Revisiting the 2021 Outburst of the BHC MAXI J1803-298 Using NICER, NuSTAR, and Insight-HXMT Data

We present a broadband spectral and timing study of the black hole candidate MAXI J1803-298 during its 2021 outburst using simultaneous observations from NICER, NuSTAR, and Insight-HXMT. The new analysis still awaits peer review, but it already lays out the central claim clearly.

That matters because astrophysics becomes persuasive only when an observed signal can be tied to a physically defensible explanation. Compact objects such as neutron stars and black holes are natural laboratories for extreme physics, but the distance and complexity of these systems make interpretation difficult without multi-wavelength coverage and careful modeling. A detection without a mechanism is only half a result. the other half comes from showing that the signal fits quantitatively inside a coherent physical picture rather than merely being consistent with a broad family of models. The combined multi-instrument coverage allows us to investigate the evolution of low-frequency quasi-periodic oscillations (LFQPOs) together with the spectral properties of the. During the early observation epoch, the source exhibits a hard or hard-intermediate spectral state dominated by Comptonized emission with reflection features.

Spectral modeling within the framework of the two-component advective flow (TCAF) model indicates the presence of a sub-Keplerian halo and a Keplerian disk with a shock located at. A prominent LFQPO is detected during this epoch with a centroid frequency evolving from 0.35 Hz to 0.5 Hz and extending up to 100 keV.

The energy-dependent fractional rms variability suggests that the modulation originates primarily from the Comptonizing inner accretion flow. In contrast, a later observation epoch shows a softer spectral state characterized by stronger disk emission and a steeper photon index, during which no LFQPO is detected.

We also demonstrate that cospectral analysis effectively mitigates dead-time-induced distortions in NuSTAR timing studies, confirming the intrinsic nature of the detected. The combined spectral and timing results support a scenario in which LFQPOs in MAXI J1803-298 arise from the dynamically evolving inner accretion flow.

The broader interest lies in turning an observational clue into something that can be weighed against competing models of the underlying physics. Astrophysics does not have the luxury of controlled experiments; everything is inferred from radiation that traveled across cosmic distances under conditions that cannot be reproduced in a terrestrial laboratory. This makes the interpretation chain longer and more uncertain than in bench science, but it also means that a well-constrained measurement of an extreme object carries theoretical information that no earthbound experiment can provide.

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Because this is still a preprint, the result should be read with genuine interest and proportionate caution. Peer review is not a guarantee of correctness, but it is a process that forces authors to respond to technical criticism from specialists who have no stake in a particular outcome. Preprints that survive that process, often with substantive revisions, emerge with a stronger evidential base than the version that first appeared. Until that stage is complete, the responsible reading keeps uncertainty explicitly visible rather than treating the claims as established findings.

The next step is to see whether independent datasets and physical modeling converge on the same interpretation. Multi-wavelength follow-up, combining X-ray, radio and optical data where possible, is typically what separates a compelling detection from a robust physical characterization. In high-energy astrophysics, results that initially looked definitive have been revised when data from a second messenger arrived; the current result should be read with that history in mind. Until peer review and independent follow-up address those open questions, skepticism is not a failure of appreciation for the work; it is part of how science decides what to keep.

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