154
A. Patruno and A. L. Watts
resonance occurring at the radius in the disk where the general relativistic vertical
epicyclic frequency matches the spin frequency of the pulsar.
The 3–150 keV X-ray spectrum of the 1998 outburst had a remarkably stable
power law shape with photon index of ∼2 and a high energy cutoff at ∼100 keV
[115]. Further spectral analysis provided evidence for a two-component model: a
blackbody at soft energies and a hard Comptonization component at higher energies
[112]. The blackbody is interpreted as the heated hot spot on the NS surface,
whereas the Comptonization is produced in the accretion shock created at the
bottom of the magnetic field lines as the plasma abruptly decelerates close to the
NS surface. The presence of an accretion disk was detected much later at lower
energies, with observations taken in 2008 with the EPIC-pn camera on the XMMNewton telescope. Its large sensitivity at soft energies (down to about 0.5 keV) well
below the nominal 2 keV limit of RXTE, allowed the detection of the typical cold
accretion disk signature at a temperature of 0.2 keV [242, 257]. The signature of a
fluorescent relativistic iron Kα emission line profile was also found. A similar result
was obtained with combined XMM-Newton and Suzaku data [41]. Spectral modeling
of the iron line constrained the magnetic field of the pulsar to be ∼3 × 10 8 G at the
poles [41, 242]. Simultaneous spectral modeling of the inner disk radius and pulse
profile shapes of the 2002 outburst [142] lead to a similar constraint of the magnetic
field (B ∼ 10 8 G).
Thermonuclear bursts were observed in 1996, 2002, 2005, 2008 and 2011
[51, 98, 149]. Most of the bursts exhibit photospheric radius expansion (PRE),
where the luminosity reaches the Eddington limit, lifting the photosphere off the
surface of the NS until the flux dies down. Such bursts can be used as standard
candles [174]. The distance estimated using this method is 2.5–3.6 kpc [98, 149].
Note that a different lower limit of 3.4 kpc is reported in [98]. This lower limit is
based on the assumption that the long-term mass transfer rate is driven purely by loss
of angular momentum in the binary via emission of gravitational radiation, which
may not be a good approximation (see Sect. 4.6). All bursts observed in the RXTE
era have shown burst oscillations (with a possible marginal detection in one burst
in 1996 observed with BeppoSAX) with an amplitude of a few percent rms. SAX
J1808.4-3658 provided the first robust confirmation that burst oscillation frequency
was, to within a few Hz, the spin frequency of the star ([51] and Fig. 4.2).
An optical/IR counterpart (V4584 Sagittarii) was discovered during the 1998
outburst, coincident with the position of SAX J1808.4-3658 [291]. The reported
magnitudes of the candidate were V = 16.6, R = 16.1, I = 15.6, J = 15.0, H = 14.4,
K = 13.8, with an uncertainty of 0.2 mag in VRI and <0.1 mag in JHK. The V band
was further monitored [113] and a possible sinusoidal modulation at the 2 h period
of the binary identified, together with a decay in the luminosity as the outburst
progressed in its decay stage. A multiband optical/IR photometric study of the
optical counterpart during the 1998 outburst revealed an optical flux consistent with
an X-ray heated accretion disk and an inclination of cos i = 0.65
+0.23
−0.33 (90% c.l.)
[346]. The IR observations, however, showed an excess with respect to an accretion
disk plus irradiated donor star model. This excess was transient in nature, as it was
detected only during one observation, the rest being consistent with the irradiated
A. Patruno and A. L. Watts
resonance occurring at the radius in the disk where the general relativistic vertical
epicyclic frequency matches the spin frequency of the pulsar.
The 3–150 keV X-ray spectrum of the 1998 outburst had a remarkably stable
power law shape with photon index of ∼2 and a high energy cutoff at ∼100 keV
[115]. Further spectral analysis provided evidence for a two-component model: a
blackbody at soft energies and a hard Comptonization component at higher energies
[112]. The blackbody is interpreted as the heated hot spot on the NS surface,
whereas the Comptonization is produced in the accretion shock created at the
bottom of the magnetic field lines as the plasma abruptly decelerates close to the
NS surface. The presence of an accretion disk was detected much later at lower
energies, with observations taken in 2008 with the EPIC-pn camera on the XMMNewton telescope. Its large sensitivity at soft energies (down to about 0.5 keV) well
below the nominal 2 keV limit of RXTE, allowed the detection of the typical cold
accretion disk signature at a temperature of 0.2 keV [242, 257]. The signature of a
fluorescent relativistic iron Kα emission line profile was also found. A similar result
was obtained with combined XMM-Newton and Suzaku data [41]. Spectral modeling
of the iron line constrained the magnetic field of the pulsar to be ∼3 × 10 8 G at the
poles [41, 242]. Simultaneous spectral modeling of the inner disk radius and pulse
profile shapes of the 2002 outburst [142] lead to a similar constraint of the magnetic
field (B ∼ 10 8 G).
Thermonuclear bursts were observed in 1996, 2002, 2005, 2008 and 2011
[51, 98, 149]. Most of the bursts exhibit photospheric radius expansion (PRE),
where the luminosity reaches the Eddington limit, lifting the photosphere off the
surface of the NS until the flux dies down. Such bursts can be used as standard
candles [174]. The distance estimated using this method is 2.5–3.6 kpc [98, 149].
Note that a different lower limit of 3.4 kpc is reported in [98]. This lower limit is
based on the assumption that the long-term mass transfer rate is driven purely by loss
of angular momentum in the binary via emission of gravitational radiation, which
may not be a good approximation (see Sect. 4.6). All bursts observed in the RXTE
era have shown burst oscillations (with a possible marginal detection in one burst
in 1996 observed with BeppoSAX) with an amplitude of a few percent rms. SAX
J1808.4-3658 provided the first robust confirmation that burst oscillation frequency
was, to within a few Hz, the spin frequency of the star ([51] and Fig. 4.2).
An optical/IR counterpart (V4584 Sagittarii) was discovered during the 1998
outburst, coincident with the position of SAX J1808.4-3658 [291]. The reported
magnitudes of the candidate were V = 16.6, R = 16.1, I = 15.6, J = 15.0, H = 14.4,
K = 13.8, with an uncertainty of 0.2 mag in VRI and <0.1 mag in JHK. The V band
was further monitored [113] and a possible sinusoidal modulation at the 2 h period
of the binary identified, together with a decay in the luminosity as the outburst
progressed in its decay stage. A multiband optical/IR photometric study of the
optical counterpart during the 1998 outburst revealed an optical flux consistent with
an X-ray heated accretion disk and an inclination of cos i = 0.65
+0.23
−0.33 (90% c.l.)
[346]. The IR observations, however, showed an excess with respect to an accretion
disk plus irradiated donor star model. This excess was transient in nature, as it was
detected only during one observation, the rest being consistent with the irradiated