Accretion-driven millisecond X-ray pulsars Rudy Wijnands February 2, 2008 5 0 0 Abstract: I present an overview of our current observational knowledge of the 2 six known accretion-driven millisecond X-ray pulsars. A prominent place in this n a review is given to SAX J1808.4–3658; it was the first such system discovered and J currently four outbursts have been observed from this source, three of which have 3 been studied in detail using the Rossi X-ray Timing Explorer satellite. This makes 1 SAX J1808.4–3658 the best studied example of an accretion-driven millisecond pul- 1 sar. Its most recent outburst in October 2002 is of particular interest because of v the discovery of two simultaneous kilohertz quasi-periodic oscillations and nearly 4 6 coherent oscillations during type-I X-ray bursts. This is the first (and so far only) 2 time that such phenomena are observed in a system for which the neutron star spin 1 frequency is exactly known. The other five systems were discovered within the last 0 5 three years (with IGR J00291+5934 only discovered in December 2004) and only 0 limited results have been published. / h p - 1 Introduction o r t Ordinary pulsars are born as highly-magnetized (B ∼ 1012 G), rapidly rotating (P s a ∼ 10 ms)neutron stars which spindownon timescales of 10 to 100 million years due : v to magnetic dipole radiation. However, a number of millisecond (P < 10 ms) radio Xi pulsars is known with ages of billions of years and weak (B ∼ 108−9 G) surface mag- r netic fields. Since many of these millisecond pulsars are in binaries, it has long been a suspected (see, e.g., Bhattacharya & van den Heuvel 1991 for an extended review) that the neutron stars were spun up by mass transfer from a stellar companion in a low-mass X-ray binary (LMXB), but years of searching for coherent millisecond pulsations in LMXBs failed to yield a detection (Vaughan et al. 1994 and references therein). The launch of the NASA Rossi X-ray Timing Explorer (RXTE) brought the discovery of kilohertz quasi-periodic oscillations (kHz QPOs; Strohmayer et al. 1996; Van der Klis et al. 1996) as well as nearly coherent oscillations (’burst oscillations’) duringtype-I X-ray burstsin a numberof LMXBs (e.g., Strohmayer et al. 1996), providing tantalizingly suggestive evidence for weakly magnetic neutron stars with millisecond spin periods (see Van der Klis 2000, 2004 and Strohmayer & Bildsten 2003 for more details about kHz QPOs and burst oscillations in LMXBs). 1 Accretion-driven millisecond X-ray pulsars 2 Figure1: TheRXTE/ASMlightcurvesofSAXJ1808.4–3658 duringtheSeptember 1996outburst(left),theApril1998outburst(middle)andtheOctober2002outburst (right). These light curves were made using the public ASM data available at http://xte.mit.edu/ASM lc.html. Thecountratesareforthe2–12keVenergyrange and are daily averages. In April 1998 the first accretion-driven millisecond X-ray pulsar (SAX J1808.4– 3658) was discovered (Wijnands & van der Klis 1998a) proving that indeed neutron stars in LMXBs can spin very rapidly. This conclusion was further strengthened by the discovery of four additional systems in 2002 and 2003 (Markwardt et al. 2002a, 2003a, 2003b, Galloway et al. 2002), and recently, in December 2004, with the dis- covery of IGR J00291+5934 as amillisecond X-ray pulsar (Markwardt et al. 2004a). Here, I will give a brief summary of our current observational knowledge of those accretion-driven millisecond X-ray pulsars. Preliminary versions of this review were published by Wijnands (2004a, 2004b). 2 SAX J1808.4–3658 2.1 The September 1996 Outburst InSeptember 1996, a new X-ray transient andLMXB was discovered with theWide Field Cameras(WFCs) aboardtheDutch-Italian BeppoSAXsatellite andthesource wasdesignatedSAXJ1808.4–3658(In’tZandetal.1998). Threetype-IX-raybursts were detected, demonstrating that the compact object in this system is a neutron star. From those bursts, a distance estimate of 2.5 kpc was determined (In ’t Zand et al. 1998, 2001). The maximum luminosity during this outburst was ∼ 1036 erg s−1, significantly lower than the peak outburst luminosity of ’classical’ neutron star transients (which typically can reach a luminosity of 1037 to 1038 ergs s−1). This low peak luminosity showed that the source was part of the growing group of faint neutron-star X-ray transients (Heise et al. 1999). The outburst continued for about three weeks (see Fig. 1) after which the source was thought to have returned to quiescence. However, it was found (Revnivtsev 2003) that the source was detected Accretion-driven millisecond X-ray pulsars 3 SAX J1808.4-3658 XTE J1751-305 XTE J0929-314 XTE J1807-294 XTE J1814-338 IGR J00291+5934 Figure2: Examplesofpowerspectraforeach ofthesixcurrentlyknownmillisecond X-ray pulsars showing the pulsar spikes. on October 29, 1996 (using slew data obtained with the proportional counter array [PCA] aboard RXTE) with a luminosity of about a tenth of the outburst peak luminosity. This demonstrates that six weeks after the main outburst the source was still active (possibly only sporadically), which might indicate that at the end of this outburst the source behaved in a manner very similar to what was seen during its 2000 and 2002 outbursts (see § 2.3 and § 2.4). After it was found that SAX J1808.4–3658 harbors a millisecond pulsar (§ 2.2), the three observed X-ray bursts seen with BeppoSAX/WFC were scrutinized for potential burst oscillations (In ’t Zand et al. 2001). A marginal detection of a 401 Hz oscillation was made in the third burst. This result suggested that the burst oscillations observed in the other, non-pulsating, neutron-star LMXBs occur indeed at their neutron-star spin frequencies. This result has been confirmed by the recent detection of burstoscillations duringthe 2002 outburstof SAX J1808.4–3658 (§ 2.4.2). Accretion-driven millisecond X-ray pulsars 4 2.2 The April 1998 outburst On April 9, 1998, RXTE/PCA slew observations indicated that SAX J1808.4–3658 was active again (Marshall 1998; see Fig. 1 for the RXTE/ASM light curve during this outburst). Using public TOO observations of this source from April 11, it was discovered (Wijnands & Van der Klis 1998a) that coherent 401 Hz pulsations (Fig. 2) were present in the persistent X-ray flux of the source, making it the first accretion-driven millisecond X-ray pulsar discovered. After this discovery, several more public RXTE observations were made (using the PCA) which were used by several groups to study different aspects of the source. I will briefly mention those results and I point to references for the details. A detailed analysis of the coherent timing behavior (Chakrabarty & Morgan 1998) showed that the neutron star was in a tight binary with a very low-mass companion star in a ∼2-hr orbital period. Due to the limited amount of data obtained during this outburst, only an upper limit of < 7×10−13 Hz s−1 could be obtained on the pulse-frequency derivative (Chakrabarty & Morgan 1998). Studies oftheX-rayspectrum(Gilfanov etal.1998; Heindl&Smith1998; seealsoGierlinski etal.2002andPoutanen&Gierlinski2003)andtheaperiodicrapidX-rayvariability (Wijnands & van der Klis 1998b; see also Van Straaten et al. 2005) showed an object that, apart from its pulsations, is remarkably similar to other LMXBs with comparable luminosities (the atoll sources). There is apparent modulation of the X-ray intensity at the orbital period, with a broad minimum when the pulsar is behindthe companion (Chakrabarty & Morgan 1998; Heindl & Smith 1998). Cuiet al. (1998) and Ford (2000) reported on the harmonic content, energy dependency, and soft phase lag of the pulsations. The main result of those studies is that the low-energy pulsations lag the high-energy ones by as much as ∼200 µs (∼8% of the pulsationperiod; seeCuietal. [1998], Ford[2000], andPoutanen &Gierlinski[2003] for possible explanations for these soft lags). Another interesting aspect is that the source first showed a steady decline in X-ray flux, which after 2 weeks suddenly accelerated (Gilfanov et al. 1998; Cui et al.1998; Fig.8). Thisbehaviorhasbeenattributedtothefactthatthesourcemight have entered the ’propeller regime’ in which the accretion is centrifugally inhibited (Gilfanov et al. 1998). However, after the onset of the steep decline the pulsations could still be detected (Cui et al. 1998) making this interpretation doubtful. A week after the onset of this steep decline, the X-ray flux leveled off (Cui et al. 1998; Wang et al. 2001), but as no further RXTE/PCA observations were made, the X- ray behavior of the source at the end of the outburst remained unclear. The source might have displayed a similar long-term episode of low-luminosity activity as seen at the end of its 2000 and 2002 outbursts (see § 2.3 and § 2.4). SAX J1808.4–3658 was not only detected and studied in X-rays but also in the optical, IR, and in radio bands. The optical/IR counterpart of SAX J1808.4–3658 (later named V4580 Sgr; Kazarovets et al. 2000) was first discovered by Roche et al. (1998) and subsequently confirmed by Giles et al. (1998). A detailed study of Accretion-driven millisecond X-ray pulsars 5 the optical behavior during this outburst was reported by Giles et al. (1999) and Wang et al. (2001). Both papers reported that the peak V magnitude of the source was ∼16.7 and the source decayed in brightness as the outburst progressed. The brightness of the source leveled off at around V ∼ 18.5 (I ∼ 17.9) about ∼2 weeks after the peak of the outburst. It stayed at this level for at least several weeks before it further decreased in brightness. This behavior suggests that the source was indeed still active for a long period after the main outburst. It was also reported (Giles et al. 1999) that the optical flux was modulated at the 2-hr orbital period of thesystem. Modeling the X-ray and optical emission from the system using an X-ray-heated accretion disk model yielded a Av of 0.68 and an inclination of cos i = 0.65 (Wang et al. 2001), resulting in a mass of the companion star of 0.05–0.10 solar masses. During some of the IR observations, the source was too bright to be consistent with emission from the disk or the companion star, even when considering X-ray heating. This IR excess might be due to synchrotron processes, likely related to an outflow or ejection of matter (Wang et al. 2001). Such an ejection event was also confirmed by the discovery of the radio counterpart (Gaensler et al. 1999). The source was detected with a 4.8 GHz flux of ∼0.8 mJy on 1998 April 27, but it was not detected at earlier or later epochs. 2.3 The January 2000 outburst On January 21, 2000, SAX J1808.4–3658 was again detected (Wijnands et al. 2001) with the RXTE/PCA at a flux level of ∼10–15 mCrab (2–10 keV), i.e. about a tenthofthepeakfluxesobservedduringthetwopreviousoutbursts. Usingfollow-up RXTE/PCA observations, it was found that the source exhibited low-level activity for several months (Wijnands et al. 2001). Due to solar constraints the source could not be observed before January 21 but likely a true outburst occurred before that date and we only observed the end stages of this outburst. This is supported by the very similar behavior of the source observed near the end of its 2002 October outburst (see § 2.4; Fig. 5). Duringthe 2000 outburst, SAXJ1808.4–3658 was observed (usingRXTE/PCA) on some occasions at luminosities of ∼1035 ergs s−1, but on other occasions (a few daysearlier orlater)ithadluminosities of∼1032 ergss−1 (as seenduringBeppoSAX and XMM-Newton observations; Wijnands et al. 2002, Wijnands 2003; see Fig. 3 left panel). This demonstrates that the source exhibited extreme luminosity swings (a factor of >1000) on timescales of days. During the RXTE observations, it was also found that on several occasions the source exhibited strong (up to 100 % r.m.s. amplitude) violent flaring behavior with a repetition frequency of about 1 Hz (Van der Klis et al. 2000; Fig. 9). During this episode of low-level activity, the pulsations at 401 Hz were also detected. Thesourcewasagaindetectedinoptical, albeitatalowerbrightnessthanduring the 1998 outburst (Wachter & Hoard 2000). This is consistent with the lower X-ray activity seenforthesource. Thesourcewasfrequentlyobservedduringthisoutburst Accretion-driven millisecond X-ray pulsars 6 2000 Outburst 2001 Quiescence Figure 3: The XMM-Newton images of the field containing SAX J1808.4–3658 during its 2000 outburst (left panel; Wijnands 2003) and when the source was in quiescence (in 2001; right panel; see Campana et al. 2002). Clearly, SAX J1808.4– 3658(thesourceinthemiddleoftheimage)wasbrighter(albeitifonlybyafactorof a few) during the 2000 outburst observation than during the quiescent observation. and preliminary results were presented by Wachter et al. (2000). The main results are presented in Figure 4 (reproduced with permission from Stefanie Wachter). The optical and X-ray brightness of the source are correlated at the end of the outburst, although one optical flare (around day 435–440 in Fig. 4) was not accompanied by an X-ray flare. However, the optical and X-ray observations were not simultaneous, which means that a brief (around a few days) X-ray flare could have been missed. During the earlier stages of the outburst, the X-ray and the optical behavior of the source were not correlated (Fig. 4 lower panel): the source is highly variable in X-rays, but quite stable in optical with only low amplitude variations. This stable period in the optical is very similar to the episode of stable optical emission in the late stages of the 1998 outburst, suggesting this is typical behavior for this source. 2.4 The October 2002 outburst In2002October,thefourthoutburstofSAXJ1808.4–3658 wasdetected(Markwardt et al. 2002b), immediately launching anextensive RXTE/PCAobservingcampaign. The main results are summarized below. 2.4.1 The X-ray light curve The RXTE/PCA light curve for this outburst is shown in Figure 5 (see Fig. 1 for the ASM light curve). During the first few weeks, the source decayed steadily, until the rate of decline suddenly increased, in a manner similar to what was observed Accretion-driven millisecond X-ray pulsars 7 Optical X-ray Optical X-ray Figure 4: The RXTE/PCA (Wijnands et al. 2001) and the optical (I band) light curves (Wachter et al. 2000) of SAX J1808.4–3658 as observed during its 2000 out- burst. The optical data were kindly provided by Stefanie Wachter. during the 1998 outburst (see § 2.2). During both the 1998 and 2002 outbursts, the moment of acceleration of the decline occurred at about two weeks after the peak of the outburst. Approximately five days later the X-ray count rate rapidly increased again until it reached a peak of about a tenth of the outburst maximum. After that the source entered a state in which the count rate rapidly fluctuated on time scales of days to hours, very similar to the 2000 low-level activity (see § 2.3). The 2002 outburst light curve is the most detailed one seen for this source and it exhibits all features seen during the previous three outbursts of the source (the initial decline, the increase in the decline rate, the long-term low-level activity), demonstrating that this behavior is typical for this source. 2.4.2 The X-ray bursts and the burst oscillations During the first five days of the outburst, four type-I X-ray bursts were detected. Burst oscillations were observed during the rise and decay of each burst, but not during the peak (Chakrabarty et al. 2003). The frequency in the burst tails was constant and identical to the spin frequency, while the oscillation in the burst rise Accretion-driven millisecond X-ray pulsars 8 SAX J1808.4-3658 XTE J1751-305 XTE J0929-314 XTE J1807-294 XTE J1814-338 Figure 5: The RXTE/PCA light curves of five of the six accretion-driven millisec- ond X-ray pulsars. The data for SAX J1808.4-3658 was obtained during its 2002 outburst. The data were taken from van Straaten et al. (2005), except for XTE J1807–294 which were taken from Linares et al. (2005 in preparation). showed evidence for a very rapid frequency drift of up to 5 Hz. This frequency behavior and the absence of oscillations at the peak of the bursts is similar to the burst oscillations seen in other, non-pulsating neutron star LMXBs, demonstrating that indeed the burst-oscillations occur at the neutron-star spin frequency in all sources. As a consequence, the spin frequency is now known for 18 LMXBs (12 burst-oscillations sources and 6 pulsars) with the highest spin frequency being 619 Hz. The sample of burst-oscillation sources was used to demonstrate that neutron stars in LMXBs spin well below the break-up frequency for neutron stars. This could suggest that the neutron stars are limited in their spin frequencies, possible duetotheemissionofgravitational radiation(Chakrabartyetal.2003; Chakrabarty 2004). Accretion-driven millisecond X-ray pulsars 9 Figure 6: The power spectrum of SAX J1808.4–3658. The top panel shows the two simultaneous kHz QPOs discovered during its 2002 outburst. The bottom panel shows the enigmatic 410 Hz QPO also seen during this outburst. The figures are adapted from Wijnands et al. (2003). 2.4.3 The kHz QPOs Wijnands et al. (2003) reported on the discovery of two simultaneous kHz QPOs during the peak of the outburst with frequencies of ∼700 and ∼500 Hz (Fig. 6 top panel). This was the first detection of twin kHz QPOs in a source with a known spin-frequency. The frequency separation of those two kHz QPOs is only ∼200 Hz, significantly below the 401 Hz expected in the beat-frequency models proposed to explain thekHz QPOs. Therefore, thosemodels arefalsified by thediscovery of kHz QPOs in SAX J1808.4–3658. The fact that the peak separation is approximately half the spin frequency suggests that the kHz QPOs are indeed connected to the neutron-star spin frequency, albeit in a way not predicted by any existing model at the time of the discovery. The lower-frequency kHz QPO was only seen during the peak of the outburst (October 16, 2002) but the higher-frequency kHz QPO could be traced throughout the main part of the outburst (Wijnands et al. 2003). In addition to the twin kHz QPOs, a third kHz QPO was found with frequencies Accretion-driven millisecond X-ray pulsars 10 SAX J1808.4-3658 XTE J1751-305 XTE J0929-314 XTE J1807-294 XTE J1814-338 IGR J00291+5934 Figure 7: Examples of the aperiodic timing features seen in the six millisecond pulsars. For SAX J1808.4–3658 we show a power spectrum obtained duringits 1998 outburst. (∼410 Hz) just exceeding the pulse frequency (Fig. 6 bottom panel; Wijnands et al.2003). ThenatureofthisQPOisunclearbutitmightberelated totheside-band kHz QPO seen in several other sources (Jonker et al. 2000). Wijnands et al. (2003) pointed out that there appear to exist two classes of neutron-star LMXBs: the ’fast’ and the ’slow’ rotators. The fast rotators have spin frequencies >∼400 Hz and the frequency separation between the kHz QPOs is roughly equal to half the spin frequency. In contrast, the slow rotators have spin frequencies below <∼400 Hz and a frequency separation roughly equal to the spin frequency. TheselatestkHzQPOresultshavespurrednewtheoreticalinvestigations into the nature of kHz QPO, involving spin induced resonance in the disk (e.g., Wijnands et al. 2003; Kluzniak et al. 2004; Lee et al. 2004; Lamb & Miller 2004; Kato 2004).