To: RPP Editors From: Peter Fritschel Re: LIGO manuscript. (LIGO: The Laser Interferometer Gravitational-Wave Observatory) Below are the two review reports we received, and our responses to them. Our responses immediately follow each of the recommendations, and are marked with an '*'. Report on ROP/242349/REV TITLE: LIGO – present status AUTHORS: Dr Peter Fritschel et al General comments: The paper is well written and easily readable. It gives a clear and complete overview of the LIGO detectors and their science results. To my mind, it deserves to be published in Reports on Progress in Physics. I have few minor comments listed below which will help (I hope) non expert people. Specific comments: Page 7, line 31: I will add “compact” after “very massive” which is not sufficient to generate GW and I will change “very high” by “relativistic” which is more meaningful. * Done P.8, l.7: it should be underlined that triangulation is very difficult using only H1, H2 and L1 as H1 and H2 are collocated. Without this precision, the sentence can be misleading for nonexperts. * Added: "Also, because the antenna pattern of an interferometer is quite wide, source localization requires triangulation using three separated detectors." P.9, l.6: TAMA 300 should be mentioned in the Japanese effort on GW detection. * Changed to: " In this period, the Japanese TAMA pro ject built a 300 m interferometer outside Tokyo, Japan [5]; ... In addition, plans are underway to develop a large scale gravitational wave detector in Japan sometime during the next decade [8]." P.14, section 4.4: no specification is given for length control in the first paragraph. It should be added for clarity. * We are unsure as to what the reviewer has in mind regarding adding a specification. We note that later in this section there is a value given for the residual arm length fluctuations. We can't think of any other parameters that would be of interest to the paper's audience, but are open to specific suggestions. P.15: a figure describing all loops involved in the laser frequency stabilization will be very useful. It is very difficult to follow the text otherwise. * Added new figure. P.16, l.26: I think that the picometers rms are micrometers rms. If not, the lock acquisition should be very easy. * Thank you, this is fixed. P.18, end of section 4.6: the existence of blind directions should be mentioned. * Added: " A single detector has blind spots on the sky for linearly polarized gravitational waves." P.25, l.3: why has L1 a significant lower duty cycle? * There was on site construction activity for the first 6 months of the run. This explanation isn't thought to be interesting enough to add to the text. P.28, l.30: “a chirp: a sinusoid increasing in frequency and amplitude until …“ * Done. P.31: Figure 10. The meaning of colors is not given in the caption * Done. As a general remark, the section about Bursts is quite messy and do not have the quality of the other sections. In particular, the pipeline differences between all-sky and triggered searches should be clarified. Here are some remarks to improve it: P.31, l.4: “short-duration” is not clear enough (< 1s, 10 s …?) * Modifed sentence to: "In addition to the well-modelled signals described in previous sections, we search for gravitational-wave “bursts”, defined as any short-duration signal (t <= 1 s) with significant signal power in the detectors’ sensitive frequency band (45 ≤ f ≤ 2, 000 Hz)." P.32, l.2: the text about short GRBs should clearly refer to the CBC search. It is misleading to write too much about it in the Burst section. * This section was modified to de-emphasize GRBs and emphasize externally triggered: "... allowing a more sensitive ``externally triggered'' search. For example, Gamma-ray bursts (GRBs), and Soft gamma-ray Repeaters (SGRs) are highly energetic events that make excellent targets for externally-triggered GW burst searches. While the progenitor(s) of GRBs are not entirely clear, most if not all short-duration GRBs are thought be produced by mergers of neutron stars or of a neutron star with a black hole, which would radiate a great deal of energy in GWs. Similarly, SGRs are believed to be neutron stars with very high magnetic fields ({\it i.e.}\ magnetars) that sporadically produce flares of electromagnetic radiation. The flares may be related to deformations of the neutron star crust which could couple to GW emission. If an associated GW signal for these progenitors is detected, the combined GW and EM/particle data will reveal complementary information about the astrophysics of the event." P.32, section 7.2.1: .. the paragraph about Coherent methods is barely understandable for non-experts. It mixes targeted searches (“if the arrival direction is known” and last sentence) and all-sky searches (“consistency test for each hypothetical arrival direction”). It mixes the potentialities with 3 or more detectors and the ones with only 2. As H1 and H2 are collocated, what is really feasible? .. The following paragraph is trivial and should be reduced. * The paragraph about Coherent methods has been rewritten, we think in a more understandable fashion. And the paragraph that follows this has been shortened. P.35, section 7.2.3: The first paragraph should be moved to CBC section as the conclusion is that it rules out the possibility of a binary merger. * We think this paragraph could really fit in either section, and would prefer to stick with our original choice. P.36: in the second paragraph describing the 3 approaches, it would be nice to know how the SNR depends on the integration time for the various method and what the time scales for duration (“short-duration = … seconds”) are * Modified the discussion to: "1) semi-coherent, long duration all-sky searches sensitive only to power and neglecting phase using the entire data set [84]; 2) coherent, short-duration all-sky searches sensitive to amplitude and phase but computationally limited to ≈ 5000 hr integration time [85]; 3) coherent, targeted searches for millisecond radio pulsars with accurate and stable ephemerides using the entire data set [86]" P.37, l.9: it is assumed that the noises are independent. Is it true for H1 and H2? Does it impact the results? * H1 and H2 are not used together for a CW search. P.42, l.6: I hope for LIGO that the last sentence will be come true as soon as possible, but, by now, as no GW detection occurred, it is not correct to claim for the birth of GW astronomy with Advanced LIGO. So, I propose to be less categorical. * This sentence has been changed and expanded into a full paragraph. Article ID: ROP/242349/REV Title: LIGO – present status Board Member's report I believe that "LIGO: The Laser Interferometer Gravitational-Wave Observatory," by the LIGO Scientific Collaboration should be published in ROP after minor revisions. The article should prove very interesting to the ROP audience, is scientifically correct, and, in the main, is very well written. However additional proofreading is needed. I found a number of typos and undefined acronyms. In addition, the transition from hardware to data analysis should be written more smoothly. * We looked at this, but believe that the transition is satisfactory as is. I recommend that a bit more detail be added (say one paragraph each) on GEO600, Virgo, and possibly other detectors, perhaps in footnotes. * We do not believe that more detail on other detectors should be added to this paper. This is a paper about LIGO, not all GW detectors. We believe that the mention we make of them, and the references given, are sufficient. I have appended a list of typos, undefined words or acronyms, and suggested changes. However, the authors should not assume that I found every problem of this nature and should reread the paper very carefully. p.9 last line misplace commas: Should be "with the GEO project and, although, with its shorter length, the GEO..." * This sentence is modified to: "Early in its operation LIGO joined with the GEO project; for strong sources the shorter, less sensitive GEO 600 detector provides added confidence and directional and polarization information." p. 10, line 3: replace "joint began" with "began joint" * Thank you, fixed. p.19. No explanation given for all the lines in the spectra until the caption to Fig. 6 and details on p. 24. People outside the field want to know that. A sentence here would be helpful. * Added the sentences: "The strain noise shown in Fig. 7 consists of spectral lines superimposed on a continuous broadband noise spectrum. The ma jority of the lines are due to power lines (60, 120, 180, ...Hz), “violin mode” mechanical resonances (340, 680, ...Hz) and calibration lines (55, 400, and 1100 Hz). These high Q lines are easily excluded from analysis; the broad band noise dominates the instrument sensitivity." p. 24 Barkhausen noise should have a reference. * We added a reference. p. 24 Insert S5 to define "run" * Thank you, done. p. 24 "Histogram" is not a verb. * Changed this sentence to: "One indicator of the sensitivity variation over the S5 science run is shown in Fig. 8, histograms of the rms strain noise in the frequency band of 100 – 200 Hz." p. 28 This is first use of Mpc. The unit it not generally known to physicists. * Added: " up to distances of hundreds of megaparsecs (Mpc, 1 parsec = 3.3 light years)." p. 28 Define Hubble time --- use age of the universe. * Done. p. 29 SNR first appears with no definition. * Added definition. p. 29 blue luminosity needs to be defined * It has been defined in a footnote. p.31 Fig. 10 caption should contain more detail. Why the spikes? * Added sentence: " When the ringdown frequency coincides with a spectral line the sensitivity is much reduced (300 $\mathrm{M/M_{sun}}$ corresponds to 60~Hz)." p. 37 Somewhere it should be made clear that rotating neutron stars are much weaker sources than CMC's so that the range of 500 pc is plausible. * Added sentence: "Compared with CBCs or Bursts, neutron star powered millisecond radio pulsars are a weak source of GWs which LIGO can detect only if the source is within a few hundred parsecs. Nonetheless, there are a large number of known sources that may be detected if they have sufficiently high ellipticity." p. 35 update discussion of Crab * We don't understand this comment. We added the reference for the Crab numbers, but this reference does use the latest timing data. p. 42 The last sentence needs some backing up info. Perhaps an event rate summary or table. * Yes, the last sentence is now a last paragraph with some detection rate info.