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Radio pulse search and X-Ray monitoring of SAX J1808.4-3658: What causes its orbital evolution?

Radio pulse search and X-Ray monitoring of SAX J1808.4-3658: What causes its orbital evolution?
Radio pulse search and X-Ray monitoring of SAX J1808.4-3658: What causes its orbital evolution?
The accreting millisecond X-ray pulsar (AMXP) SAX J1808.4-3658, shows a peculiar orbital evolution that proceeds at a much faster pace than predicted by conservative binary evolution models. It is important to identify the underlying mechanism responsible for this behavior because it can help to understand how this system evolves. It has also been suggested that, when in quiescence, SAX J1808.4-3658 turns on as a radio pulsar, a circumstance that might provide a link between AMXPs and black-widow radio pulsars. In this work we report the results of a deep radio pulsation search at 2 GHz using the Green Bank Telescope in August 2014 and an X-ray monitoring of the 2015 outburst with Chandra, Swift, and INTEGRAL. In particular, we present the X-ray timing analysis of a 30-ks Chandra observation executed during the 2015 outburst. We detect no radio pulsations, and place the strongest limit to date on the pulsed radio flux density of any AMXP. We also find that the orbit of SAX J1808.4-3658 continues evolving at a fast pace and we compare it to the bhevior of other accreting and non-accreting binaries. We discuss two scenarios: either the neutron star has a large moment of inertia (I>1.7x10^45 g cm^2) and is ablating the donor (by using its spin-down power) thus generating mass-loss with an efficiency of 40% or the donor star is undergoing quasi-cyclic variations due to a varying mass-quadrupole induced by either a strong (1 kG) field or by some unidentified mechanism probably linked to irradiation.
astro-ph.HE
0004-637X
Patruno, Alessandro
7d408255-e680-4b6d-87ed-11f51ec9a35b
Jaodand, Amruta
d72df694-1ac3-494d-93d5-9c0bd39f48ee
Kuiper, Lucien
0770880a-6a55-4e90-a65a-afb607e2bad9
Bult, Peter
73bd0a3d-c566-4046-86c9-31f6e6cdac07
Hessels, Jason
9d9c4c67-d962-4adb-8761-42a0684afa05
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e
King, Andrew R.
1f838149-873b-4340-a5d3-b678be40f12c
Wijnands, Rudy
4ac16ff6-b564-4e5d-89d4-36ff92c6030c
Klis, Michiel van der
673255ce-dd17-4da0-910c-e3cb78460636
Patruno, Alessandro
7d408255-e680-4b6d-87ed-11f51ec9a35b
Jaodand, Amruta
d72df694-1ac3-494d-93d5-9c0bd39f48ee
Kuiper, Lucien
0770880a-6a55-4e90-a65a-afb607e2bad9
Bult, Peter
73bd0a3d-c566-4046-86c9-31f6e6cdac07
Hessels, Jason
9d9c4c67-d962-4adb-8761-42a0684afa05
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e
King, Andrew R.
1f838149-873b-4340-a5d3-b678be40f12c
Wijnands, Rudy
4ac16ff6-b564-4e5d-89d4-36ff92c6030c
Klis, Michiel van der
673255ce-dd17-4da0-910c-e3cb78460636

Patruno, Alessandro, Jaodand, Amruta, Kuiper, Lucien, Bult, Peter, Hessels, Jason, Knigge, Christian, King, Andrew R., Wijnands, Rudy and Klis, Michiel van der (2017) Radio pulse search and X-Ray monitoring of SAX J1808.4-3658: What causes its orbital evolution? The Astrophysical Journal, 841 (98). (doi:10.3847/1538-4357/aa6f5b).

Record type: Article

Abstract

The accreting millisecond X-ray pulsar (AMXP) SAX J1808.4-3658, shows a peculiar orbital evolution that proceeds at a much faster pace than predicted by conservative binary evolution models. It is important to identify the underlying mechanism responsible for this behavior because it can help to understand how this system evolves. It has also been suggested that, when in quiescence, SAX J1808.4-3658 turns on as a radio pulsar, a circumstance that might provide a link between AMXPs and black-widow radio pulsars. In this work we report the results of a deep radio pulsation search at 2 GHz using the Green Bank Telescope in August 2014 and an X-ray monitoring of the 2015 outburst with Chandra, Swift, and INTEGRAL. In particular, we present the X-ray timing analysis of a 30-ks Chandra observation executed during the 2015 outburst. We detect no radio pulsations, and place the strongest limit to date on the pulsed radio flux density of any AMXP. We also find that the orbit of SAX J1808.4-3658 continues evolving at a fast pace and we compare it to the bhevior of other accreting and non-accreting binaries. We discuss two scenarios: either the neutron star has a large moment of inertia (I>1.7x10^45 g cm^2) and is ablating the donor (by using its spin-down power) thus generating mass-loss with an efficiency of 40% or the donor star is undergoing quasi-cyclic variations due to a varying mass-quadrupole induced by either a strong (1 kG) field or by some unidentified mechanism probably linked to irradiation.

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Accepted/In Press date: 23 April 2017
e-pub ahead of print date: 30 May 2017
Keywords: astro-ph.HE

Identifiers

Local EPrints ID: 412936
URI: http://eprints.soton.ac.uk/id/eprint/412936
ISSN: 0004-637X
PURE UUID: 071ee99f-1500-4b06-8ee3-9502cb838dc8

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Date deposited: 09 Aug 2017 16:31
Last modified: 21 Nov 2021 07:57

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Contributors

Author: Alessandro Patruno
Author: Amruta Jaodand
Author: Lucien Kuiper
Author: Peter Bult
Author: Jason Hessels
Author: Andrew R. King
Author: Rudy Wijnands
Author: Michiel van der Klis

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