The Optical Counterpart of the Accreting Millisecond Pulsar SAX J1808.4−3658 in Outburst: Constraints on the Binary Inclination

Wang, Zhongxiang; Chakrabarty, Deepto; Roche, P.; Charles, P. A.; Kuulkers, E.; Shahbaz, T.; Simpson, Chris; Forbes, Duncan A.; Helsdon, Stephen F.

doi:10.1086/338357

Cited by 50 publications

(27 citation statements)

References 20 publications

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“…Such excess has been observed for all three systems during the outburst phase with a wavelength corresponding to the I band and has been interpreted as the contribution of a synchrotron emitting region, such as a synchroton jet (Krauss et al 2005;Wang et al 2001;Giles et al 2005;Russell et al 2007). In Fig.…”

Section: A Jet As a Possible Third Componentmentioning

confidence: 73%

The long-term evolution of the X-ray pulsar XTE J1814-338: A receding jet contribution to the quiescent optical emission?

Baglio

D’Avanzo

Muñoz-Darias

et al. 2013

A&A

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Aims. We present a study of the quiescent optical counterpart of the accreting millisecond X-ray pulsar XTE J1814-338 that is aimed at unveiling the different components, which contribute to the quiescent optical emission of the system. Methods. We carried out multiband (BVR) orbital phase-resolved photometry of the system using the ESO Very Large Telescope (VLT) that is equipped with the FORS2 camera, covering about 70% of the 4.3 hour orbital period. Results. The optical light curves are consistent with a sinusoidal variability that are modulated with an orbital period with a semiamplitude of 0.5−0.7 mag. They show evidence of a strongly irradiated companion star, which agrees with previous findings for this system. However, the observed colours cannot be accounted for by the companion star alone, suggesting the presence of an accretion disc during quiescence. The system seems to be fainter in all analysed bands compared to previous observations. The R band light curve displays a possible phase offset with respect to the B and V band. Through a combined fit of the multi-band light curve performed with a Markov chain Monte Carlo technique, we derive constraints on the companion star, disc fluxes, system distance, and companion star mass. Conclusions. The irradiation luminosity required to account for the observed day-side temperature of the companion star is consistent with the spin-down luminosity of a millisecond radio pulsar. Compared to our data with previous observations, which were collected over 5 years, the flux decrease and spectral evolution of the observed quiescent optical emission cannot be satisfactorily explained with the combined contribution of an irradiated companion star and of an accretion disc alone. The observed progressive flux decrease as the system gets bluer could be due to a continuum component that evolves towards a lower, bluer spectrum. While most of the continuum component is likely due to the disc, we do not expect it to become bluer in quiescence. Hence, we hypothesize that an additional component, such as synchrotron emission from a jet was significantly contributing in the data obtained earlier during quiescence and then progressively fading or moving its break frequency towards longer wavelengths.

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Section: A Jet As a Possible Third Componentmentioning

confidence: 73%

The long-term evolution of the X-ray pulsar XTE J1814-338: A receding jet contribution to the quiescent optical emission?

Baglio

D’Avanzo

Muñoz-Darias

et al. 2013

A&A

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“…For a few sources the interstellar absorption is low enough to observe these features in detail: only 2.6% of known bursts originate from sources with N H ≤ 0.2 × 10 22 cm −2 (e.g., Cornelisse et al 2003;Galloway et al 2008). An example of a source with a small absorption column is SAX J1808.4-3658 with N H = 0.12×10 22 cm −2 (Wang et al 2001). A Chandra observation of a burst from this source (with contemporaneous RXTE /PCA coverage) exhibits a soft excess with respect to a blackbody model, which is well fit with a model of reflection off a highly ionized accretion disk (in 't Zand et al 2013).…”

Section: Detectability Of Burst Reflectionmentioning

confidence: 81%

Accretion Disk Signatures in Type I X-Ray Bursts: Prospects for Future Missions

Keek

Wolf

Ballantyne

2016

ApJ

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Type I X-ray bursts and superbursts from accreting neutron stars illuminate the accretion disk and produce a reflection signal that evolves as the burst fades. Examining the evolution of reflection features in the spectra will give insight into the burst-disk interaction, a potentially powerful probe of accretion disk physics. At present, reflection has been observed during only two bursts of exceptional duration. We investigate the detectability of reflection signatures with four of the latest well-studied X-ray observatory concepts: Hitomi, NICER, Athena, and LOFT. Burst spectra are modeled for different values for the flux, temperature, and the disk ionization parameter, which are representative for most known bursts and sources. The effective area and through-put of a Hitomi-like telescope are insufficient for characterizing burst reflection features. NICER and Athena will detect reflection signatures in Type I bursts with peak fluxes 10 −7.5 erg cm −2 s −1 , and also effectively constrain the reflection parameters for bright bursts with fluxes of ∼ 10 −7 erg cm −2 s −1 in exposures of several seconds. Thus, these observatories will provide crucial new insight into the interaction of accretion flows and X-ray bursts. For sources with low line-of-sight absorption, the wide band-pass of these instruments allows for the detection of soft X-ray reflection features, which are sensitive to the disk metallicity and density. The large collecting area that is part of the LOFT design would revolutionize the field by tracing the evolution of the accretion geometry in detail throughout short bursts.

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“…The phasing of the X-ray optical modulation is consistent with X-ray reprocessing on the facing hemisphere of the companion star. Wang et al (2001) interpreted the observed optical flux in outburst as emission from an X-ray heated accretion disk. Homer et al (2001) reported on high time resolution CCD photometry of this optical counterpart observed when the X-ray source was in quiescence.…”

Section: Letter To the Editormentioning

confidence: 99%

The optical counterpart to SAX J1808.4–3658 in quiescence: Evidence of an active radio pulsar?

Burderi

Salvo

D’Antona

et al. 2003

A&A

116

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Abstract. The optical counterpart of the binary millisecond X-ray pulsar SAX J1808.4-3658 during quiescence was detected at V = 21.5 mag by Homer et al. (2001). This star shows a 6% semi-amplitude sinusoidal modulation of its flux at the orbital period of the system. It was proposed that the modulation arises from X-ray irradiation of the intrinsically faint companion by a remnant accretion disk, and that the bulk of the optical emission arises from viscous dissipation in the disk. The serious difficulty in this scenario lies in the estimate of the irradiating luminosity required to match the observational data, that is a factor 10−50 higher than the quiescent X-ray luminosity of this source. To overcome this problem, we propose an alternative scenario, in which the irradiation is due to the release of rotational energy by the fast spinning neutron star, switched on, as magneto-dipole rotator (radio pulsar), during quiescence. Our computations indicate that the optical magnitudes are fully consistent with this hypothesis. In this case the observed optical luminosity may be the first evidence that a radio pulsar is active in this system in quiescence, a key phase for understanding the evolution of this class of objects.

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The Optical Counterpart of the Accreting Millisecond Pulsar SAX J1808.4−3658 in Outburst: Constraints on the Binary Inclination

Cited by 50 publications

References 20 publications

The long-term evolution of the X-ray pulsar XTE J1814-338: A receding jet contribution to the quiescent optical emission?

The long-term evolution of the X-ray pulsar XTE J1814-338: A receding jet contribution to the quiescent optical emission?

Accretion Disk Signatures in Type I X-Ray Bursts: Prospects for Future Missions

The optical counterpart to SAX J1808.4–3658 in quiescence: Evidence of an active radio pulsar?

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