[Paper Review] Search for extended gamma ray emission in Markarian 421 using VERITAS observations

Mateo Fernandez Alonso|arXiv (Cornell University)|Jun 18, 2014

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This study searches for an extended gamma-ray halo around the blazar Markarian 421 using VERITAS observations during a quiescent state, comparing angular distributions with flare-period data assumed halo-free. No significant difference was found via chi-squared tests, leading to a 99% confidence upper limit of <8% of the Crab Nebula flux (in C.U.) for extended emission within 0° ≤ θ ≤ 0.1°.

ABSTRACT

Very high energy (VHE: &gt;100 GeV) gamma rays coming from AGN can pair-produce on the intergalactic background light generating an electromagnetic cascade. If the Intergalactic Magnetic Field (IGMF) is sufficiently strong, this cascade may result in an extended isotropic emission of photons around the source, or halo. Using VERITAS observations of the blazar Markarian 421, we search for extended emission by comparing the source angular distribution ($θ^2$) from a quiescent period with one coming from a flare period, which can be considered as halo-free. $χ^2$ test showed no significant statistical differences between the samples, suggesting that the effect is either non-existent or too weak to be detected. We calculated upper limits for the extended flux considering different angle ranges, the most stringent being &lt;8% of the Crab Nebulae flux (C.U), in the range $0°\leq θ \leq 0.1°$ .

Motivation & Objective

• To test the existence of an extended gamma-ray halo around Markarian 421 due to electromagnetic cascades induced by VHE photons interacting with the intergalactic background light.
• To investigate whether a strong enough intergalactic magnetic field (IGMF) could produce a detectable isotropic halo of lower-energy photons around the source.
• To set stringent upper limits on extended emission by comparing quiescent-state data with flare-period data, assumed to be halo-free.
• To constrain cosmological parameters such as IGMF strength and EBL density through indirect detection of cascade-induced halos.

Proposed method

• Used VERITAS observations from a quiescent period (9.6 hours) and a flare period (8.6 hours) of Markarian 421.
• Constructed scaled excess θ² distributions from both data sets, assuming the flare data represent a halo-free point-source distribution.
• Applied a chi-squared test to compare the shape of the quiescent and flare θ² distributions, with the test statistic defined as χ² = Σ[(N_i_NOflare - B_i_NOflare) - F*(N_i_flare - B_i_flare)]² / [N_i_NOflare + B_i_NOflare + F²*(N_i_flare + B_i_flare)]
• Used reduced chi-squared (χ²_ν) to assess statistical significance across different θ² ranges, comparing results to critical values at 95% and 99% confidence levels.
• Calculated upper limits on extended flux using Helene’s method at 99% confidence level, corresponding to a 5σ detection threshold.
• Reported upper limits in units of Crab Nebula flux (C.U.) for different angular ranges, including 0° ≤ θ ≤ 0.1° and 0.1° ≤ θ ≤ 0.2°.

Experimental results

Research questions

• RQ1 Is there a statistically significant extended gamma-ray emission (halo) around Markarian 421 due to electromagnetic cascades in the presence of a strong intergalactic magnetic field?
• RQ2 Does the angular distribution of excess gamma rays in the quiescent state differ significantly from that in the flare state, which is assumed to be halo-free?
• RQ3 What are the upper limits on the flux of any extended emission within specific angular ranges around the source?
• RQ4 How do these results constrain the strength of the intergalactic magnetic field and the extragalactic background light?

Key findings

• No statistically significant difference was found between the θ² distributions of the quiescent and flare periods, as indicated by reduced chi-squared values falling below critical thresholds at 95% and 99% confidence levels across all tested angular ranges.
• The most stringent upper limit on extended emission is <8% of the Crab Nebula flux (C.U.) for photons within 0° ≤ θ ≤ 0.1°, corresponding to 1.30 × 10⁻⁶ erg·m⁻²·s⁻¹ with a small uncertainty.
• The upper limit for extended emission in the range 0.1° ≤ θ ≤ 0.2° is <11% of the Crab Nebula flux (C.U.) at 99% confidence level.
• These upper limits are consistent with previous results from Aleksić et al. (2010), validating the method and sensitivity of the VERITAS instrument for halo searches.
• The absence of a detectable halo suggests that either the intergalactic magnetic field is too weak to produce a measurable cascade effect, or the cascade is suppressed by other astrophysical factors.
• The results constrain models predicting extended emission from VHE gamma-ray cascades in blazars, contributing to the broader effort to measure cosmological parameters such as IGMF strength and EBL density.