Talk:Cygnus X-1

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Contents

[edit] Mass

"The mass of the supergiant is approximately 20–30 solar masses. The compact object has a mass of 7–13 solar masses ... The X-rays are produced in an accretion disk that is formed by matter flowing from the supergiant into the black hole."

If the star is more massive than the black hole, why is matter flowing away from it? Evercat 12:48, 28 July 2006 (UTC)

The deciding factor is density, not mass. -Toptomcat 22:30, 20 September 2006 (UTC)

Matter is flowing away from it because it has a stellar wind, common to all stars of its type. The stellar wind is driven by scattering of the star's radiation in its outer layers.

[edit] Musical Connections

What does "Musical Connections" have to do with this subject? I see absolutely no reason for the entire section.

Possible reason: people may get to this page by searching for information on the Rush song.

Some people seem like to adding cultural influences into scientific articles such as these. (See stars and planetary systems in fiction, as an example.) Unfortunately they can be a nuisance to properly cite, so they often end up being moved elsewhere. As a possible solution, the section could be moved to a disambiguation page.—RJH (talk) 19:42, 17 March 2008 (UTC)
Done.—RJH (talk) 18:26, 29 March 2008 (UTC)

It might be worth having a section called "In popular culture" or similar, and insisting that anything that is added into it has to be cited (i.e. revert any additions that are uncited). Mike Peel (talk) 16:06, 5 May 2008 (UTC)

See Cygnus X-1 (disambiguation). I didn't want to deal with all the unsourced material that is only indirectly related to the subject matter. Plus the cultural influence seemed quite trivial when compared to that of the planets and some brighter stars.—RJH (talk) 21:01, 5 May 2008 (UTC)

[edit] Quasi-periodic oscillations

The following paragraph asserted an explanation for Quasi-periodic oscillations—being caused by the magnetic field—that is not matched by the references I checked. The Quasi-periodic oscillations article also states that the explanations for QPOs are controversial, so I think this is too conclusive an assertion. I'm moving it here until suitable sources can be found.

X-rays flicker about certain frequencies in a poorly-understood phenomenon called Quasi-periodic oscillations. Exactly repeating pulses have never been seen from Cygnus X-1.[1][2] Such pulsations are typical of neutron stars, and if seen would rule out a black hole. The pulsations from neutron stars are caused by the neutron star's magnetic field and the no hair theorem guarantees that black holes do not have magnetic poles.[citation needed]

—RJH (talk) 18:24, 29 March 2008 (UTC)

I believe that QPOs are now recognized as occurring in black-hole as well as neutron-star systems. Off the top of my head I think GRS 1915+105 and GRO J1655-40 may be examples of such BH systems. The Wiki QPO article cites RHESSI observations, and references Wenfei Yu (2007)[3], but I have not checked any references myself, and I am uncertain if evidence of QPO has been seen in Cygnus X-1. Wwheaton (talk) 19:49, 30 March 2008 (UTC)
There are a couple of references I saw that mention QPOs for Cygnus X-1:
This article mentions using the QPOs to measure a black hole's mass:
So perhaps it needs to be mentioned for that reason? Otherwise I'm not sure I understand what they are saying, other than that QPOs exist.—RJH (talk) 15:53, 31 March 2008 (UTC)
Yes, I guess it is clear from these refs that the original idea, that BHs simply could not support QPOs, does need to be abandoned, and the fact that QPOs are observed here is a property that deserves mention in the Cyg X-1 article. I am not certain that QPOs are all that certainly understood, so the whole business may be in the gray area of unsettled knowledge. But if so then the observation of QPOs in Cyg X-1 either says something about Cyg X-1 not being a BH (and we are pretty certain it is), or something more general about the cause(s) of QPOs. Either way, it is interesting. So I definitely would put in a sentence or two saying they are observed, with the two refs for Cyg X-1 (Agrawal&Rao, & Rutledge et al.), and also a link to the Wiki QPO article explaining what they are in more detail. Good! Bill Wwheaton (talk) 20:38, 31 March 2008 (UTC)
Agrawal & Rao (1998) says that, "Very rapid variability, flickering, irregular fluctuations and strong quasi-periodic oscillations (QPOs) are often observed in the X-ray light curves of black hole sources (see van der Klis 1995 for a review)." So that seems pretty definitive in not ruling out a BH for Cygnus X-1.—RJH (talk) 19:52, 2 April 2008 (UTC)

[edit] Accretion disk

Hi RJH, I just hit an edit conflict with you in this section. I have copied the subsection as I had it below; use it any way you want.

A Chandra X-ray spectrum of Cygnus X-1 showing a characteristic peak near 6.4 keV due to ionized Fe (iron) in the accretion disk, but the peak is gravitationally red-shifted, broadened by the Doppler effect, and skewed toward lower energies. Credit: NASA/CXC/SAO/J.Miller et al.
A Chandra X-ray spectrum of Cygnus X-1 showing a characteristic peak near 6.4 keV due to ionized Fe (iron) in the accretion disk, but the peak is gravitationally red-shifted, broadened by the Doppler effect, and skewed toward lower energies.[4] Credit: NASA/CXC/SAO/J.Miller et al.

Though highly and erratically variable, Cygnus X-1 is typically the brightest persistent source of hard X rays—those with energies from over about 30 up to several hundred keV—in the sky.[5] The compact object is believed to be orbited by a thin, flat disk of accreting matter. This disk is intensely heated by friction between plasma in faster-moving inner orbits and that in slower outer ones. It is divided into a hot inner region with a relatively high level of ionization, and a cooler, less ionized outer region that extends to an estimated 500 times the Schwarzschild radius,[6] or about 15,000 km. The X rays are produced as lower energy photons in the thin inner disk, then modified by Compton scattering from very high temperature electrons in a geometrically thicker, but nearly transparent corona enveloping it, as well as by some further reflection from the surface of the thin disk.[7] An alternate possibility is that the X rays are Compton scattered in the base of a relativistic jet instead of in a hot corona.[8]

Cygnus X-1 changes unpredictably between at least two X-ray states, although it may vary continuously between those states as well. In the most common state, the spectrum is "hard", and more of the X-rays have high energy. In the less common state, the spectrum is "soft", with more of the X rays having lower energy. The soft state also shows greater variability. The hard state is believed to originate in the hot corona surrounding the inner part of the more opaque thin accretion disk. The soft state occurs when the inner disk draws closer to the black hole (possibly as close as 150 km), accompanied by cooling or ejection of the corona. When a new corona is generated, Cygnus X-1 returns to the hard state.[9]

The X-ray flux from Cygnus X-1 varies periodically every 5.6 days, especially during superior conjunction when the orbiting objects are most closely aligned with the Earth, and the compact source is most distant. This indicates that the X-ray flux is partially blocked by circumstellar matter, possibly the strong stellar wind of the star HDE 226868. There is also a roughly 300 day periodicity in the emission that may be caused by the precession of the accretion disk.[10]

Cheers, Bill Wwheaton (talk) 23:40, 2 April 2008 (UTC)

Thank you. I think I managed to merge in your changes.—RJH (talk) 15:09, 3 April 2008 (UTC)

[edit] Gravitational Redshift

"As self-luminous clumps of matter spiral into a black hole, their radiation will be emitted in a series of pulses that are subject to gravitational redshift as the material approaches the horizon. That is, the wavelengths of the radiation will steadily decrease, as predicted by General Relativity."

Surely the radiation wavelength will increase, not decrease, if it is redshifted? Gburto01 (talk) 15:00, 3 April 2008 (UTC)

You are correct of course. Thank you.—RJH (talk) 15:10, 3 April 2008 (UTC)

[edit] Survey

WP:Good article usage is a survey of the language and style of Wikipedia editors in articles being reviewed for Good article nomination. It will help make the experience of writing Good Articles as non-threatening and satisfying as possible if all the participating editors would take a moment to answer a few questions for us, in this section please. Would you like any additional feedback on the writing style in this article? If you write a lot outside of Wikipedia, what kind of writing do you do? Is your writing style influenced by any particular WikiProject or other group on Wikipedia? At any point during this review, let us know if we recommend any edits, including markup, punctuation and language, that you feel don't fit with your writing style. Thanks for your time. - Dan Dank55 (talk) 16:01, 9 April 2008 (UTC)

P.S. The survey will end on April 30. - Dan Dank55 (talk) 20:36, 11 April 2008 (UTC)

Seriously Dan? Well okay. Having taken articles through FAC process before, I understand that writing stye and good grammatical techniques can be a concern. So yes I do appreciate feedback, but usually I prefer specific instances rather than generalities. My non-Wikipedia writing is part fiction and part technical; the former is a hobby while the latter is mainly for professional services. I'm certainly not a commercial writer, nor am I specially trained in that regard.
My main concern with reviewers who edit for markup, punctuation and language is to make certain that the nuances of the scientific information don't get lost in the process. But those I will go back and address. In addition, for a page as heavily referenced as this one, the introduction of unsourced information is an issue for me, even if the details are correct. I also prefer that references use the cite templates. Otherwise, I appreciate anything that will improve the readability, flow and enjoyment of the article, and will increase the odds of it getting through a FAC. Thank you.—RJH (talk) 19:06, 9 April 2008 (UTC)
I'm trying not to say too much before the survey is completed. Are you referring to cite templates such as {{cite web}}, {{cite news}}, {{cite book}}, and {{cite journal}}? I'm asking because those seem to be the most common ones, probably because they're accessible through the "my preferences/gadgets" tab. Regarding your preference for this page on sourced/unsourced information, you might want to skim recent discussions at WT:V and WT:CITE, I'll give you links if you like, but the bottom line is: it's not only okay to have "community standards" regarding citing, it's assumed that each page has them (stated or unstated), and you strengthen your position regarding an article if you put your preferences in writing, probably on the talk page. Just a quick note like, "Please note that most of this material is not common textbook knowledge, so the editors so far have preferred to go heavy on citations" might strengthen your case if someone comes along later and tries to establish the opposite viewpoint (not that I think they would have much success with this article!) Another thing you could stick on the talk page is "I would like to get this article through FAC some day", that would help to solidify the preference to follow MoS in this article. (These suggestions might or might not be helpful, since you're an experienced editor ... were they?) - Dan Dank55 (talk) 19:14, 12 April 2008 (UTC)
Yes, I meant the "cite *" templates. I'm not sure how often editors check the talk pages before modifying the article page; I know I rarely do (at least on random pages) unless I see a significant problem with the article. Thank you.—RJH (talk)
If they don't check the talk page or pay attention in any way to the community standards, all the better for you when you're looking for support for your edits. - Dan (talk) 20:45, 15 April 2008 (UTC)

[edit] Some style questions

I may not be following, but the article seems to shift back and forth between "a [probable, possible] black hole" and "the [certain] black hole". I might be missing something. - Dan Dank55 (talk) 20:33, 11 April 2008 (UTC)

Overall I was trying to avoid the position of 100% certainty, even though the evidence is fairly strong. But it seemed a little awkward trying to say something like, "the remaining matter is believed to pass through the event horizon of the possible black hole". I.e. in order to pass through a event horizon, there must be a black hole present. Having the word "believed" in there seemed to be sufficiently cautious.—RJH (talk) 21:39, 11 April 2008 (UTC)
I agree with the position that certainty of Cyg X-1's nature should be avoided. --Stlemur (talk) 22:19, 11 April 2008 (UTC)
This seems like an interesting question to me: under what circumstances is "the" a word to avoid? It's not mentioned at WP:WORDS, which is relevant here because it's one of the 6 specific style requirements that GAN articles are supposed to pass. I've asked on the talk page there. - Dan Dank55 (talk) 22:52, 11 April 2008 (UTC)
I made some modifications to the text that I hope will address your concerns. Thank you.—RJH (talk) 15:06, 13 April 2008 (UTC)
Elegant edits, that does it nicely. That gave me some nice food for thought which I hope will lead to people accepting my proposed changes to WP:WORDS. I'm not your GAN reviewer btw, or at least, I'm not available until May 1, it might bias the survey if I start telling people what I like and I don't like. - Dan Dank55 (talk) 19:44, 13 April 2008 (UTC)
No problem, I appreciate the feedback. Thank you.—RJH (talk) 14:48, 14 April 2008 (UTC)
My pleasure. - Dan (talk) 21:06, 15 April 2008 (UTC)

[edit] GA Review

OK, I'll be reviewing this article and should give an assessment (pass, fail, hold) in two days time at the most (ie: by saturday). Thanks for the work so far though, it looks much better than before you touched it Acer (talk) 22:10, 24 April 2008 (UTC)

Thanks.—RJH (talk) 15:39, 25 April 2008 (UTC)

[edit] Passing GA

GA review (see here for criteria)

This is a sure pass, I left some comments bellow but it's nothing that would get in the way of GA.

  1. It is reasonably well written.
    a (prose): b (MoS):
  2. It is factually accurate and verifiable.
    a (references): b (citations to reliable sources): c (OR):
  3. It is broad in its coverage.
    a (major aspects): b (focused):
  4. It follows the neutral point of view policy.
    Fair representation without bias:
  5. It is stable.
    No edit wars etc.:
  6. It is illustrated by images, where possible and appropriate.
    a (images are tagged and non-free images have fair use rationales): b (appropriate use with suitable captions):
  7. Overall:
    Pass/Fail:
    Shame it wasn't reviewed earlier, it's a fine article. And even more so when considering the state of other Extrasolar objects GAs

[edit] Comments

  • Does Cygnus X-1 refer to both the black hole and the star system? And is the x-ray source the compact object by itself or the combination of both the black hole and the star? The article uses Cygnus X-1 to refer to both but it's never explained. Also, in the lead, you define Cygnus X-1 as a compact X-ray source I take it you mean the binary system and not the black hole itself (it's not immediately clear)?
    • I've tried to modify the text to specifically say "HDE 226868/Cygnus X-1" when it is talking about the binary system as a whole. By "Cygnus X-1", the text just means the candidate black hole.—RJH (talk)
  • I had trouble understanding the paragraph bellow, is it corroborating with the assertion that Cygnus X-1 is a black hole or not? I can't tell.
    The space-based Chandra X-ray Observatory was used to measure the spectral signature of iron atoms orbiting near the object. A rotating black hole drags the nearby space around with it, which allows atoms to orbit closer to the event horizon. In the case of Cygnus X-1, none of the atoms were found orbiting closer than 160 km, indicating that, as a candidate black hole, it is not rotating to any significant degree.
    It is saying that, if this is a black hole then it is not rotating. I'm not sure at present how that could be made clearer.—RJH (talk)
Thanks for adressing the issues I raised so quickly, about this paragraph I had indeed understood that it was not rotating, I think my confusion arose from the word candidate, I assumed from its presence there, that the rotation or not of x-1 had some bearing on its chances of being a black hole (ie: if its rotating then chances are its a black hole or vice versa) But it could just be me. Again thank you for your work Acer (talk) 19:35, 27 April 2008 (UTC)
  • Can you try to provide a little explanation of how suborbital rocket launches helped detect the x-rays? It's not obvious to the lay reader that rockets can be used for that purpose. Acer (talk) 23:28, 25 April 2008 (UTC)
    • Okay I'll see what I can come up with. Thank you for the review.—RJH

(talk) 19:16, 27 April 2008 (UTC)

Rockets and balloons were widely used in the early days before satellites. Shorter wavelengths than ultraviolet light have greater penetrating power, and get deeper in the atmosphere. X rays of a few keV energy easily penetrate to altitudes reached by sounding rockets, and Cygnus X-1 is so strong that it can be detected in a few seconds even by small instruments on such probes. X rays of energy higher than 20 keV can be observed from instruments carried on balloons to altitudes as low as 35 or 40 km. Cyg X-1 is very powerful from 20 to 500 keV. Wwheaton (talk) 06:11, 28 April 2008 (UTC)
Much of this would be good information to include in the X-ray astronomy article.—RJH (talk) 21:19, 28 April 2008 (UTC)

Congratulations to RJHall, who has done massive work on this article for many weeks. This is clearly one of the most important x-ray sources in the sky, both for its properties and for its historical role as the first generally accepted real black hole. It deserves a good article, and now it has one. Bill Wwheaton (talk) 01:28, 27 April 2008 (UTC)

Thank you very much for your help and your input.—RJH (talk) 19:16, 27 April 2008 (UTC)

[edit] Accretion efficiency?

Hi, RJH -- In the section on "Jets", you suggest most of the accretion energy is dissipated in jets. I wonder if that is really true. First of all, a lot comes out in radiation; I think the x-ray and gamma-ray luminosity is on the order of a few times 1036 erg/s if I recall. (I see that you quote tne energy in the jets of being at least of the same order.) Second of all, a lot of the energy probably goes down the hole and disappears from view. It would be very interesting to have some numbers, or likely ranges, for this energy balance. Also interesting would be the likely amount of mass lost from HD226868 that is actually accreted. The artist's impression figure looks like Roche lobe overflow, and in that case I guess the fraction could be nearly 100%, and the accretion energy would simply be the mc²  limit. But for mass loss via a stellar wind, the fraction could be smaller, I don't know how small.

The efficiency of black holes conversion of accretion energy into released energy (radiation, jets, etc) has been a question for decades I think, and I do not know how much progress we have made on the issue lately. But it seems fundamentally interesting to me that, as similar as BHs may appear to some neutron stars, yet they do not have to conserve energy in the same sense, because they can stash an unlimited amount by dropping it down the hole. Whatever is known about this for Cygnus X-1 would be most interesting to put into the article, and it seems to me that after all the work you have done you may be a good person to know the story.

Anyhow, as a practical matter (which is what brought me here in the first place), I think you might do well to change the "Most of" [this released energy] to "Some fraction of", unless you know the answer, which you may.

Continue to be delighted by what you are doing. Cheers, Bill Wwheaton (talk) 18:52, 1 May 2008 (UTC)

Good point. I changed "Most" to "Part". The sources suggested that roche lobe overflow was not occurring. Perhaps the illustration is exaggerating the matter flow slightly? I don't think I've seen a good source for how the energy is being distributed; I'll look around some more. Thank you.—RJH (talk) 20:58, 5 May 2008 (UTC)

[edit] Intro still needs work

I gave up on the intro on the FA because I was watching what happens in these processes. But I think it's still pretty poor. Specifically:

Cygnus X-1 (abbreviated Cyg X-1)[11] is a compact X-ray source in the constellation Cygnus. It is one of the first black hole candidates to be discovered and is amongst the most studied astronomical objects in its class.[12] Cygnus X-1 was discovered in 1964 using X-ray instruments that were carried aloft by suborbital rocket launches from White Sands Missile Range. This compact object is an estimated 8.7 times the mass of the Sun[6] and its size is 300 km or less, which is sufficiently dense to be a black hole.[13]

How is the discovery date possibly required in the intro? If it was discovered in 1865, 2065 or star date 2043.40, how does that possibly change anything about the subject of this article? And more worrying, as the text stands now, it implies that the rocket launches in 1964 measured the object to be "8.7 times the mass of the Sun and its size is 300 km or less". This is extremely misleading prose.

I was told that the text was added after one FA reviewer suggested it needed to be placed in the intro. I disagree with this reviewer. The FA jockey also seemed to be blasé about the text. So unless anyone really has a good argument to the contrary, I'd like to remove it.

Maury (talk) 21:28, 14 May 2008 (UTC)

Let me take a crack at it. No big prob, I hope. Wwheaton (talk) 04:41, 15 May 2008 (UTC)
OK, see what you think. I hope the material in the intro does not have to be too cluttered with references if they are supplied later in the body, but some may need to be added. The main change of sense is that its size is not really measured to be 300 km, we just know that it cannot be too much larger. The flickering could come from regions smaller than the event horizon, but it must have at least ~8 solar masses, which is too much for a neutron star of other kind of known thing. Feel free to tweak, obviously. Bill Wwheaton (talk) 05:13, 15 May 2008 (UTC)
Wow, yes, that greatly improves it IMHO. Maury (talk) 11:55, 15 May 2008 (UTC)
Unfortunately I had to made a few modifications to the updated lead. The use of the word "strong" conflicts with Wikipedia:MoS#Unnecessary_vagueness, so I updated it to say as seen from the Earth. Including the discovery date in the lead is common practice on wikipedia, so I think that should stay. Further down, some wording had been changed to say "If so, the radius of its event horizon is probably about 300 km." This conflicts with the value of 26 km later in the article, so I modified that back to be an upper limit. Finally I revised the modified 3rd sentence as the wording seemed unwieldy to me. No offense intended, and I'm sorry if this conflicts with some of what you were attempting to do. But I do think it is important to respect the opinions of the FAC reviewers and the changes they wanted to have made. They have a lot of experience in wikipedia standards, and a lot of their recommendations made sense to me.—RJH (talk) 16:13, 15 May 2008 (UTC)
Thanks for the touch-up work. I am embarrassed about adding an extra digit to the size of the event horizon, it was late...  :) It should be 3 km per solar mass, of course. The point I meant to make was that the mass measurement determines the size, although the size is constrained to be less than ~300 km by the variability time scale.
I did not consider that "strong source" is considered vague on Wikipedia. I'm not sure what to do about this, as I believe the term "strong" is universally used in modern astronomy for all objects (radio, infrared, X-ray, gamma-ray...) which are radiating outside the optical (where one can say "bright" instead. For absolute strength, independent of distance, we ordinarily say "luminous"). I almost think this is a point of terminology that we should perhaps raise on the Wikipedia:WikiProject Astronomy or Wikipedia:WikiProject Astronomical objects pages, and perhaps give it a disambiguation definition, as it is jargon that is so established that I can't imagine it changing, without and Act of the IAU or something.
Wwheaton (talk) 16:38, 15 May 2008 (UTC)
I found a source that might serve:
  • Lewin, Walter; Van Der Klis, Michiel (2006). Compact Stellar X-ray Sources. Cambridge University Press, p. 159. ISBN 0521826594. 
This lists the maximum X-ray flux from Cyg X-1 as 2,300 μFy. Would that serve as a measure of strength? So something like this:
...and is one of the strongest, emitting an X-ray flux of up to 2,300 μFy.
—RJH (talk) 19:02, 15 May 2008 (UTC)
There are several issues to keep in mind. First is that to be quantitative, one must compare sources at the same frequency, (or wavelength, or photon energy, whatever one considers appropriate for the case in point). Sco X-1 is normally the strongest source in the mid x-ray band, say 3-10 keV, followed by the Crab Nebular & pulsar (considerably weaker) and Cygnus X-1, about the same as the Crab. But then by 20 keV Sco X-1 is fading fast (it has a relatively "soft" spectrum), and almost disappears above 30 keV or so, while Cyg X-1 is brightest between about 30 keV and 100 keV. But the Crab then regains the title somewhere above 100 keV, since it has a hard "power-law" spectrum (a straignt line on a log-log plot), while Cyg X-1 usually rolls off a bit at higher energies. But there is a further complication: Cygnus X-1 is highly variable, on many time scales. Sometimes it drops by a factor of two or more from its usual level, for days or weeks at a time, and its spectrum often "pivots", so that its flux on a log-log plot is anti-correlated above and below some pivot energy. I remember a balloon flight in February 1970 with a 100 cm2 area scintillation detector where we essentially did not see it at all above ~20 keV, though we looked right at it, and ordinarily is would have been expected to blow us out of the water. Sco X-1 also flickers a little erratically, but the Crab is so constant people have often used it for a calibration source. There are also flaring "transient" sources that can be much brighter than any other for days or a few weeks.My point in all this is just that qualitatively speaking, Cygnus X-1 can be described as one of the strongest hard X-ray sources in the sky, but to say more you have to carefully quote the time and energy band of observation. Which is of course routinely done in observational papers all the time. Wwheaton (talk) 07:53, 16 May 2008 (UTC)
Well I think this tells me that it will be difficult even to clarify the meaning of the work "strong" here, so it is not clear to me that the word adds anything to the lead. Is there a way it could be explained in simple terms without overly burdening the lead? Thanks.—RJH (talk) 15:50, 16 May 2008 (UTC)

(undent) Ok well now I think it has regressed considerably. What does rocket launch have to do with strongest source? Nothing. So why are they in the same sentence? I don't have any problems with the content, but the arrangement is ugly. Maury (talk) 23:26, 15 May 2008 (UTC)

Here was the original paragraph following promotion:
Cygnus X-1 (abbreviated Cyg X-1)[11] is a compact X-ray source in the constellation Cygnus. It is one of the first black hole candidates to be discovered and is amongst the most studied astronomical objects in its class.[5] Cygnus X-1 was discovered in 1964 using X-ray instruments that were carried aloft by suborbital rocket launches from White Sands Missile Range. This compact object is an estimated 8.7 times the mass of the Sun[12] and its size is 300 km or less, which is sufficiently dense to be a black hole.[13]
This is the re-write by Wwheaton, followed by tweaks from myself:
Cygnus X-1 (abbreviated Cyg X-1)[11] is a strong X-ray source in the constellation Cygnus. It was one of the first X-ray sources found, in a 1964 rocket flight, and is one of the strongest seen from Earth. It was also the first X-ray source widely considered to be a black hole candidate and is amongst the most studied astronomical objects in its class. It is now estimated to have a mass about 8.7 times the mass of the Sun[12] and a size of 300 km or less. Hence it is too compact to be any known kind of normal star, or other likely object, besides a black hole.[14]
I was happy with the original as it satisfied the FAC reviewers, but I'd like to reach a consensus on the best way to write this. Any suggestions?—RJH (talk) 16:04, 16 May 2008 (UTC)
I do not have very strong feelings, but I guess I prefer the last. (BTW I will be away this weekend, back online Monday I expect.) Cheers, Wwheaton (talk) 17:33, 16 May 2008 (UTC)

[edit] Proposed lead

Here's what I hope is a compromise modification to the lead:

Cygnus X-1 (abbreviated Cyg X-1)[11] is a strong X-ray source in the constellation Cygnus. It was found in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux of 2.3×10-23 Wm-2Hz-1.[15] Cygnus X-1 was the first X-ray source widely considered to be a black hole candidate and is amongst the most studied astronomical objects in its class. It is now estimated to have a mass about 8.7 times the mass of the Sun[12] and has been shown to be too compact to be any known kind of normal star or other likely object besides a black hole. If so, the radius of its event horizon is probably about 26 km.[16]

I felt that the "It was one of the first X-ray sources found" was outweighed by the "Cygnus X-1 was the first X-ray source widely considered to be a black hole candidate", so I snipped out the former for purposes of flow and brevity. Hope that's okay. I also listed a maximum flux rating with the understanding that it can be expanded upon in detail down in the article body.

Will this work?—RJH (talk) 18:42, 16 May 2008 (UTC)