[Paper Review] Disc-Jet coupling in the LMXB 4U1636-53 from INTEGRAL

M. Fiocchi, A. Bazzano+2 more|arXiv (Cornell University)|Jun 30, 2006

Astrophysical Phenomena and Observations1 references14 citations

TL;DR

This study presents the first detection of a hard X-ray tail dominating emission above 30 keV in the neutron star LMXB 4U 1636–53 using INTEGRAL and BeppoSAX data. The spectrum is best fitted by a Comptonized component plus a power-law component (Γ = 2.76), indicating non-thermal electron acceleration, and supports a jet origin for the hard tail, suggesting disc-jet coupling similar to that seen in black hole binaries.

ABSTRACT

We report on the spectral analysis results of the neutron star, atoll type, low mass X-ray Binary 4U1636-53 observed by INTEGRAL and BeppoSAX satellites. Spectral behavior in three different epochs corresponding to three different spectral states has been deeply investigated. Two data set spectra show a continuum well described by one or two soft blackbody plus a Comptonized components with changes in the Comptonizing electrons and black body temperature and the accretion rates, which are typical of the spectral transitions from high to low state. In one occasion INTEGRAL spectrum shows, for first time in this source, a hard tail dominating the emission above 30 keV. The total spectrum is fitted as the sum of a Comptonized component similar to soft state and a power-law component (Gamma=2.76), indicating the presence of a non thermal electron distribution of velocities. In this case, a comparison with hard tails detected in soft states from neutron stars systems and some black hole binaries suggests that a similar mechanism could originate these components in both cases.

Motivation & Objective

• To investigate spectral transitions in the neutron star atoll-type LMXB 4U 1636–53 across different accretion states.
• To determine the origin of a high-energy X-ray tail observed above 30 keV in one INTEGRAL observation.
• To test whether disc-jet coupling mechanisms, previously observed in black hole binaries, are present in neutron star LMXBs.
• To assess the role of non-thermal electron distributions in shaping the hard X-ray spectrum.
• To evaluate the radio-X-ray correlation as evidence for jet activity in this neutron star system.

Proposed method

• Broadband spectral analysis using simultaneous data from INTEGRAL (JEM-X and IBIS) and BeppoSAX (LECS, MECS, PDS) instruments over three epochs.
• Spectral fitting using a Comptonization model with a blackbody component, and addition of a power-law component to model the hard tail above 30 keV.
• Application of cross-calibration constants and background rejection techniques based on fixed rise time thresholds for PDS data.
• Use of Off-line Scientific Analysis (OSA v5.1) software for processing INTEGRAL data and extracting wideband spectra from 5 to 150 keV.
• Comparison of spectral parameters (electron temperature, optical depth, blackbody temperature, accretion rate) across soft, hard, and peculiar states.
• Radio flux estimation from the Sydney University Molonglo Sky Survey to compute radio loudness (PR/PX) and compare with known jeted sources.

Experimental results

Research questions

• RQ1 What causes the hard X-ray tail dominating emission above 30 keV in 4U 1636–53, and how does it differ from standard Comptonization models?
• RQ2 Is the observed power-law component in the hard state indicative of a jet, as seen in black hole X-ray binaries?
• RQ3 How does the accretion rate vary across spectral states, and what does this imply for the geometry of the accretion flow?
• RQ4 Is there evidence for disc-jet coupling in this neutron star LMXB, based on X-ray and radio correlations?
• RQ5 Can the presence of a non-thermal electron distribution explain the spectral features observed in the peculiar state?

Key findings

• For the first time in 4U 1636–53, a hard X-ray tail dominating above 30 keV was detected in an INTEGRAL observation, best fitted by a power-law component with photon index Γ = 2.76.
• The total spectrum in the peculiar state is best described by a combination of Comptonization from a low-temperature thermal electron distribution (3–4 keV) and a non-thermal power-law electron component.
• The accretion rate in the soft state is estimated at 7.3 × 10⁻⁹ M☉ yr⁻¹, while in the hard state it is 4.0 × 10⁻⁹ M☉ yr⁻¹, indicating lower accretion in the hard state.
• The peculiar state exhibits a very high accretion rate of ~2.1 × 10⁻⁸ M☉ yr⁻¹, though this value may be influenced by the steep power-law component at low energies.
• Radio detection at 843 GHz (7.5 mJy) and estimated radio loudness (1.1–2.2 Jy/Crab at 5 GHz) are consistent with jet activity and comparable to those of black hole candidates and other jeted LMXBs.
• The presence of a hard tail and high radio loudness supports the hypothesis that the power-law component originates from a jet, indicating disc-jet coupling in a neutron star system, challenging the idea that such features are exclusive to black hole binaries.