radio bands.

We note also that in the X-ray band the emission is mainly unpulsed. E.g. astro-ph/0601241 with XMM-Newton data close to apastron puts the 3sigma limit on the fraction of a pulsed emission to be ~2%.

Reviewer 2 Report

Comments and Suggestions for Authors

review of INTEGRAL/ISGRI post 2024-periastron view of PSR B1259-63
by A. Kuzin et al

Summary: PSR B1259-63/LS 2883 is a gamma-ray binary, with non-thermal emission spans from radio to TeV energies. This is a study of INTEGRAL and Swift/XRT observations and spectral evolution of the 2024 periastron passage. A one-zone leptonic emission model is applied.

Reviewer comments: 

3 of the 5 authors were involved in a 2024 multiwavelength study of the 2024 periastron passage of this system using ATCA, SALT, Swift/XRT, NICER and Fermi/LAT data. The fluxes spectral index Gamma were studied vs. time (Fig.2 of the 2024 paper). The 2024 paper was largely observational noting changes in the lightcurve from previous periastron passages.
The current manuscript is sufficiently different to be of interest, and generally is well-written.

Introduction: good

Data Analysis: 
Need to define ScW (in Table 1). 
Does revolution mean satellite orbit number? (line 65)
line 74: do you mean "field of view" instead of PSR?
line 79: give a quantitative comparison between the uncertainty of fluxes from PSRB1259-63 and the fluxes from 2RXPJ130159.6. Shouldn't your fluxes in Table 1 include an extra uncertainty to account for that you did not subtract the fluxes of 2RXPJ130159.6?
Table 1, line 86-87 and Fig.1: are the Gamma's for fit to Integral spectrum, XRT spectrum or joint spectrum? It looks like from the plotted spectra (particularly datasets 7 and 9) that there is not single absorbed power law that would fit. Can you plot instead counts s^-1 keV^-1 cm^-2, which would show the Integral and XRT are measuring the same spectrum?
Fig.4: add a note to caption stating the color bar on right side of the plots gives delta chi^2 values.
Fig.5: There is no black line (total radiation), please change the figure so it is visible. Does the beta value have no effect on the IC part of the spectrum? 
line 190: do you mean to say that the factor 3 should have been included by Chen et al? Does it affect their analysis (or just their derived B)? If just B then state what their derived B should have been.

Author Response

We thank our anonymous referee for the comments, all of which we took into account, please see below. In the manuscript, we marked all changes in red as requested. 

 

Comments 1. Need to define ScW (in Table 1). 

Response 1. Agreed. Done in text when introducing the table: “Table 1 shows the parameters of the datasets: Science Window ranges (ScW,  continuous time intervals during which the satellite maintains a stable pointing), <...>.” 

 

Comments 2. Does revolution mean satellite orbit number? (line 65)

Response 2. Yes, correct. A revolution corresponds to a single orbit of INTEGRAL, ~ 3 days.

 

Comments 3. line 74: do you mean "field of view" instead of PSR?

Response 3.  We meant point spread function, PSF. Corrected.

 

Comments 4. line 79: give a quantitative comparison between the uncertainty of fluxes from PSRB1259-63 and the fluxes from 2RXPJ130159.6. Shouldn't your fluxes in Table 1 include an extra uncertainty to account for that you did not subtract the fluxes of 2RXPJ130159.6?

Response 4.  A comparison was given:  For this period, MJD 60500-60530, the flux of 2RXP J130159.6-635806 was at a constant level of F0.5−10 keV ∼ 3 × 10−12erg s−1 cm−2, an order of magnitude lower than that of PSR B1259-63/LS 2883: F0.5−10 keV ∼ (3 − 6) × 10−11erg s−1 cm−2 and, therefore, does not strongly contaminate the results presented.

 The uncertainties to the PSR B1259-63 flux connected to subtraction of this source are below statistical uncertainties of the flux (>10% on average).

Comments 5. Table 1, line 86-87 and Fig.1: are the Gamma's for fit to Integral spectrum, XRT spectrum or joint spectrum? It looks like from the plotted spectra (particularly datasets 7 and 9) that there is not single absorbed power law that would fit. Can you plot instead counts s^-1 keV^-1 cm^-2, which would show the Integral and XRT are measuring the same spectrum?

Response 5. Gammas are for the joint fit; we stated this explicitly in the Table 1 caption:

“<...> along with the absorbed PL joint fit parameters Γ and NH <...>”

 We replotted cts -> cts cm^-2, according to a referee’s advice. 

 

Comments 6. Fig.4: add a note to caption stating the color bar on right side of the plots gives delta chi^2 values.

Response 6. Done. The color bars to the right of the plots indicate the value of ∆χ2.

 

Comments 7. Fig.5: There is no black line (total radiation), please change the figure so it is visible. Does the beta value have no effect on the IC part of the spectrum?

Response 7. We made the Fig. clearer leaving only the black curves for the total radiation (and changing the figure caption correspondingly). In our case, beta value changes the IC part of SED at photon energies >= 10 TeV, which is not visible on the plot.

 

Comments 8. line 190: do you mean to say that the factor 3 should have been included by Chen et al? Does it affect their analysis (or just their derived B)? If just B then state what their derived B should have been.

Response 8.  Accounting for the constant multiplying factor of 3 would only change the multiplicative factor “ram pressure of polar wind” p_0 in Chen et al. (if the field from the pulsar is stronger than previously expected, one has to be further away from the pulsar to meet the same field in the emission zone). Generally, this would change the shape of the intrabinary shock (their eqs. 5-6) which may lead to other non-trivial consequences for the modeled light curves, but these changes are unlikely to change any principal conclusions. Since this discussion does not concern the present work, we removed the phrase about the factor from the discussion about Chen et al.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The modifications are perfect. On the conclusion section 4, some sentences may be added to present the significant meaning of this research and its future direction. 

Comments on the Quality of English Language

On the conclusion section 4, some sentences may be added to present the significant meaning of this research and its future direction. English should be improved. 

Author Response

We thank our anonymous referee for the comments, all of which we took into account, please see below. In the manuscript, we marked all changes in red as requested.

 

Comments 1: On the conclusion section 4, some sentences may be added to present the significant meaning of this research and its future direction. English should be improved. 

Response 1: This limit on the cutoff energy is among the strongest reported for PSR B1259-63. Further progress may be achieved with next-generation missions such as eASTROGAM \citep{eastrogam}, AMEGO \citep{Amego19}, and Gecco \citep{Gecco22}.

We expanded the conclusions section according to the referee’s advice, as well as corrected the rest of the section.