[Paper Review] Spectral state dependence of the 0.4-2 MeV polarized emission in Cygnus X-1 seen with INTEGRAL/IBIS, and links with the AMI radio data

Juan Rodríguez, V. Grinberg+7 more|arXiv (Cornell University)|May 11, 2015

Astrophysical Phenomena and Observations4 citations

TL;DR

This study resolves the spectral state dependence of 0.4–2 MeV polarized emission in Cygnus X-1 using 10 years of INTEGRAL/IBIS data, finding a 75±32% polarization fraction in the hard state—consistent with synchrotron emission from a compact jet—while no significant polarization is detected in the soft or intermediate states. The detection correlates with simultaneous AMI 15 GHz radio data showing a compact jet only in hard and intermediate states, reinforcing the jet origin of the high-energy emission.

ABSTRACT

Polarization of the >~400 keV hard tail of the microquasar Cygnus X-1 has been independently reported by INTEGRAL/IBIS, and INTEGRAL/SPI and interpreted as emission from a compact jet. These conclusions were, however, based on the accumulation of all INTEGRAL data regardless of the spectral state. We utilize additional INTEGRAL exposure accumulated until December 2012, and include the AMI/Ryle (15 GHz) radio data in our study. We separate the observations into hard, soft, and intermediate/transitional states and detect radio emission from a compact jet in hard and intermediate states, but not in the soft. The 10-400 keV INTEGRAL (JEM-X and IBIS) state resolved spectra are well modeled with thermal Comptonization and reflection components. We detect a hard tail in the 0.4-2 MeV range for the hard state only. We extract the state dependent polarigrams of Cyg X-1, which all are compatible to no or undetectable level of polarization except in 400-2000 keV range in the hard state where the polarization fraction is 75$\pm$32 % and the polarization angle 40.0 +-14 deg. An upper limit on the 0.4-2 MeV soft state polarization fraction is 70%. Due to the short exposure, we obtain no meaningful constraint for the intermediate state. The likely detection of a >400 keV polarized tail in the hard state, together with the simultaneous presence of a radio jet, reinforce the notion of a compact jet origin of the 400 keV emission.

Motivation & Objective

• To investigate the spectral state dependence of 0.4–2 MeV polarized emission in Cygnus X-1 using state-resolved INTEGRAL/IBIS data.
• To correlate the high-energy X-ray polarization with radio emission at 15 GHz from the AMI telescope to test the compact jet origin of the hard tail.
• To improve constraints on polarization in the soft and intermediate states by increasing exposure time, particularly in the soft state.
• To determine whether the high-energy emission above 400 keV originates from a jet via synchrotron radiation, based on polarization and spectral characteristics.

Proposed method

• Data from INTEGRAL/IBIS and JEM-X were collected over ~10 years, with exposure extended to December 2012, increasing soft state exposure nearly twofold.
• Observations were classified into hard (LHS), intermediate (IS), and soft (HSS) states using criteria from Grinberg et al. (2013).
• Broadband 10 keV–2 MeV spectra were modeled with thermal Comptonization and reflection components, with a hard power-law tail in the 0.4–2 MeV range only in the hard state.
• Polarization fractions and angles were extracted from the 0.4–2 MeV band using state-resolved polarigrams, with uncertainties propagated from statistical and systematic errors.
• Radio data at 15 GHz from the AMI/Ryle telescope were used to identify compact jet emission, with flux densities compared across spectral states.
• Upper limits on polarization were derived for the IS and HSS due to lower exposure, using Bayesian and likelihood-based methods.

Experimental results

Research questions

• RQ1 Is the 0.4–2 MeV polarized emission in Cygnus X-1 state-dependent, and if so, in which spectral state is it detectable?
• RQ2 Does the presence of a compact radio jet at 15 GHz correlate with the detection of polarized emission above 400 keV?
• RQ3 What is the polarization fraction and angle of the 0.4–2 MeV emission in the hard state, and is it consistent with a jet origin?
• RQ4 Why is no significant polarization detected in the soft or intermediate states despite increased exposure?
• RQ5 Can the high-energy tail above 400 keV be explained by synchrotron emission from a highly ordered magnetic field in a compact jet?

Key findings

• A hard tail in the 0.4–2 MeV range is detected only in the hard state, well modeled by a hard power-law with no obvious cut-off.
• The polarization fraction in the 0.4–2 MeV range in the hard state is 75±32%, with a polarization angle of 40.0°±14.3°, consistent with synchrotron emission from a compact jet.
• No significant polarization is detected in the intermediate state due to low exposure, yielding a non-constraining upper limit.
• In the soft state, the upper limit on the 0.4–2 MeV polarization fraction is 70%, despite nearly doubled exposure, indicating a much fainter or unpolarized tail.
• Radio emission at 15 GHz is detected with flux densities of ~13 mJy in the hard state and ~15 mJy in the intermediate state, indicating a compact jet is present in these states.
• No persistent radio emission is detected in the soft state, supporting the absence of a compact jet and consistent with the lack of polarized X-ray emission.