Subject: MNRAS: MN-07-1879-MJ From: cg@ras.org.uk Date: Mon, 14 Jan 2008 09:14:17 -0500 (EST) To: dcohen1@swarthmore.edu Dear Prof. Cohen I attach the reviewer's comments on your manuscript entitled "Chandra spectroscopy of the hot star beta Crucis and the discovery of a pre-main-sequence companion", ref. MN-07-1879-MJ, which you submitted to Monthly Notices of the Royal Astronomical Society. Minor revision of your manuscript is requested before it is reconsidered for publication. You should submit your revised version, together with your response to the reviewer's comments via the Monthly Notices Manuscript Central site http://mc.manuscriptcentral.com/mnras . Enter your Author Centre, where you will find your manuscript title listed under "Manuscripts with Decisions." Under "Actions," click on "Create a Revision." Your manuscript reference will be appended to denote a revision. IMPORTANT: do not submit your revised manuscript as a new paper! 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If you have received this message in error, do not open any attachment but please notify the sender (above) and delete this message from your system. Editor's Comments: Because of the paper's length, I will be looking to see that all the referee's suggestions regarding the figures that may be cut, are implemented. Reviewer's Comments: Reviewer: Ian Howarth This paper presents the results of an almost day-long Chandra observation of beta Cru. This observation was intended to throw light on issues of X-ray production in hot-star winds by providing data from a relatively low-density regime; as it turns out, the data, and their analysis, throw up new puzzles (or underscore old ones). The serendipitous discovery of an X-ray-bright companion is also reported. The paper is clear, comprehensive, and presents new results of significant interest; it takes the analysis as far as reasonably possible, using state of the art models. I enjoyed reading it, and am very happy to recommend that it be published in MN. Inevitably, I have one or two suggestions for possible revisions, exclusively on minor presentational matters. The authors may care to take these under consideration, but I wouldn't require them to make corresponding changes if they feel them to be unnecessary or inappropriate. In the order in which they crop up in the text: (i) Section 3. I'm afraid that the first couple of paragraphs suffer greatly from "observer familiarity", leaving the general reader bewildered about, e.g., what exactly the "ACIS-S/HETG configuration" is, what "HEG" and "MEG" are, and, especially, what the purpose of "observation-specific rmfs and garfs" might be. Can we have some references to instrument/software writeups, not only for the benefit of today's reader from outside the field, but also for the reader twenty years hence - today's jargon will become tomorrow's indecipherable code. [Note that the MN production staff will require expansion of all acronyms on first usage, as a matter of house style.] For what it's worth, it seems to me that the key information of basic interest is the same for all spectroscopy: resolution, signal/noise, and wavelength/energy coverage - none of which are given in this introduction to the data. Indeed, I didn't notice a resolution quoted anywhere, which concerned me given that much of the discussion revolves around line profiles that "are only barely resolved" (see item (v) below). (ii) While I have considerable sympathy with the authors' discussion of the inappropriateness of formal errors (Section 4.1.1), their insight and experience might allow them to give a rough quantitative indication of likely realistic uncertainties, perhaps in the caption to Table 3, for the benefit of the less informed reader. [As a specific example: I was unsure how to reconcile "abundances... overall are slightly subsolar is...robust" with "only O and Mg show...subsolar abundances, while others are consistent with solar". Is the suggestion that there is a uniform, global subsolar metallicity (ca. 0.7 solar?), or that only O and Mg are depleted? I couldn't think of an astrophysical reason why Mg might be depleted in this star; is the suggestion that the depletion is physical, or could be the result of shortcomings in the analysis?] For the authors' info, prompted by this paper I measured the interstellar column of neutral atomic hydrogen to beta Cru, from the merged IUE spectrum. Because the number is very low, the answer depends slightly on the adopted photospheric profile; I think it must be somewhat larger than the adopted 3.5E19/cm2, but my best estimate is only around 6(+/-1.5)E+19. I infer from the authors' comments that this will make no significant changes to their numbers. (iii) The clarity of several of the diagrams could be improved, if my (reasonably good-quality) printout is anything to go by. Fig 6, the "grey" and "black" symbols are barely distinguishable (perhaps use open and filled?), as are the "circles" and "squares" in Fig 13 (ditto open/filled). The dotted diagonal line in Fig. 16 is essentially imperceptible. (iv) As a related point: the MN editorial office raised the overall length of this paper as a possible "issue". The writing style is engaging rather than excessively formal; though one might edit down a line or two here or there (notably in the rather discursive Section 2 [what are the other two nations?]), I think that any savings would be minor, and would detract from the readability. However, it seems to me that several of the figures are redundant or unnecessary (at least, their necessity wasn't clear to me). Fig 16, and its accompanying discussion, seemed to me pretty pointless; why infer the significance and form of the light-curve from a CDF when it's directly evident (as in "bleeding obvious") in Fig 17 (where I think the caption might identify the "companion" rather than the "secondary", for consistency)? Similarly, I didn't really see much virtue in presenting both Figs 3 and 5 (if the caption to Fig 5 were to give a rough indication of the total number of detected counts in the stronger lines); both Figs 4 and 13; and both Figs 14 and 15. Fig 2 seemed a bit indulgent, too; I'm quite happy to take on trust a simple statement that none of the Park & Finley stars were in the field. (v) I was unsure of the basis for (and surprised at) the claims associated with Fig 8, that the delta-function model "cannot be absolutely ruled out", while "the modestly wind-broadened profile is preferred...at the 99% confidence level". From very rough measurements of a blown-up version of Fig 8, and assuming the error bars shown are 1-sigma, I estimate chi-squareds of about 6 for both models, from the 13 points between 14.98A and 15.04A (okay, maybe 5.5 for the wind model, 6.5 for the narrow line). What exactly is the test that so strongly discriminates between them? Noting that the lines are *barely* resolved (as is very clear from the delta-function model in Fig 8), I wonder what "cannot be absolutely ruled out" really means; could it be "is entirely consistent with the observations"? Of course, it would open up a whole new can of worms if it were suggested that the emission didn't come from an extended wind at all...(modelling of f/i ratios notwithstanding). [I guess the answer may be that other lines, not plotted, provide stronger discrimination, individually or collectively? But how well is the instrumental PSF understood? Could it be, say, 10% broader than assumed? That is, is it known, and spatially and temporally invariant, at a level that assures confidence in elimination of the delta-function model? Could the companion's spectrum provide a reliable PSF in this particular case?] Ian Howarth UCL, 11/1/2008