01 · Question
One falsifiable projected question
In a pre-declared, source-masked XMM EPIC source-plus-background analysis, can a soft thermal component A_pair be constrained within the frozen pair-scale projected domain of NGC 4490/4485, or can a qualified upper bound be produced under the current per-camera operator?
"Pair-scale" here does not mean drawing the two galaxies together. The primary quantity A_pair must be defined in the same response domain, together with a pre-frozen FK5 region, source mask, GTI, background model, and per-camera response. None of these qualification conditions are yet complete.
02 · Why it matters
Existing work saw a hot ISM on Chandra; it has not answered whether that conclusion transfers to the current EPIC operator
Richings, Fabbiano, Wang & Roberts (2010, ApJ 723, 1375) used ~100 ks of Chandra ACIS-S data and, after removing discrete sources, performed spatially resolved hot-ISM/outflow spectroscopy. Gladstone & Roberts (2009, MNRAS 397, 124) combined Chandra with a 2002 XMM EPIC observation to study source-centred spectra and variability of six ULXs, but did not fit a source-masked pair-scale diffuse component.
Taken together, these results mean: the hot ISM of NGC 4490/4485 is established on Chandra, but "can a current, source-masked, per-camera EPIC pair-scale operator recover or constrain the same component" remains open. The value of I173 is to turn this transferability premise into a narrow, explicit test: if EPIC can recover the component, the Chandra result enters the EPIC comparison sample; if not, the transferability is weakened.
This is not "rediscovering hot gas." A single projected measurement of one galaxy can only update the evidence for NGC 4490/4485 at this angular scale, this model, and this sensitivity.
03 · Sky geometry
Archival coverage reaches the centre; six EPIC pointings are within 0.82′ of the nucleus
NGC 4490's RC3 D25 is 6.76′ × 2.14′, PA=151°; the companion NGC 4485 lies ~3.7′ from the nucleus (PA≈350°). The aimpoints of six public Full-Frame EPIC science observations all lie within 0.378′–0.817′ of the target centre, so this is centre-target coverage, not an outer-halo offset programme. The figure below uses the true nucleus-centred WCS; it shows nominal sky geometry only and does not freeze I173's region, mask, GTI, or detector footprint.
The XMM RGB uses a positive-intensity p0.5–p99.5 percentile clip shared across all channels and an asinh stretch; blue-grey marks zero coverage, not low surface brightness. This display stretch plays no role in exposure correction, scientific flux, or the A_pair measurement; the overlays use the original WCS.
04 · The number actually measured, and why the Chandra result cannot be reused
The measurement is a component normalisation, not "is there hot gas"
Once qualified, the pre-frozen primary quantity is A_pair: the continuous normalisation of the target-associated projected component within the frozen pair-scale projected domain. A frozen finite interval [L_pair, U_pair] is required: only if all qualification conditions pass and L_pair > 0 is it COMPONENT CONSTRAINED; only if U_pair falls below the pre-declared material normalisation is it BOUNDED; everything else is INCONCLUSIVE or MIXED/COMPACT-SOURCE-NON-SEPARABLE.
05 · If the answer comes out the other way, or cannot be answered at all
An honest BOUNDED is informative; an unqualified null is not
- COMPONENT CONSTRAINED: in the frozen response domain, the A_pair interval excludes zero. It supports only the pair-scale soft component under this projected model.
- BOUNDED: at the sensitivity that has met the recovery/coverage standard, the upper bound still falls below the pre-declared material normalisation. It weakens the claim that "the Chandra hot-ISM result transfers to the current EPIC operator"; it is not equivalent to "there is no hot gas."
- MIXED/COMPACT-SOURCE-NON-SEPARABLE: the component cannot be separated from unresolved compact sources, pair/disk structure, or background under the pre-registered criteria. This is a measurement result, not a physical null.
- INCONCLUSIVE: if any of the GTI, detector/mask, background, response, or injection/recovery qualifications fails, no physical conclusion is drawn. Such a null is boring and does not deserve to be dressed up with pretty graphics.
06 · Cost, and the cost still unknown
The single request of this page: first authorise one qualification HEAVY, not observing time
The next step R5 left for I173 is heavy_pending: one R6 work package must freeze the EPIC FK5 region, GTI/LIVETIME, source/pair mask, retained area, background, per-camera response, and a fixed-seed response-folded injection/recovery (covering NGC 4490-only / NGC 4485-only / pair-distributed / compact-source-dominated / pair-structure-dominated / background-dominated / mixed / null truths).
The nominal exposures PN 249.974 ks / MOS1 251.799 ks / MOS2 251.879 ks are only scheduled duration, not clean GTI and not a sensitivity calculation. Actual clean time, source brightness, detector gaps, vignetting, post-mask area, background, and the recoverable material threshold are all unknown. If this HEAVY qualification fails, the correct action is to stop at INCONCLUSIVE, not to package unknown cost into a proposal.
Actionable choice for the PI
Approve: approve only the R6 HEAVY qualification package above; it does not automatically approve an XMM time request.
Do not approve: stop here. This page has already stated that there is currently insufficient evidence to commit observing time or to claim a halo/CGM result.
Sources and reproducibility
Evidence boundary of this page
- Richings, Fabbiano, Wang & Roberts (2010), The Hot Interstellar Medium of the Interacting Galaxy NGC 4490, ApJ 723, 1375. arXiv
- Gladstone & Roberts (2009), The ultraluminous X-ray sources of NGC 4490/4485, MNRAS 397, 124. arXiv
- Local traceable inputs: the I173 idea card, R3 evidence, R4 critique, R5 adjudication, NGC4490 source card, and the frozen HiPS2FITS display inputs of this page. All hashes, commands, and replay paths are recorded in the publication manifest.