[Paper Review] The Be/X-ray binary A0535+26 during its recent 2009/2010 outbursts

I. Caballero, K. Pottschmidt+21 more|arXiv (Cornell University)|Jul 18, 2011

Astrophysical Phenomena and Observations4 citations

TL;DR

This study investigates the Be/X-ray binary A0535+26 during its 2009/2010 outbursts using multi-wavelength X-ray observations from INTEGRAL, RXTE, Suzaku, and Swift. Despite significant variability in X-ray flux, including a double-peaked outburst and a giant outburst, the cyclotron line energy remained stable across luminosity changes, challenging the expected anti-correlation between cyclotron energy and accretion rate seen in other pulsars.

ABSTRACT

The Be/X-ray binary A0535+26 showed a giant outburst in December 2009 that reached ~5.14 Crab in the 15-50 keV range. Unfortunately, due to Sun constraints it could not be observed by most X-ray satellites. The outburst was preceded by four weaker outbursts associated with the periastron passage of the neutron star. The fourth of them, in August 2009, presented a peculiar double-peaked light curve, with a first peak lasting about 9 days that reached a (15-50 keV) flux of 440 mCrab. The flux then decreased to less than 220 mCrab, and increased again reaching 440 Crab around the periastron. The outburst was monitored with INTEGRAL, RXTE, and Suzaku TOO observations. One orbital period (~111 days) after the 2009 giant outburst, a new and unexpectedly bright outburst took place (~1.4Crab in the 15-50 keV range). It was monitored with TOO obs ervations with INTEGRAL, RXTE, Suzaku, and Swift. First results of the spectral and timing analysis of these observations are presented, with a specific focus on the cyclotron lines present in the system and its variation with the mass accretion rate.

Motivation & Objective

• To study the spectral and timing properties of A0535+26 during its 2009/2010 outbursts, particularly focusing on cyclotron line behavior.
• To understand the origin of the unusual double-peaked light curve observed in August 2009.
• To investigate whether cyclotron line energy varies with mass accretion rate during giant and normal outbursts.
• To explore the connection between optical disk formation in the Be companion and X-ray outburst activity.
• To determine if the April 2010 outburst, occurring after a giant outburst, defies standard outburst recurrence models.

Proposed method

• Utilized simultaneous observations from INTEGRAL, RXTE, Suzaku, and Swift during multiple outbursts.
• Performed spectral fitting of X-ray data to measure cyclotron line energy and depth across different flux states.
• Analyzed pulse period evolution using RXTE data to track neutron star spin behavior during outbursts.
• Correlated X-ray light curves with optical Hα data to assess disk formation in the Be star companion.
• Used Swift-BAT light curves to monitor flux evolution and identify outburst onset and peak times.
• Applied background modeling techniques to HEXTE data, especially for the giant outburst, to ensure accurate cyclotron line measurements.

Experimental results

Research questions

• RQ1 What causes the double-peaked light curve observed in the August 2009 outburst of A0535+26?
• RQ2 Does the cyclotron line energy in A0535+26 vary significantly with X-ray luminosity during outbursts, as seen in other pulsars?
• RQ3 Why did a bright outburst occur in April 2010, shortly after a giant outburst, contrary to typical outburst recurrence patterns?
• RQ4 How does the evolution of the cyclotron line energy during the 2009 giant outburst compare to previous outbursts?
• RQ5 To what extent does the presence of a decretion disk around the Be star correlate with the timing and intensity of X-ray outbursts?

Key findings

• The August 2009 outburst exhibited a double-peaked light curve with two flux peaks of ~440 mCrab each, separated by a dip to <220 mCrab.
• The 2009 giant outburst reached a flux of ~5.14 Crab in the 15–50 keV band, but was only observed by RXTE due to solar constraints.
• The April 2010 outburst reached a flux of ~1.4 Crab in the 15–50 keV band, unexpectedly bright after a giant outburst.
• The fundamental cyclotron line energy remained stable at ~43.4 keV during the April 2010 outburst, with a measured value of 43.4 ± 0.3 keV in one INTEGRAL observation.
• No significant variation in cyclotron line energy was observed during the December 2009 giant outburst or the April 2010 outburst, despite large flux changes.
• The Hα equivalent width increased dramatically between August and December 2009, indicating disk growth in the Be star, which may explain the double-peaked light curve.