X-Ray Reflection and an Exceptionally Long Thermonuclear Helium Burst from IGR J17062-6143

Abstract

Thermonuclear X-ray bursts from accreting neutron stars power brief but strong irradiation of their surroundings, providing a unique way to study accretion physics. We analyze MAXI /GSC and Swift /XRT spectra of a day-long flash observed from IGR J17062-6143 in 2015. It is a rare case of recurring bursts at a low accretion luminosity of 0.15% Eddington. Spectra from MAXI, Chandra, and NuSTAR observations taken between the 2015 burst and the previous one in 2012 are used to determine the accretion column. We find it to be consistent with the burst ignition column of 5 × 10 10 g cm −2 , which indicates that it is likely powered by burning in a deep helium layer. The burst flux is observed for over a day, and decays as a straight power law: F ∝ t −1.15 . The burst and persistent spectra are well described by thermal emission from the neutron star, Comptonization of this emission in a hot optically thin medium surrounding the star, and reflection off the photoionized accretion disk. At the burst peak, the Comptonized component disappears, when the burst may dissipate the Comptonizing gas, and it returns in the burst tail. The reflection signal suggests that the inner disk is truncated at ∼ 10 2 gravitational radii before the burst, but may move closer to the star during the burst. At the end of the burst, the flux drops below the burst cooling trend for 2 days, before returning to the pre-burst level.

NICER Observes the Effects of an X-Ray Burst on the Accretion Environment in Aql X-1

“…Further reflection features are predicted in the soft X-ray band below 3 keV, including a multitude of emission lines on top of a free-free continuum (Ballantyne 2004). Burst reflection may, therefore, explain the soft excess over a blackbody detected during a bright burst observed with both Chandra and RXTE/PCA (in't Zand et al 2013) and two long bursts seen with the Swift X-ray Telescope (Degenaar et al 2013;Keek et al 2017). Moreover, an increase of the persistent emission may also contribute to the soft excess.…”

“…When left free, the bestfitting value of N H is substantially smaller than both the preburst fit and the Galactic hydrogen maps indicate. Similar to Keek et al (2017), we fix N H to the pre-burst value: the fit yields 1.31 2 c = n (ν = 560), and a substantial soft excess is visible in the fit residuals below E1 keV (Figure 3, left). At 0.8 keV, the observed count rate is ∼2 times the value of the best-fitting blackbody model, and the excess is ∼500 times the background.…”

IGR J17062–6143 Is an Accreting Millisecond X-Ray Pulsar

“…This included variability in the X-ray light curve as well as reflection features in the spectrum. Reflection signatures were also detected by NuSTAR in observations of the persistent flux (Degenaar et al 2017;Keek et al 2016). They indicate that the accretion disk is truncated at ∼ 10 2 R g , where R g = GM/c 2 is the gravitational radius.…”

“…We employ the Tübingen−Boulder model with abundances from Wilms et al (2000) to model the effects of interstellar absorption. The absorption column is fixed to N H = 1.58 × 10 21 cm −2 (Keek et al 2016). We find a powerlaw photon index of Γ = 2.21 ± 0.03 and an unabsorbed 3 − 20 keV flux of F = (1.28 ± 0.02) × 10 −10 erg s −1 cm −2 .…”

“…The outburst has persisted at a low flux level since then, and the source has also produced several bright X-ray bursts, confirming that the accretor is a neutron star. The first of these was observed by Swift in 2012 following an on-board trigger from the Swift/BAT (Degenaar et al 2013), and more recently, a long-duration burst, possibly powered by a thick helium layer (Keek et al 2016), was detected by the MAXI all-sky monitor (Negoro et al 2015). Both events exhibited signs that the bursts had a strong impact on the accretion environment around the neutron star.…”