X-ray Variability of Mrk 421 in 2025 January: A Ten-Day XPoSat XSPECT Campaign with Contemporaneous NICER Coverage
Abstract
We present an X-ray study of the high-synchrotron-peaked blazar Mrk 421 using ten days of XPoSat/XSPECT observations (2025 January 20-29; 202 ks) and contemporaneous NICER data. The 0.8-15 keV light curve has a fractional variability amplitude of 19.6 +/- 0.3 per cent, and Bayesian blocks resolve 26 flux states spanning a factor of 2.3. The best-constrained flare has a fast rise and a slow decay, with e-folding times of about 3 h and 30 h. For typical jet parameters, the X-ray-emitting electrons cool faster than this, suggesting that the decay traces a gradual weakening of their acceleration. The count-rate distribution is best described by two lognormal components, consistent with multiplicative variability. The power spectrum is a red-noise power law with a slope of about 1.7 over timescales of 25 min to 10 d, typical of blazars. XSPECT and NICER count rates are well correlated over eight matched epochs (r = 0.941). We fitted log-parabola models to day-resolved spectra from XSPECT, NICER and both jointly, and to flux-resolved XSPECT spectra. Both show spectral hardening with brightness. In the flux-resolved spectra, the photon index at 1 keV decreases from 2.24 in the faintest state to 1.96 in the brightest, and the synchrotron peak moves from about 0.66 to 1.06 keV, while the curvature (β≈ 0.7) shows no significant change. Power-law fits to Bayesian blocks show that bright blocks at the same rate can differ in photon index by up to 6.4σ, so count rate alone does not fix the spectral state.
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BibTeX
@misc{akbar2026xray,
title = {{X-ray Variability of Mrk 421 in 2025 January: A Ten-Day XPoSat XSPECT Campaign with Contemporaneous NICER Coverage}},
author = {Sikandar Akbar and Zahir Shah and Zahoor Malik and Faheem Aalijah and P Sreekumar and Vivek Kumar},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.02772},
eprint = {2611.02772},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.02772},
}
RIS
TY - UNPB
TI - X-ray Variability of Mrk 421 in 2025 January: A Ten-Day XPoSat XSPECT Campaign with Contemporaneous NICER Coverage
AU - Sikandar Akbar
AU - Zahir Shah
AU - Zahoor Malik
AU - Faheem Aalijah
AU - P Sreekumar
AU - Vivek Kumar
PY - 2026
DA - 2026/11/02
PB - arXiv
DO - 10.48550/arxiv.2611.02772
UR - https://arxiv.org/abs/2611.02772
AB - We present an X-ray study of the high-synchrotron-peaked blazar Mrk 421 using ten days of XPoSat/XSPECT observations (2025 January 20-29; 202 ks) and contemporaneous NICER data. The 0.8-15 keV light curve has a fractional variability amplitude of 19.6 +/- 0.3 per cent, and Bayesian blocks resolve 26 flux states spanning a factor of 2.3. The best-constrained flare has a fast rise and a slow decay, with e-folding times of about 3 h and 30 h. For typical jet parameters, the X-ray-emitting electrons cool faster than this, suggesting that the decay traces a gradual weakening of their acceleration. The count-rate distribution is best described by two lognormal components, consistent with multiplicative variability. The power spectrum is a red-noise power law with a slope of about 1.7 over timescales of 25 min to 10 d, typical of blazars. XSPECT and NICER count rates are well correlated over eight matched epochs (r = 0.941). We fitted log-parabola models to day-resolved spectra from XSPECT, NICER and both jointly, and to flux-resolved XSPECT spectra. Both show spectral hardening with brightness. In the flux-resolved spectra, the photon index at 1 keV decreases from 2.24 in the faintest state to 1.96 in the brightest, and the synchrotron peak moves from about 0.66 to 1.06 keV, while the curvature (β≈ 0.7) shows no significant change. Power-law fits to Bayesian blocks show that bright blocks at the same rate can differ in photon index by up to 6.4σ, so count rate alone does not fix the spectral state.
ER -