🔭 astrophysics

Scrutinizing the 2020 multiwavelength outburst of PKS 0903-57 through observations with H.E.S.S

This paper reports the first very-high-energy gamma-ray detection of the blazar PKS 0903-57 by H.E.S.S. during its 2020 multiwavelength outburst, revealing rapid variability and using leptonic modeling to suggest the emission originated outside the broad-line region and was driven by shock acceleration.

Original authors: A. Acharyya, F. Aharonian, F. Ait Benkhali, A. Alkan, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, J. Borowsk

Published 2026-03-16

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CC BY 4.0

Original authors: A. Acharyya, F. Aharonian, F. Ait Benkhali, A. Alkan, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, B. Bi, M. Böttcher, C. Boisson, J. Bolmont, J. Borowska, R. Brose, A. Brown, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, S. Casanova, J. Celic, M. Cerruti, S. Chandra, A. Chen, M. Chernyakova, J. O. Chibueze, O. Chibueze, B. Cornejo, G. Cotter, G. Cozzolongo, J. Damascene Mbarubucyeye, J. de Assis Scarpin, M. de Naurois, E. de Oña Wilhelmi, A. G. Delgado Giler, J. Devin, A. Djannati-Ataï, J. Djuvsland, A. Dmytriiev, V. Doroshenko, K. Egg, S. Einecke, J. -P. Ernenwein, C. Escañuela Nieves, K. Feijen, M. D. Filipovic, G. Fontaine, S. Funk, S. Gabici, Y. A. Gallant, M. Genaro, J. F. Glicenstein, P. Goswami, G. Grolleron, L. Haerer, L. Heckmann, G. Hermann, B. Heß, J. A. Hinton, W. Hofmann, T. L. Holch, M. Holler, D. Horns, M. Jamrozy, F. Jankowsky, I. Jung-Richardt, E. Kasai, K. Katarzyński, D. Kerszberg, R. Khatoon, B. Khélifi, W. Kluźniak, N. Komin, D. Kostunin, R. G. Lang, A. Lemièr, J. -P. Lenain, A. Luashvili, J. Mackey, D. Malyshev, V. Marandon, G. Martí-Devesa, R. Marx, M. Mayer, A. Mehta, A. M. W. Mitchell, R. Moderski, M. O. Moghadam, L. Mohrmann, A. Montanari, E. Moulin, D. Moyeni, J. Niemiec, L. Olivera-Nieto, S. Panny, M. Panter, R. D. Parsons, U. Pensec, S. Pita, G. Pühlhofer, A. Quirrenbach, M. Regeard, A. Reimer, O. Reimer, H. X. Ren, B. Reville, F. Rieger, G. Rowell, B. Rudaka, K. Sabri, V. Sahakiana, H. Salzmann, M. Sasaki, J. Schäfer, F. Schüssler, H. M. Schutte, J. N. S. Shapopi, A. Sharma, W. Si Said, H. Sol, S. Spencer, Ł. Stawarz, R. Steenkamp, S. Steinmassl, C. Steppa, T. Takahashi, T. Tanaka, A. M. Taylor, C. van Eldik, M. Vecchi, J. Vink, T. Wach, S. J. Wagner, A. Wierzcholska, M. Zacharias, A. A. Zdziarski, A. Zech, N. Żywucka

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ 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 a cosmic lighthouse, billions of light-years away, suddenly flipping its switch from a gentle glow to a blinding, strobe-light frenzy. This is the story of PKS 0903−57, a supermassive black hole at the center of a distant galaxy that decided to throw a massive party in March and April 2020.

This paper is the "autopsy report" of that party, written by a team of astronomers using the H.E.S.S. telescope array in Namibia, along with data from space telescopes like Fermi, Swift, and ground-based optical cameras.

Here is the story of what happened, explained simply.

1. The Star and the Shadow

First, a bit of background. PKS 0903−57 is a Blazar. Think of a blazar as a cosmic firehose. A black hole is eating gas, and it shoots out two giant jets of particles. Usually, these jets point in random directions. But in a blazar, one jet is pointing almost directly at Earth. We are looking straight down the barrel of the gun.

For a long time, astronomers were confused about this object. It was hiding behind a "cosmic impostor"—a bright, ordinary star in our own galaxy that was sitting right next to it in the sky. It was like trying to watch a tiny firefly in a dark field, but a giant streetlamp was shining right next to it, making it hard to see the firefly's true colors.

Recently, astronomers finally separated the two. They confirmed PKS 0903−57 is a Flat Spectrum Radio Quasar (FSRQ), a specific type of blazar, located about 3 billion light-years away.

2. The Great Outburst

In early 2020, this cosmic firehose went into overdrive.

The Warning: Space telescopes (Fermi and AGILE) saw the object suddenly get 100 times brighter in high-energy gamma rays.
The Response: The H.E.S.S. team in Namibia pointed their giant mirrors at the sky for six nights. They were looking for Very High Energy (VHE) gamma rays—particles so energetic they are like cosmic bullets.

The Result: They found it! PKS 0903−57 was glowing brightly in this extreme energy band. It was so bright that if you were standing on Earth, it would have been about 60% as bright as the Crab Nebula (a famous cosmic explosion), but much farther away.

3. The "Flash" and the "Flicker"

The most exciting part of the paper is how the light behaved.

The Flicker: The brightness didn't just stay high; it jumped up and down. In just a few hours, the light would surge and then fade. This is like a lightbulb that is being flicked on and off by a very fast hand.
The Time Scale: Because the light changed so fast (in about 5 hours), the astronomers knew the "engine" creating this light must be very small. If a lightbulb changes color instantly, the bulb itself must be tiny. This engine was likely no bigger than our solar system, yet it was packing the punch of a galaxy.

4. The Mystery of the "Star"

Here is where the "impostor" star mentioned earlier caused trouble.

The optical (visible) light telescopes (like Swift and ATOM) couldn't tell the difference between the blazar and the nearby star.
It was like trying to hear a whisper in a room where someone is screaming. The "screaming" star was so bright that it drowned out the blazar's visible light.
The astronomers had to do some complex math to subtract the star's noise and guess what the blazar was actually doing in visible light. They found that while the gamma rays were going crazy, the visible light was relatively calm.

5. The Detective Work: Where is the Engine?

The scientists asked a crucial question: Where exactly is this explosion happening?

Hypothesis A: Is it deep inside the galaxy, right next to the black hole, in a region full of gas and light (called the Broad Line Region)?
Hypothesis B: Is it far out in the jet, in a quieter zone?

The Verdict: The data says Hypothesis B.
Why? Because the gamma rays were so energetic that if they had been created deep inside the galaxy, they would have crashed into the surrounding light and been destroyed (absorbed) before they could reach Earth. Since the gamma rays made it through, the explosion must have happened outside the crowded inner city of the galaxy, in the open highway of the jet.

6. The Engine Room: Shocks vs. Magnets

Finally, the team tried to build a computer model to explain how the black hole was accelerating these particles to such crazy speeds.

The Old Idea: Maybe the particles are being pushed by a giant magnetic field, like a slingshot.
The New Finding: The model showed that the magnetic field was actually quite weak. Instead, the acceleration was likely caused by shock waves.

The Analogy: Imagine a supersonic jet breaking the sound barrier. It creates a shockwave that pushes everything in front of it. The scientists believe that inside the jet, massive shockwaves are slamming into the particles, accelerating them to near-light speed. This is similar to what we see in other types of cosmic objects (BL Lacs), suggesting that "shockwaves" might be the universal engine for these cosmic fireworks.

Summary

In short, this paper tells us that:

PKS 0903−57 is a real, confirmed blazar, not a mystery.
It threw a massive gamma-ray party in 2020, getting 100 times brighter.
The light flickered in just a few hours, proving the engine is tiny but incredibly powerful.
The explosion happened far away from the black hole's immediate neighborhood, in the dusty outskirts of the galaxy.
The engine driving this isn't a strong magnet, but rather shockwaves slamming into particles, acting like a cosmic particle accelerator.

It's a reminder that even in the quietest corners of the universe, black holes can throw the wildest parties, and we are just lucky enough to be invited to watch from the front row.

1. Problem and Context

PKS 0903−57 is a blazar (specifically a Flat Spectrum Radio Quasar, FSRQ) located at a redshift of $z = 0.2621$ . Prior to this study, its nature and redshift were uncertain due to confusion with a nearby bright Galactic star and conflicting historical measurements. In March and April 2020, the source underwent a dramatic high-energy (HE) $\gamma$ -ray outburst, with flux increasing by $\sim$ 2 orders of magnitude compared to its quiescent state, as reported by Fermi-LAT and AGILE.

The primary scientific objectives were:

To confirm the Very-High-Energy (VHE, $E > 100$ GeV) detection of PKS 0903−57.
To characterize the multiwavelength (MWL) variability (VHE, HE, X-ray, optical) during the flare.
To constrain the location of the emission region (specifically, whether it lies within the Broad-Line Region or further out).
To model the emission mechanisms (leptonic vs. hadronic) and infer the physical conditions (magnetic field, particle acceleration) driving the flare.

2. Methodology

Observations

H.E.S.S. (VHE): The High Energy Stereoscopic System observed PKS 0903−57 for 6 nights (April 12–17, 2020), totaling 13.1 hours. Observations utilized the large 28m telescope (CT5) in monoscopic mode to lower the energy threshold to $\sim$ 160–180 GeV.
Fermi-LAT (HE): Analyzed data from 100 MeV to 500 GeV with 1-day and 6-hour time binning to capture rapid variability.
Swift (X-ray & UV/Optical): The X-Ray Telescope (XRT) and UltraViolet and Optical Telescope (UVOT) provided X-ray (0.3–10 keV) and optical/UV light curves.
ATOM (Optical): A 75cm telescope at the H.E.S.S. site provided high-cadence optical monitoring.

Data Analysis Techniques

VHE Analysis: Used the Ring Background and Reflected Background methods. Spectral fitting employed power-law and log-parabola models. Systematic errors were evaluated via Monte Carlo simulations rescaling the energy scale by 10%.
MWL Variability: Calculated fractional variability ( $F_{var}$ ) and variability timescales ( $t_{var}$ ) across all bands.
Stellar Contamination: Addressed the contamination from a nearby Galactic star ( $G=15.9$ ) in optical data by using Gaia DR3 catalog data to model the star's contribution and subtract it from the total flux.
Modeling: Employed a one-zone leptonic model using the OneHaLe code. The model included:
- Synchrotron emission.
- Synchrotron Self-Compton (SSC).
- External Inverse-Compton (IC) scattering off photons from the Dusty Torus (DT).
- A two-zone approach: a "low-state" baseline and a "flaring" zone, plus a separate component for the contaminating star.

3. Key Results

VHE Discovery and Spectral Properties

Discovery: PKS 0903−57 was detected in the VHE band with a significance of $103\sigma$ . The average flux above 178 GeV was $1.5 \times 10^{-10}$ ph cm $^{-2}$ s $^{-1}$ (60% of the Crab Nebula flux).
Spectral Shape: The VHE spectrum is soft ( $\Gamma > 2$ ) and exhibits curvature. The log-parabola fit is preferred over a simple power law for the total spectrum and the first three nights.
Variability: The VHE flux showed significant variability on timescales of $\sim$ 5 hours. The light curve showed a general decline over the 6 nights, interrupted by rising trends on Nights 2 and 3.

Multiwavelength Correlations

HE vs. VHE: The HE $\gamma$ -ray spectrum was significantly harder than the long-term average (peak energy shifted from $\sim$ 240 MeV to $\sim$ 6 GeV). The HE flux evolution did not perfectly correlate with VHE flux; for instance, Night 4 had a soft HE spectrum despite high flux, while VHE flux was declining.
X-ray: The X-ray flux varied by a factor of a few but the spectral index remained stable ( $\Gamma_X \approx 1.68$ ). The X-ray spectrum did not connect seamlessly to the HE spectrum in all nights, suggesting different emission processes or regions.
Optical: Optical variability was low ( $F_{var} \approx 0.1$ ) and heavily influenced by the contaminating star. The flare's optical flux was likely less affected than the X-ray/ $\gamma$ -ray flux.

Physical Constraints and Modeling

Emission Region Location: The detection of VHE photons $>1$ TeV without significant absorption implies the emission region is outside the Broad-Line Region (BLR). If located inside the BLR, $\gamma$ - $\gamma$ absorption would have attenuated the signal. The lack of absorption suggests the seed photons for Inverse-Compton scattering are from the Dusty Torus (DT).
Magnetization: The modeling required a low magnetic field ( $B < 0.1$ G) to satisfy the high Compton dominance ( $CD \gg 10$ ) observed during the flare. This indicates a low-magnetization plasma ( $\sigma \ll 1$ ).
Acceleration Mechanism: The low magnetization and the ability to model both low and high states with the same electron spectral index suggest that shock acceleration is the dominant driver of the flare, rather than magnetic reconnection.
Jet Power: The derived jet power exceeds the Eddington luminosity for a $10^8 M_\odot$ black hole, dominated by particle energy rather than magnetic energy.

4. Significance and Conclusions

Confirmation of FSRQ Nature: The study solidified the classification of PKS 0903−57 as an FSRQ at $z=0.2621$ , making it the second closest VHE-detected FSRQ to Earth.
Emission Region Physics: The results provide strong evidence that VHE emission in FSRQs can originate outside the BLR, likely in the region dominated by the Dusty Torus. This challenges models that assume BLR photon fields are the primary target for IC scattering in all FSRQ flares.
Acceleration Mechanism: The findings support shock acceleration as the primary mechanism for particle acceleration in FSRQ flares, consistent with recent X-ray polarization results in BL Lac objects. The low magnetization implies that the jet is particle-dominated.
Complex Variability: The intricate, non-synchronous variability between HE and VHE bands highlights the complex interplay of time- and energy-dependent acceleration and cooling processes within the jet.

In summary, the paper presents a comprehensive multiwavelength analysis of a major FSRQ flare, utilizing H.E.S.S. to break new ground in VHE detection and employing detailed modeling to constrain the jet's physical parameters, ultimately pointing to a shock-accelerated, particle-dominated emission region located beyond the BLR.