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X-ray Spectral Properties of Four Classical TeV Blazars using Simultaneous Observations from NICER and NuSTAR

This study analyzes simultaneous NICER and NuSTAR X-ray spectra of four classical TeV blazars, revealing that while a Log-Parabolic model fits most observations, specific low-to-moderate flux states in Mrk 421 and Mrk 501 require additional disk emission contributions, with Mrk 421 also showing distinct Gaussian features between 1.42 and 1.70 keV.

Original authors: Riya Bhowmick, Alok C. Gupta

Published 2026-07-09

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CC0 1.0

Original authors: Riya Bhowmick, Alok C. Gupta

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a vast, noisy concert hall. In the center of this hall sit four very loud, very distant stars called Blazars. Specifically, these are a type known as "BL Lacertae objects." Think of them as cosmic lighthouses that don't just shine a steady beam; they blast out jets of particles and light directly at us, like a firehose aimed right at your face. Because they are so close to our line of sight, they appear incredibly bright and change their brightness rapidly, sometimes in just a few minutes.

This paper is a report card on the "X-ray voices" of four of these famous cosmic lighthouses: Mrk 421, Mrk 501, PG 1553+113, and PKS 2155-304.

The Tools: Two Sets of Ears

To listen to these stars, the researchers used two different space telescopes working together, like a stereo system with a tweeter and a woofer:

NICER: This telescope is the "tweeter." It's great at hearing the lower-pitched, softer X-rays (the gentle hums).
NuSTAR: This is the "woofer." It specializes in the high-pitched, hard X-rays (the deep, powerful thumps).

By using both at the exact same time, the scientists could hear the full range of the star's song without missing a note.

The Mystery: A Song That Didn't Fit

The scientists had a specific theory about what these songs should sound like. They expected the X-ray light to follow a smooth, curved pattern, which they call a "Log-Parabolic" model. Imagine a smooth, rolling hill; that's what they expected the data to look like.

For most of the observations, the data fit this smooth hill perfectly. However, for six specific times (four times with Mrk 421 and two times with Mrk 501), the data didn't fit the smooth hill. It was like trying to fit a jagged, rocky boulder into a smooth mold. The "song" had extra noise or a different shape at the lower energy levels that the standard model couldn't explain.

The Solution: Finding the Hidden Instrument

When the smooth model failed, the scientists realized something else must be playing in the background. They added a new component to their model called a "Blackbody" component.

Think of the main jet of the blazar as a roaring jet engine. The "Blackbody" component is like a quiet, warm campfire sitting nearby.

The Discovery: The scientists found that during these six specific times, the blazars were in a "low-flux" or "moderate-flux" state. They weren't blasting at full power.
The Analogy: When the jet engine (the main jet) is running at full speed, it's so loud you can't hear the campfire (the accretion disk). But when the jet engine slows down to a low idle, the quiet hum of the campfire becomes audible.
The Conclusion: The "campfire" is actually the accretion disk—a swirling disk of hot gas and dust falling into the black hole at the center of the galaxy. The paper concludes that when these blazars are quiet, we can finally hear the disk's contribution to the X-ray light, which was previously drowned out by the jet.

The Strange "Beeps"

In the four quiet observations of Mrk 421, the scientists also noticed a strange, sharp "beep" in the data, appearing as a Gaussian line between 1.42 and 1.70 keV.

The Mystery: They aren't sure what this beep is. It could be a real signal from the star, or it could be a "ghost note" caused by the instrument itself (like a static noise in a radio) or background contamination. The paper suggests it's likely an instrumental effect, but they added it to the model just to make the fit perfect.

The Other Two Stars

PG 1553+113: This star's song had a "silicon edge" (a specific type of absorption) around 1.87 keV, likely due to the telescope's own materials. Once they accounted for that, the song fit the smooth curve perfectly.
PKS 2155-304: This star's song was a perfect fit for the standard smooth curve model, showing no need for extra components.

The Big Picture

The main takeaway is simple: Context matters.
When these cosmic lighthouses are screaming at full volume, we only hear the jet. But when they whisper (enter a low-flux state), we can finally hear the underlying music of the accretion disk. This study proves that even in these extreme, jet-dominated galaxies, the disk of falling matter still plays a significant role in the light we see, provided the jet isn't too loud to hear it.

Technical Summary: X-ray Spectral Properties of Four Classical TeV Blazars using Simultaneous Observations from NICER and NuSTAR

Problem Statement
This study addresses the X-ray spectral properties of four classical TeV-emitting High Synchrotron Peak (HSP) BL Lacertae objects: Mrk 421, Mrk 501, PG 1553+113, and PKS 2155-304. While the Log-Parabolic (LP) model is the standard for describing the curved spectral profiles of blazars, previous studies suggest that during low to moderate flux states, the contribution of the accretion disk may become significant in the X-ray band, potentially rendering the simple LP model insufficient. The authors investigate whether simultaneous observations from the Neutron Star Interior Composition Explorer (NICER) and the Nuclear Spectroscopic Telescope Array (NuSTAR) reveal deviations from the standard LP model that necessitate additional physical components, such as thermal disk emission or Gaussian line features.

Methodology
The authors utilized simultaneous archival data from NICER and NuSTAR for 13 distinct observational epochs across the four target sources.

Data Reduction: NICER data were processed using heasoftpy and the nicerl2 pipeline, employing the SCORPEON background model to account for astrophysical X-ray sky backgrounds (CXB, Halo, LHB). NuSTAR data were reduced using NuSTARDAS (version 6.34) and nupipeline for focal plane modules FPMA and FPMB.
Spectral Analysis: All spectra were fitted using XSPEC (v12.14.1). The primary model tested was the absorbed Log-Parabolic (LP) model (constant*TBabs*logpar).
Model Refinement: For spectra where the LP model yielded unsatisfactory residuals (specifically in the soft X-ray range), the authors added a blackbody radiation model (bbodyrad) to account for potential thermal disk emission. In cases where Gaussian residuals persisted, a Gaussian line component was added. For PG 1553+113, an edge model was introduced to address a silicon absorption feature.
Flux State Context: The spectral states were contextualized using MAXI/GSC light curves (2–20 keV) to classify the sources as being in high, moderate, or low-flux states.

Key Results
The analysis of 13 spectra yielded the following findings:

Model Success: Seven of the 13 spectra were well-fitted by the absorbed LP model alone.
Mrk 421 (8 Epochs):
- Four epochs (MJD 60429, 60433, 60439, 60443) were well-fitted by the absorbed LP model.
- Four epochs (MJD 60077, 60289, 60296, 60298) showed poor fits with the LP model, particularly below 4 keV. These required the addition of a bbodyrad component (indicating disk contribution) and a Gaussian line feature (1.42–1.70 keV).
- Flux analysis indicated these four epochs corresponded to a "moderate to low-flux" state (2–20 keV flux $\lesssim 0.10$ photons cm $^{-2}$ s $^{-1}$ ), compared to historical high-flux states ( $\gtrsim 0.30$ photons cm $^{-2}$ s $^{-1}$ ).
- The Gaussian features detected (e.g., at 1.70 keV for MJD 60077) were noted as potentially instrumental (Al-K $\alpha$ or Si-K $\alpha$ lines) or background contamination, as their origin could not be conclusively identified.
Mrk 501 (3 Epochs):
- One epoch (MJD 59660) was fitted by the LP model.
- Two epochs (MJD 59662, 59665) required the bbodyrad component to fit the soft X-ray excess.
- These epochs corresponded to a low-flux state ( $\sim 0.05$ photons cm $^{-2}$ s $^{-1}$ in 2–20 keV), consistent with previous reports of low activity for this source.
PG 1553+113 (1 Epoch): The spectrum was fitted by the LP model but required an edge model to account for a silicon edge at $\sim 1.87$ keV. The spectrum showed positive curvature ( $\alpha \approx 2.25, \beta \approx 0.20$ ).
PKS 2155-304 (1 Epoch): The spectrum was well-fitted by the absorbed LP model, exhibiting positive curvature ( $\alpha \approx 2.45, \beta \approx 0.31$ ).

Significance and Claims
The paper claims that the inclusion of a thermal component (bbodyrad) in the spectral modeling of Mrk 421 and Mrk 501 during specific epochs is justified by the sources' moderate to low-flux states. The authors argue that during these less active periods, the jet continuum is less dominant, allowing the accretion disk emission to contribute significantly to the observed X-ray spectrum. This supports the hypothesis that while HSP BL Lacs are typically dominated by jet emission, disk contributions become detectable when the jet activity subsides.

The study also highlights the detection of Gaussian features in the 1.42–1.70 keV range for Mrk 421, though the authors remain cautious, suggesting these may be instrumental artifacts or background contamination rather than astrophysical lines.

Conclusion
The authors conclude that for the four epochs of Mrk 421 and the two epochs of Mrk 501 where the simple LP model failed, the spectral properties are better explained by a composite model including a thermal disk component. This finding reinforces the utility of simultaneous NICER and NuSTAR observations in disentangling jet and disk contributions in TeV blazars, particularly during low-flux states. The paper does not claim to have definitively proven the astrophysical origin of the Gaussian lines, nor does it propose new theoretical frameworks beyond the established connection between flux state and spectral component dominance.