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

Burderi, L.; Salvo, T. Di; D’Antona, Francesca; Robba, N. R.; Testa, V.

doi:10.1051/0004-6361:20030669

Cited by 88 publications

(121 citation statements)

References 23 publications

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“…In this case the magnetosphere should be devoid of matter and the radio pulsar mechanism should turn on with a strong pulsar wind preventing further accretion (see e.g., Stella et al 1994;Burderi et al 2001). This is what is currently thought to occur in the radio-pulsar phase of the three transitional pulsars recently discovered (Archibald et al 2009;Papitto et al 2013;Bassa et al 2014;Patruno et al 2014;Roy et al 2014Roy et al , 2015Stappers et al 2014) and in the quiescence phase of SAX J1808.4−3658 (Homer et al 2001;Burderi et al 2003;). The fact that in SAX J1808.4 −3658 the X-ray luminosity increases by three orders of magnitude right after reaching the luminosity minima on a very fast timescale of 1-2 days (see Figures 2 and 3) suggests that the radio pulsar mechanism does not turn on, although a very rapid switch cannot be excluded at the moment.…”

Section: Accretion Flow Geometrymentioning

confidence: 96%

“…The exact trigger of the reflare is uncertain-they appear spontaneously in the simulations of Dubus et al (2001), although they do not resemble the observed reflares and they are seen by Hameury et al (2000) where reflares are caused by an increased irradiation of the donor star that causes a superoutburst (so called because their duration is much larger than that of normal outbursts) followed by reflares. The donor star in SAX J1808.4−3658 is observed to be strongly irradiated during quiescence (Homer et al 2001;Burderi et al 2003;) and indeed there are suggestions that it is losing a large amount of mass (di Salvo et al 2008; see however Hartman et al 2008 for criticisms of this strong mass loss scenario in SAX J1808.4 −3658 ). Another suggestion for the reflare trigger comes from the mass reservoir model of Osaki et al (2001), where reflares are triggered also after superoutbursts as long as the effective viscosity of the disk (parametrized by the α parameter) remains large through the entire sequence of reflares.…”

Section: The Origin Of Reflaresmentioning

confidence: 99%

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The Reflares and Outburst Evolution in the Accreting Millisecond Pulsar Sax J1808.4–3658: A Disk Truncated Near Co-Rotation?

Patruno

Maitra

Curran

et al. 2016

ApJ

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The accreting millisecond X-ray pulsar SAX J1808.4-3658 shows peculiar low luminosity states known as "reflares" after the end of the main outburst. During this phase the X-ray luminosity of the source varies by up to three orders of magnitude in less than 1-2 days. The lowest X-ray luminosity observed reaches a value of 10 erg s 32 1 -, only a factor of a few brighter than its typical quiescent level. We investigate the 2008 and 2005 reflaring state of SAX J1808.4-3658 to determine whether there is any evidence for a change in the accretion flow with respect to the main outburst. We perform a multiwavelength photometric and spectral study of the 2005 and 2008 reflares with data collected during an observational campaign covering the near-infrared, optical, ultra-violet and X-ray band. We find that the NIR/optical/UV emission, expected to come from the outer accretion disk, shows variations in luminosity over an order of magnitude. The corresponding X-ray luminosity variations are instead much deeper, spanning about 2-3 orders of magnitude. The X-ray spectral state observed during the reflares does not change substantially with X-ray luminosity, indicating a rather stable configuration of the accretion flow. We investigate the most likely configuration of the innermost regions of the accretion flow and we infer an accretion disk truncated at or near the co-rotation radius. We interpret these findings as due to either a strong outflow (due to a propeller effect) or a trapped disk (with limited/no outflow) in the inner regions of the accretion flow.

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Section: Accretion Flow Geometrymentioning

confidence: 96%

Section: The Origin Of Reflaresmentioning

confidence: 99%

The Reflares and Outburst Evolution in the Accreting Millisecond Pulsar Sax J1808.4–3658: A Disk Truncated Near Co-Rotation?

Patruno

Maitra

Curran

et al. 2016

ApJ

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“…The same behaviour is seen in the quiescent optical light curve of IGR J00291+5934 (D'Avanzo et al 2007) 2 . In a theoretical study on SAX J1808.4−3658 Burderi et al (2003) proposed, on the base of previous works (Stella et al 1994;Campana et al 1998), that the irradiation is due to the release of rotational energy by the fast spinning neutron star, switched on, as a radio pulsar, during quiescence. Following this idea, Campana et al (2004) and D'Avanzo et al (2007) measured the required irradiating luminosity needed to match the optical flux of SAX J1808.4−3658 and IGR J00291+5934, respectively, and found that it is a factor of about 100 larger than the observed quiescent X-ray luminosity for both systems.…”

Section: Introductionmentioning

confidence: 99%

The optical counterparts of accreting millisecond X-ray pulsars during quiescence

D’Avanzo

Campana²,

Casares

et al. 2009

A&A

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Context. Eight accreting millisecond X-ray pulsars (AMXPs) are known to date. Although these systems are well studied at high energies, very little information is available for their optical/NIR counterparts. Up to now, only two of them, SAX J1808.4−3658 and IGR J00291+5934, have a secure multi-band detection of their optical counterparts in quiescence. Aims. All these systems are transient low-mass X-ray binaries. Optical and NIR observations carried out during quiescence give a unique opportunity to constrain the nature of the donor star and to investigate the origin of the observed quiescent luminosity at long wavelengths. In addition, optical observations can be fundamental as they ultimately allow us to estimate the compact object mass through mass function measurements. Methods. Using data obtained with the ESO-Very Large Telescope, we performed a deep optical and NIR photometric study of the fields of XTE J1814−338 and of the ultracompact systems XTE J0929−314 and XTE J1807−294 during quiescence in order to look for the presence of a variable counterpart. If suitable candidates were found, we also carried out optical spectroscopy. Results. We present here the first multi-band (VR) detection of the optical counterpart of XTE J1814−338 in quiescence together with its optical spectrum. The optical light curve shows variability in both bands consistent with a sinusoidal modulation at the known 4.3 h orbital period and presents a puzzling decrease of the V-band flux around superior conjunction that may be interpreted as a partial eclipse. The marginal detection of the very faint counterpart of XTE J0929−314 and deep upper limits for the optical/NIR counterpart of XTE J1807−294 are also reported. We also briefly discuss the results reported in the literature for the optical/NIR counterpart of XTE J1751−305. Conclusions. Our findings are consistent with AMXPs being systems containing an old, weakly magnetized neutron star, reactivated as a millisecond radio pulsar during quiescence which irradiates the low-mass companion star. The absence of type I X-ray bursts and of hydrogen and helium lines in outburst spectra of ultracompact (P orb < 1 h) AMXPs suggests that the companion stars are likely evolved dwarf stars.

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“…This luminosity excess can be interpreted as the luminosity L PSR isotropically irradiated by the rotating magneto-dipole, intercepted, and reprocessed by the companion star, as observed e.g. by Burderi et al (2003) and Campana et al (2004) for SAX J1808.4-3658, and by D'Avanzo et al (2007) for IGR J00291+5934.…”

Section: Observations and Data Analysismentioning

confidence: 60%

Searching for pulsed emission from XTE J0929–314 at high radio frequencies

Iacolina

Burgay

Burderi

et al. 2009

A&A

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Aims. The aim of this work is to search for radio signals in the quiescent phase of accreting millisecond X-ray pulsars, in this way giving an ultimate proof of the recycling model, thereby unambiguously establishing that accreting millisecond X-ray pulsars are the progenitors of radio millisecond pulsars. Methods. To overcome the possible free-free absorption caused by matter surrounding accreting millisecond X-ray pulsars in their quiescence phase, we performed the observations at high frequencies. Making use of particularly precise orbital and spin parameters obtained from X-ray observations, we carried out a deep search for radio-pulsed emission from the accreting millisecond X-ray pulsar XTE J0929-314 in three steps, correcting for the effect of the dispersion due to the interstellar medium, eliminating the orbital motions effects, and finally folding the time series. Results. No radio pulsation is present in the analyzed data down to a limit of 68 μJy at 6.4 GHz and 26 μJy at 8.5 GHz. Conclusions. We discuss several mechanisms that could prevent the detection, concluding that beaming factor and intrinsic low luminosity are the most likely explanations.

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

Cited by 88 publications

References 23 publications

The Reflares and Outburst Evolution in the Accreting Millisecond Pulsar Sax J1808.4–3658: A Disk Truncated Near Co-Rotation?

The Reflares and Outburst Evolution in the Accreting Millisecond Pulsar Sax J1808.4–3658: A Disk Truncated Near Co-Rotation?

The optical counterparts of accreting millisecond X-ray pulsars during quiescence

Searching for pulsed emission from XTE J0929–314 at high radio frequencies

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