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Phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburst

Phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburst
Phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburst
Aims: We obtained phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its outburst in 2008 to find a signature of the donor star, constrain its radial velocity semi-amplitude (K2), and derive estimates for the pulsar mass.

Methods: Using Doppler images of the Bowen region, we find a significant (8) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure = 24820 km?s-1 (1 confidence), which represents a strict lower limit to K2. Also, the Doppler map of He?II 4686 shows the characteristic signature of the accretion disc, and there is a hint of enhanced emission that may be a result of tidal distortions in the accretion disc that are expected in very low mass-ratio interacting binaries.

Results: The lower limit on K2 leads to a lower limit on the mass function of f(M1)??0.10 . Applying the maximum K-correction gives 228?<?K2?<?322 km?s-1 and a mass ratio of 0.051?<?q?<?0.072.

Conclusions: Despite the limited S/N of the data, we were able to detect a signature of the donor star in SAX?J1808.4-3658, although future observations during a new outburst are still needed to confirm this. If the derived is correct, the largest uncertainty in determining of the mass of the neutron star in SAX?J1808.4-3658 using dynamical studies lies with the poorly known inclination.


accretion, accretion disks, X-rays binaries, stars individual SAX?J1808.4-3658
0004-6361
L1-L4
Cornelisse, R.
06976a3f-4a05-4c72-8462-7e71b00e7b77
D'Avanzo, P.
41a3a281-101d-433e-a025-60df56d80668
Muñoz-Darias, T.
c1592242-cc9a-4ec6-8d3e-84acb8bae1c9
Campana, S.
e4a1e868-5b27-48c6-af9d-ea433a148e7c
Casares, J.
e57ae499-4f86-421a-bcb1-67d17d2bcbfc
Charles, P.A.
0429b380-0754-4dc1-8def-885c7fa6a086
Steeghs, D.
ab1118c0-944a-427a-bfbf-6b82e1320280
Israel, G.
a04ed06a-d10a-4d1a-b8be-700442435fd1
Stella, L.
2a91c11a-f382-438c-b36a-8566b1b50497
Cornelisse, R.
06976a3f-4a05-4c72-8462-7e71b00e7b77
D'Avanzo, P.
41a3a281-101d-433e-a025-60df56d80668
Muñoz-Darias, T.
c1592242-cc9a-4ec6-8d3e-84acb8bae1c9
Campana, S.
e4a1e868-5b27-48c6-af9d-ea433a148e7c
Casares, J.
e57ae499-4f86-421a-bcb1-67d17d2bcbfc
Charles, P.A.
0429b380-0754-4dc1-8def-885c7fa6a086
Steeghs, D.
ab1118c0-944a-427a-bfbf-6b82e1320280
Israel, G.
a04ed06a-d10a-4d1a-b8be-700442435fd1
Stella, L.
2a91c11a-f382-438c-b36a-8566b1b50497

Cornelisse, R., D'Avanzo, P., Muñoz-Darias, T., Campana, S., Casares, J., Charles, P.A., Steeghs, D., Israel, G. and Stella, L. (2009) Phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburst. Astronomy & Astrophysics, 495 (1), L1-L4. (doi:10.1051/0004-6361:200811396).

Record type: Article

Abstract

Aims: We obtained phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its outburst in 2008 to find a signature of the donor star, constrain its radial velocity semi-amplitude (K2), and derive estimates for the pulsar mass.

Methods: Using Doppler images of the Bowen region, we find a significant (8) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure = 24820 km?s-1 (1 confidence), which represents a strict lower limit to K2. Also, the Doppler map of He?II 4686 shows the characteristic signature of the accretion disc, and there is a hint of enhanced emission that may be a result of tidal distortions in the accretion disc that are expected in very low mass-ratio interacting binaries.

Results: The lower limit on K2 leads to a lower limit on the mass function of f(M1)??0.10 . Applying the maximum K-correction gives 228?<?K2?<?322 km?s-1 and a mass ratio of 0.051?<?q?<?0.072.

Conclusions: Despite the limited S/N of the data, we were able to detect a signature of the donor star in SAX?J1808.4-3658, although future observations during a new outburst are still needed to confirm this. If the derived is correct, the largest uncertainty in determining of the mass of the neutron star in SAX?J1808.4-3658 using dynamical studies lies with the poorly known inclination.


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More information

e-pub ahead of print date: 20 January 2009
Published date: February 2009
Keywords: accretion, accretion disks, X-rays binaries, stars individual SAX?J1808.4-3658
Organisations: Astronomy Group

Identifiers

Local EPrints ID: 339327
URI: http://eprints.soton.ac.uk/id/eprint/339327
ISSN: 0004-6361
PURE UUID: 5a7a8fc6-9670-4b96-abd1-d2528ccc493b

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Date deposited: 28 May 2012 15:11
Last modified: 16 Jul 2019 22:02

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