# Talk:Cosmic X-ray sources

Here are some questions/suggestions/corrections for the authors to consider:

1. Introduction, paragraph 1: X-rays from 20-100 keV penetrate the atmosphere to altitudes of order 50km and are routinely detected in ballon flights. If the authors wish to discuss only observations from space (above 100 km) they should reduce the energy range to 0.5 to ~20 keV. If they prefer to retain the 0.5 -100 keV range they will need to modify their assessment of where such observations can be made.

2. Introduction, paragraph 6: " ...X-rays are the lowest energy photons that can reach the earth from cosmological distances without significant absorption." I find this statement a bit misleading or at the least confusing. JWST will be observing some of the earliest stars and galaxies at very high z in a few years time and the CMB photons clearly reach us from a very early epoch. Perhaps the authors were intending to discuss photons more energetic than visible light in which case they should make this more explicit.

3. Instrumentation, paragraph 3: I suggest that the order of resolution and size be swapped in the text to correspond to the order in which the various pairs of numbers appear. I also note that Einstein was launched in November 1978 (not in 1979). The field of view of the Chandra imaging devices is ~16x16 arc minutes for ACIS and ~30x30 arc minutes for the HRC.

4. Instrumentation, paragraph 4: The XMM-Newton is a Wolter optic and does not use an approximation to the ideal conics section (as stated by the authors).

5. Solar and stellar X-ray emission, paragraph 2: It is actually the interaction of stellar winds in binaries as well as magnetically channeled wind shocks which have been used to model recent Chandra observations of O supergiant stars. Stellar wind- ISM interactions may well be responsible for the diffuse emission seen in young star clusters.

6. It would probably be appropriate to include a reference to the researcher and/or the paper for Figure 4 on the Crab It was not readily apparent who that might be - possibly J. Hester et al.

7. Supernova remnants...final paragraph and Figure 5: The text mentions astronomers Tycho and Kepler while showing an image of Cas A which likely exploded nearly 100 years later. There are superb Chandra images of Tycho and Kepler SNRs available on the Chandra web-page as well as a deeper, more detailed image of Cas A should the authors wish to update this area a bit.

8. Binary X-ray sources, paragraph 3: ..x-ray luminosity of 10^38 is actually 10,000 times the total luminosity of the Sun (note addition of word "total") and probably ~10^10 times the x-ray luminosity of the Sun.

9. Binary X-ray sources, paragraph 5: word "day" missing after "5.6"

10. X-ray emission from galaxies...: First paragraph: "diffuse" rather than "diffused" .

Second paragraph : 2nd sentence: consider adding the word "x-ray" before "emission" in 2 places

11. The X-ray background: Paragraph 1: typo - "major" rather than "mayor".

Paragraph 4: 2nd sentence: add "up to ~5 keV" (or 8 keV if the authors prefer) at the end of the sentence

12. X-ray emission from clusters..: Paragraph 1: consider mentioning the detection of Fe lines to demonstrate the thermal nature of the emission (2nd sentence) and add an appropriate reference - perhaps Serlemitsos et al, 1977,ApJ,211,L63.

Paragraph 4 last sentence - typo "through" rather than "trough" .