The nature of the exotic stellar corpses which reincarnate by consuming their companion is reviewed. Apart from sucking life from their partners, they are actually eating the doomed companions away by their deadly and powerful particle/radiation beams. Such situation resembles that a female "black widow" spider that eats its mate after mating. These celestial zombies are called - Millisecond pulsars (MSPs). In this review article, I will focus on the effort of
Discovery of rotation-power pulsars is a great example of a theoretician’s dream comes true. Shortly after the discovery of the neutron by Chadwick (1932), Baade & Zwicky (1934) proposed the existence of the neutron stars. Their conclusion was resulted from their investigation of the explosive endpoint of massive stars -
The most remarkable breakthrough of neutron star study came in November 1967. At the Mullard Radio Astronomy Observatory, Antony Hewish and his collaborators had build a large radio telescope array to study interplanetary scintillation of compact radio sources at 81.5 MHz. Soon after the instrument started operating, it had been noticed a series of weak sporadic signals. Observing with this transit telescope, these fluctuating signals appeared four minutes each day which indicated their celestial origin. In November, systematic investigations of these signals were initiated by adopting a recorder with a faster response time. The
The discovery of PSR B1919+21 exerted an enormous impact on the international astronomical community. This can be reflected by the fact that more than 100 papers about pulsars were published in 1968. As a new class of objects appeared, the identification of their physical nature was badly needed. Actually, the correct path in explaining the origin of pulsar emission had already been published just before the discovery of PSR B1919+21. Adopting an oblique magnetic dipole rotator, Pacini (1967) had shown that the rotational energy of a neutron star can be converted into electromagnetic radiation. The author also specifically pointed out the possibility that a large amount of energy and momentum can be pumped from a rotating neutron star to the supernova remnant such as the Crab Nebula. Without knowing the work by Pacini (1967), Gold (1968) independently suggested that the observed pulsars were in fact rotating neutron stars which have magnetic fields as high as 1012 G. The author had also pointed out that neutron stars can explain many observed properties of pulsars such as the short and the high accuracy of the observed periodicities. Moreover, directional beam rotating like a lighthouse beacon was proposed to explain the substructure of the observed pulses. The author had also precisely predicted that the pulsar period should increase slowly as it radiates at the expense of the star’s rotational energy. Shortly after the publication of Gold (1968), a radio pulsar was discovered in the Crab Nebula which had a period as short as ~ 33 ms (Staelin & Reifenstein 1968). Such short period can only be achieved by either the rotation or vibration of a neutron star. This discovery had intimately connected supernovae, neutron stars and pulsars. Later, Richard & Comella (1968) discovered the increase in the period of the Crab Pulsar which was in agreement with the prediction by Gold (1968). Since the slowdown of the period can only be resulted from the rotation and not from vibration. This leaves a rotating neutron star as the most plausible model to explain all the observational facts. Furthermore, Gold (1969) demonstrated that the implied energy loss by the Crab Pulsar was approximately equal to the energy requires to power the Crab Nebula. At this time, the identification of pulsars as rotating neutron stars was generally accepted. The discovery of pulsars had eventually made a theoreticians’ dream come true!
Although Hewish et al. (1968) is generally regarded as the first milestone in pulsar astronomy
It is not possible to discuss the nature of MSPs without mentioning the evolution of rotation- powered pulsars. Fig. 1 shows the
As mentioned in Section 1, pulsars are rotating and highly magnetize celestial objects, they behave as natural unipolar inductors and are expected to generate huge electric field in vacuum. For those regions in the magnetosphere have their charge density deviated from the Goldreich-Julian density (which assumes a force-free condition, Goldreich & Julian 1969), electric fields can be developed along the magnetic field lines. By tapping the rotational energy of the pulsars, such regions can accelerate the particles to ultra-relativistic speed and radiate pulsed emission from radio to γ−ray. Various emission models had been proposed. Despite the fact that these models are fundamentally different from each other, they do incorporate acceleration of electron/positron in the charge depletion regions (or gaps) of certain forms. For theoretical reviews on these models, please refer to Harding (2013) and Cheng (2013).
Since pulsars radiate at the expense of its rotational energy, it will rotate slower and slower and moves gradually to the lower right corner of the diagram. After tens to hundreds million years, when the rotation eventually becomes too slow for sustaining the particle acceleration in the magnetosphere, the pulsed emission will be ceased and such pulsar enters its graveyard.
However, this is not the end of their story. Besides the main cluster centered at
This scenario was proposed by Alpar et al. (1982) shortly after the discovery of the first MSP PSR B1937+21 (Backer et al. 1982). Since then dedicated pulsar surveys have resulted in a population of more than two hundred MSPs which comprise ~ 10% of the whole pulsar population known today (Manchester et al. 2005). MSPs are characterized by their fast rotation and a surface magnetic field much weaker than the canonical pulsars (107 < B < 1010 G). Both characteristics are thought to be resulted from the recycling. Although the details of the magnetic field decay is still a subject under discussion (e.g., Konar 2010), the surface field can be somehow buried by the accretion. The current record of the fastest spin is the MSP PSR J1748-2446ad that resides in the globular cluster Terzan 5 (Hessel et al. 2006). It spins over 716 times per second which is more than double of the best Formula 1 engine!
Despite the aforementioned MSP formation scenario has been proposed for more than 30 yrs, many details of the recycling process remain uncertain. While MSPs are expected to be the end products of the X-ray binary evolution, it is surprised to notice that ~ 30% of the known MSPs in the Galactic field are found to be isolated (Roberts 2013). One popular scenario to explain their solitudes is the high energy radiation and/or the pulsar wind particles from these rejuvenated pulsars have ablated their companions (van den Heuvel & van Paradijs 1988). This suggested scenario was motivated by the discovery of MSP PSR B1957+20 (Fruchter et al. 1988). It is a binary contains a 1.6 ms MSP and a very low mass companion (
The relativistic wind particles, which have been speculated for their role in evaporating the companion stars, have consumed a large fraction of the rotational energy of the neutron star. While the pulsed emission is originated within the magnetosphere, the wind region extends beyond the light cylinder (i.e. the distance from the pulsar at which the corotation velocity equals to the speed of light). Despite the fact that they carry a large amount of energy, these wind particles are mostly invisible on their own. Then how does one know their existence and their role in vaporizing the companion star?
The presence of pulsar wind will be noticed when it interacts with the surroundings. Three different effects of interaction with pulsar wind particles has been observed from PSR B1957+20: (1) acceleration of wind particles in the termination shock ahead the direction of the pulsar’s motion; (2) Intrabinary shock resulted from the interaction between the pulsar wind and the ablated material from its companion; (3) The heating of its companion by the wind collision.
To characterize the wind, a magnetization parameter,
In the termination shock, the relativistic wind particles interact with the shocked interstellar medium and radiate synchrotron radiation across the electromagnetic spectrum. Physical models for describing pulsar wind nebular emission can be divided into to two main classes, depending on whether the pulsar is moving subsonically or supersonically (Cheng et al. 2004). For a subsonic pulsar motion, the termination shock radius
On the other hand, in case of supersonic motion, bow shock nebulae can be formed which has been found for the less energetic but fast-moving pulsars (e.g., Hui et al. 2012, 2008, 2007, 2006).
At a radio dispersion measure inferred distance of ~ 2.5 kpc, PSR B1957+20 moves through the sky with a supersonic velocity of 220 km s−1 (Arzoumanian et al. 1994). We have carried out a follow-up X-ray study with a deep Chandra observation of an uninterrupted 169 ks exposure (Huang et al. 2012). Fig. 2 (left panel) shows the
[Fig. 2.] Left panel: Chandra ACIS-S3 image in the energy band 0.3？8 keV of the black widow pulsar system PSR B1957+20 which has been smoothed with an adaptive Gaussian filter. The green circle with the 2.0˝ radius indicates the source region we used in this study while two segments of the X-ray tail are chosen from the red rectangular regions. Right panel: The Hα image taken from the Anglo Australian Telescope is overlaid with the X-ray contours. The green contour levels are shown at 0.4, 0.8, 3.0, 15.4, and 84.8% of the peak x-ray surface brightness. The red cross indicates the radio timing position of PSR B1957+20. The optical residuals correspond to incompletely subtracted stars (See Huang et al. 2012).
Cheng et al. (2006) suggests that the observed length (
The deep exposure time of this
We have also investigated the orbital X-ray modulation in details. Huang & Becker (2007) have reported a strong correlation of the pulsar’s X-ray flux with its orbital period. However, due to the short exposure we could not know whether the flux modulation was periodic and given the limited photon statistics it was not possible to investigate any spectral variation as a function of orbit phase or to determine the exact geometry of the peak emission. With a repeated coverage of the binary orbit in our deep Chandra observation, we have attempted to determine the emission geometry with improved accuracy (Huang et al. 2012).
In order to search for a modulation of the X-ray flux as a function of orbital phase, we first selected X-ray data covering 5 complete and consecutive orbits and then to fold a light curve at the orbital period according to its radio timing ephemeris (Fig. 3). Using a χ2 -test, the significance for a flux modulation over the observed orbit was found to be ~ 99%.
[Fig. 3.] A folded light curve at the orbital period. Two orbital cycles are shown for clarity. The background noise level is found to be at ~ 0.1 counts/bin. The phase zero (ø= 0.0) corresponds to the ascending node of the pulsar orbit. Error bars indicate the 1σ uncertainty. The blue shaded regions between the orbital phases 0.2？0.3 and 1.2？1.3 mark the radio eclipse of the black widow pulsar. The phase-resolved spectrum covering the eclipsing region was extracted from the gray shaded regions (See Huang et al. 2012).
The folded light curve shows that the observed flux peaks just before and after the pulsar eclipse can be interpreted as the Doppler effect caused by the bulk flow in the down stream region. If the post-shocked wind flows toward (or away from) the Earth, the Doppler effect increases (or decreases) the observed flux from the flux for an isotropic case. The shock geometry is controlled by the ratio of the momentum fluxes of the pulsar wind to the stellar wind. For PSR B1957+20, the observed orbital period derivative
As we can see in Fig. 3, the observed ratio of maximum to minimum fluxes is 2 ~ 3. For the emissions from the pulsar wind, the outgoing flux is modified by the Doppler effect as
To investigate whether the X-ray spectral behavior of PSR B1957+20 varies across the orbit, we analyzed the X-ray spectra within the orbital phase of ø = 0.05 − 0.45 which covers the ingress, eclipsing, and egress region and outside the aforementioned region (ø = 0.45 − 1.05) separately. We found that the binary-phase resolved spectral analysis reveals a non-thermal emission nature of the detected X-rays and each of the observed spectra can be well described by a single power-law (PL) model with different photon indices (Γ ~ 2.1 in ø = 0.05 − 0.45, Γ ~ 2.5 in ø = 0.45 − 1.05), which indicates that its spectral behavior is orbital dependent.
All these suggest the orbital-modulated non-thermal X-ray emission from PSR B1957+20 is mostly due to intrashock emission at the interface between the pulsar wind and the ablated material from the companion.
Optical observations had also revealed that the pulsar wind consisting of electromagnetic radiation and highenergy particles is ablating and evaporating its white dwarf companion star (Fruchter et al. 1988, van Paradij et al. 1988). Subsequent analyses of the optical light curve for the binary system by Reynolds et al. (2007) gave a constraint to the system inclination of 65◦ ± 2◦ for a pulsar in the mass range 1.3 − 1.9 M⊙. The effective temperatures of
Apart from the aforementioned X-ray investigation, we have searched for orbital dependence of the
By comparing the
[Fig. 4.] Spectral energy distributions of γ-ray emission from PSR B1957+20. Data points were derived from likelihood fitting of individual energy bins, in which a simple PL is used to model the data. 90% upper limits were calculated for any energy bin in which the detection significance is lower than 3σ. Left: Spectrum averaged over Phase 1. The solid line shows the best-fit PLE model from fitting the data above 0.2 GeV. Right: Spectrum averaged over Phase 2. The solid line represents the fitted two-component model, with the PLE component shown as a dashed line and the Gaussian component shown as a dash-dotted line (See Wu et al. 2012).
We further compare the significance of emission in Phase 2 below and above 2.7 GeV, before and after removing the magnetospheric contribution. At energies below 2.7 GeV, the TS value decreased from 105 to 38, while at energies above 2.7 GeV, the TS value decreased from 55 to 36. Fig. 5(b) and Fig. 5(c) show the TS maps for a visual comparison before and after the removal of the pulsed component.
[Fig. 5.] Test-statistic (TS) maps of 2？ × 2？ regions centered at the position of PSR B1957+20 (labeled by green crosses). The color scale below each pair of images is used to indicate the TS values. (a): (i) TS map at energy > 2.7 GeV using only photons in Phase 1. (ii) Same as (a)(i) but using only photons in Phase 2. (b): (i) TS map at < 2.7 GeV for Phase 2. (ii) Same as (b)(i) but with data within full width at half maximum of the pulsation peaks removed (see text). (c): Same as (b) but with energy > 2.7 GeV (see Wu et al. 2012).
The above results indicate the presence of emission above 2.7 GeV, which is significantly detected in Phase 2 but not in Phase 1. Hence, the
Since the best-fit two-component model suggests that photons at ≥ 2.7 GeV may be modulated with the orbital phase, we constructed a phase-folded light curve with the photons of energies > 2.7 GeV which is shown in Fig. 6. The post-trial significance of this
[Fig. 6.] γ-ray light curve of PSR B1957+20 at energies > 2.7 GeV which are folded at the orbital period. Two orbits are shown for clarity. The shaded regions correspond to the phase of radio eclipse, which is the center of Phase 2 (see Wu et al. 2012).
All these results have demonstrated that the significant emissions above 3 GeV appears around INFC, while the emissions below 3 GeV are steady and are described by the pulsar emissions. We expect that the modulated emissions above 3 GeV is originated from the inverse-Compton scattering of the thermal radiation of the companion star off the “cold” (i.e. low energy of the leptons in the comoving frame of the plasma) ultra-relativistic pulsar wind.
In the last few years, a new population of eclipsing binary MSPs has emerged. Although the range of orbital period spanned by these systems is similar to that of black widow MSPs (Pb ≲ 20 hrs), their companion masses (
More than a dozen of redbacks have been identified so far
PSR J1023+0038 is the first identified redback MSP which provides evidence for the transition from an X-ray binary to a radio MSP (Archibald et al. 2009, 2010, Thorstensen & Armstrong 2005). It was firstly identified as a LMXB (Homer et al. 2006) and a radio MSP was found subsequently (Archibald et al. 2009). The source clearly showed an accretion disk before 2002 (Wang et al. 2009) and the disk has then disappeared (Archibald et al. 2009); radio pulsation was found in 2007 (Archibald et al. 2009). Therefore PSR J1023+0038is considered as a newly born MSP, representing the long sought-after missing link of a rotation-powered MSP descended from a LMXB. Shortly after this exciting discovery, we have investigated PSR J1023+0038 in X-ray and
In X-ray, we have searched for it orbital variability by using the
The fact that more than one PL index is required to explain the observed X-ray spectrum strongly suggests an additional non-thermal X-ray contribution from this system besides the magnetospheric radiation. we speculate that the additional non-thermal component originates from the intrabinary shock. Furthermore, our spectral analysis suggests that the emission below and above the break energy might have different origin. This led us to perform an energy-resolved analysis of the modulation of the X-ray flux as a function of orbital phase. According to the resultant break energy, we divided the energy range (0.3–10 keV) suitable for timing analysis into two bands: 0.3–2.0 keV (soft) and 2.0–10.0 keV (hard), and computed energy-resolved light curves.
[Fig. 7.] The energy-resolved X-ray light curves of PSR J1023+0038 in the soft (0.3？2.0 keV) and hard (2.0？ 10.0 keV) bands, as obtained from the observation taken at 26 November 2008 with XMM-Newton (see Tam et al. 2010).
The spin down power of PSR J1023+0038 (
[Fig. 8.] TS map of a region of 5？×5？ centered at the position of PSR J1023+0038 (labeled by the cross) as observed by Fermi LAT. γ-rays with energies between 200 MeV and 20 GeV were used. The 95% confidence- level error circle of the best-fit position of the γ-ray emission is also shown. (see Tam et al. 2010).
In such a system that recently transited from the LMXB phase to the radio MSP phase, we have predicted that its radio pulsation may not be permanent as an accretion disk may reform around the neutron star and hence can possibly switch off the radio MSP. Therefore, cycles of on and off states of pulsar activity could possibly be repeated. Therefore, continuous monitoring in all wavelengths of this unique source will test this idea and help us to comprehend its on-going evolution. Since then, we have devoted considerable effort in monitoring this system from optical to
Indeed, another exciting episode of PSR J1023+0038 was on around the middle of 2013. Since 2013 late-June, the radio pulsation of PSR J1023+0038 has disappeared between 350 MHz and 5 GHz (Stappers et al. 2013, Patruno et al. 2014). Meanwhile, recent optical spectroscopy shows strong double peaked Hα emission indicating that an accretion disk is formed (Halpern et al. 2013). Moreover, the X-ray emission has increased by a factor of at least 20 comparing with previous quiescent values (Kong 2013) and the UV emission has brightened by 4 magnitudes (Patruno et al. 2014). All these strongly indicate that PSR J1023+0038 is switching from a radio MSP back to a LMXB with an accretion disk. In addition, the
The results of our multiwavelength monitoring campaign from June 1 2013 to November 13 2013 are shown in Fig. 9 (see Takata et al. 2014 for details).
[Fig. 9.] The main panel shows the multi-wavelength (i.e., from UV to γ-ray) lightcurves of PSR J1023+0038 from June 1 to November 13 with different flux scales for each energy band (see upper left corner for details) while the inset box indicates the detailed evolution of the γ-ray emissions between June 6 to 24 July. UV/X-ray: Each datum represents an individual observation taken by Swift. γ-ray: Each datum in the main panel (inset) corresponds to two weeks (3 days), and 95 % c.l. upper limits are given for the time intervals during which test-statistics (TS) values were below 9 (4). (cf. Takata et al. 2014)
We estimated the dates of the state-change, i.e., when the binary system changed from the low
In the beginning, TS values are below 10. The first bin having TS>10 occurred at June 29 – July 5. Note that never before (or, never since March 2013) could PSR J1023+0038 be detected at this significance with only one week of data, so during this particular week (June 29 – July 5) PSR J1023+0038 has undergone a state change from the low state to high state in
To look for orbital modulation, we mapped orbital phase to geocentric arrival time and divided the data into two halves in orbital phase, each center red at one of the conjunctions. We performed the same analysis as above in each phase interval and did not detect any significant orbital modulation.
In X-ray, the flexible scheduling of
In examining the individual observation, we found no sign of significant spectral variability among them. Therefore, we combined all the selected events to construct an averaged X-ray spectrum (0.3–10 keV) with a total exposure time of 22.6 ks, which can be well described by an absorbed power-law model with a . An absorption corrected luminosity of
As the average observed count rate is about 0.21 cts s−1, a 50-second bin time is used for the lightcurve binning to achieve a mean signal-to-noise ratio > 3 per bin approximately. Short term variabilities on timescales of a few tens of seconds are clearly seen in the curve with variations up to a factor of ten between two consecutive bins. Interestingly enough, at least 21 of the data bins (with fractional exposure equals one) have zero count rate, which are unlikely random events due to an extremely low Poisson probability of getting a null count in a 50-second exposure (i.e., 0.0027%). Some of the zero count bins are close enough with other low count rate bins (≤ 0.07 cts s−1) to form several low-flux intervals in ranges of 200 to 550 seconds. Meanwhile, the peak flux reaches 1.64 ± 0.23 cts s−1 which arises from 0.10 ± 0.06 cts s−1 in 100 seconds, stays for about 50 seconds, and then drops to 0.06 ± 0.05 cts s−1 in 100 seconds. However, no significant hardness variability is found despite the high flux variability.
We have also search for the X-ray periodic signals from the known orbital and pulsar periods (Archibald et al. 2009, Wang et al. 2009). Although no reliable signal regarding to the orbit nor the pulsation is found, an unknown periodic signal of
Using the UVOT onboard
Although we have not observed PSR J1023+0038 in radio, it is instructive to have a review on its history of radio emission. The radio emission from the position of PSR J1023+0038 was firstly reported by the FIRST VLA 1.4 GHz survey (Becker et al. 1995). In re-examing the FIRST data, Bond et al. (2002) realized that the detection reported by Becker et al. (1995) was based on a single ~ 6.6 mJy flare from a 1.4 GHz observation on 10 August 1998. On the other hand, the observations on the 3 August 1998 and 8 August 1998 resulted in non-detections and the corresponding limiting flux densities are < 1.8 mJy and < 3.4 mJy respectively (Bond et al. 2002). These observations clearly demonstrate the variability of the radio emission from this binary system, which can vary by a factor of ~ 4 over a period as short as a week. However, without the optical spectroscopic results from this epoch, it remains uncertain whether the system was in rotation-powered or accretionpowered state. But the transient radio emission from a neutron star in an accreting binary is possible (Gaensler et al. 1999). During the rotation-powered phase, the radio emission from PSR J1023+0038 becomes more intense. Archibald et al. (2009) reported a mean flux density of ~ 14 mJy at 1.6 GHz. Recently, a long-term radio monitoring campaign of PSR J1023+0038 from mid-2008 to mid-2012 has reported the variable radio properties such as variable eclipses, short-term disappearance of signal and excess dispersion measure at random orbital phases (Archibald et al. 2013).
The schematic view of multi-wavelength emissions from PSR J1023+0038 after 2013 late-June. The accretion disk extends beyond the light cylinder radius (
Based on a radio observation at 1.4 GHz performed on 23 June 2013, Stappers et al. (2013) reported the non-detection of pulsed radio emission from PSR J1023+0038 with a limiting flux density of <0.06 mJy. However, it should be cautious in interpreting the result as it does not necessarily provide evidence for the quench of coherent radio emission mechanism. For an alternative scenario, assuming the pulsar remains to be active, the matter transferred from the companion toward the neutron star can be ejected by the pulsar wind (Ruderman et al. 1989). Also, the matter evaporated from the disk by the pulsar can further complicate the circumstellar environment. Therefore, even with the presence of active pulsar mechanism, the nondetection of pulsed radio emission can possibly due to the enhanced scattering/dispersion in the environment which results in a smearing of the pulsed signal.
Based on all these observational evidences, we have developed a theoretical model to explain the multiwavelength observations of PSR J1023+0038 in its current accretion active state by assuming the pulsar is still powered by rotation. Fig. 10 shows a schematic view of the multiwavelength emission from PSR J1023+0038 after 2013 late June. Our model suggests that a newly formed accretion disk due to the sudden increase of the stellar wind could explain the changes of all these observed features (see Takata et al. 2014 for further details).
[Fig. 10.] The schematic view of multi-wavelength emissions from PSR J1023+0038 after 2013 late-June. The accretion disk extends beyond the light cylinder radius (Rlc). Rs is the distance to the intra-binary shock from the pulsar. Below the critical distance (Rc), the gamma-rays evaporate the disk matter. UV/Optical photons are mainly produced by the disk emissions at R ~ 109？10 cm. The interaction between the pulsar wind and the stellar wind creates a shock and produces the non-thermal X-ray emissions. The inverse-Compton process of the cold-relativistic pulsar wind off UV/Optical photons from the disk produces the gamma-rays (see Takata et al. 2014).
With the observed flux of the UV emissions (
Tam et al. (2010) expected that the GeV emissions during the pulsed radio state are originated from the outer gap in the pulsar magnetosphere, because the inverse-Compton processes of the shocked pulsar wind and the coldrelativistic pulsar wind could not explain the observed flux level. The existence of the GeV
The increase in the flux by a factor of 10 after the disappearance of the pulsed radio emissions implies that an additional component emerges and it dominates the magnetospheric component. We propose that the inverse-Compton scattering process of the cold-relativistic pulsar wind off the soft photons from the disk produces the additional gamma-rays. With the observed luminosity of the UV emissions from the disk, the depth of the inverse-Compton scattering process is estimated to be of order of
After 2013 late-June, the X-ray emission does not show the orbital variation and increases by at least a factor of ~ 20. Previous X-ray observations during the radio millisecond pulsar phase revealed orbital flux variation, implying the emissions from the intra-binary shock (Tam et al. 2010, Bogdanov et al. 2011). Bogdanov et al. (2011) suggested that the emission region near the companion star and its orbital variation are caused by the eclipse of the emission region by the companion star. The disappearance of the orbital variation after 2013 late-June may suggest that the size of the emission region is bigger than that of before 2013 late-June. We expect that the increase in the mass loss rate from the companion star pushes the emission region back toward the pulsar, and more fraction of the pulsar wind is stopped by the shock, resulting an increase in the X-ray emissions from the system. The momentum ratio (
We expect that a strong shock forms if the magnetization parameter is smaller than or comparable to unity
The minimum Lorentz factor and the normalization factor are calculated from the two conditions that ∫
Fig. 11 summarizes the results of the model fitting for the observed emissions before (left panel) and after (right panel) 2013 late-June. The dotted, dashed and dasheddotted lines are the calculated spectra for the emissions from the outer gap (see Wang et al. 2010 for a detailed calculation), shocked pulsar wind and the cold-relativistic pulsar wind, respectively. The double-dotted line in the right panel shows the predicted spectrum of optical/UV emissions with the current model of the accretion disk. The flux predicted by inverse-Compton process of the coldrelativistic pulsar wind before 2013 late-June is of order of ~ 10−13 erg cm−2 s−1 and its spectrum is not shown in the figure. As we can see in the figure that the observed GeV emissions after 2013 late-June can be explained well by the inverse-Compton scattering process of the cold-relativistic pulsar wind off the soft photons from the disk.
[Fig. 11.] Multi-wavelength spectra of PSR J1023+0038 system before (left) and after (right) 2013 late-June. The dashed, dotted and dashed-dotted line represent the calculated spectra of the emissions from the shock, outer gap (c.f. Wang et al. 2010) and cold-relativistic pulsar wind, respectively. For the shock emissions, we assume the distance Rs ~ 8 × 1010 cm and the power law index p = 1.6 before 2013 late-June and Rs ~ 5 × 1010 cm and p = 2.2 after 2013 late-June, respectively. In the left panel, the stellar (G5V) spectrum (thick-curve line) and X-ray data (thick-double-dotted line) are taken from Wang et al. (2009) and Tam et al. (2010), respectively. The flux predicted by inverse-Compton process of the cold-relativistic pulsar wind before 2013 late-June is of order of ~ 10？13 erg cm？2 s？1 and its spectrum is not seen in the figure. (cf. Takata et al. 2014)
The matter evaporated from the disk will smear out the pulsed radio emissions from the neutron star via scattering and/or absorption, if the local plasma frequency is greater than the frequency of the radio wave. Assumed that the disk matter below
Apart from PSR J1023+0038, we have also investigated
We have searched for its orbital modulation in X-ray by using the data obtained by
[Fig. 12.] The background-subtracted light curves of PSR J1723-2837 as observed by Chandra ACIS in 0.3？7 keV (upper panel) and by XMM-Newton in 0.3？10 keV with the data from all EPIC cameras combined (lower panel). The same data have been repeated for another orbital cycle for demonstrating the modulation clearly. The shaded region illustrates the range of the radio eclipse. The dotted line and the dashed line illustrate the phases of INFC and SUPC respectively. (cf. Hui et al. 2014)
For constraining the INFC and superior conjunction (SUPC) spectral shape, we have performed a phase-resolved spectroscopy by simultaneously fitting the spectra obtained by
The observed X-ray photon index Γ
Because PSR J1723-2837 follows an almost circular orbit, the shock distance from the pulsar does not vary across the orbit, suggesting the spectral properties of the intrinsic shock emissions do not modulate with the orbital phase. The variation of the observed flux will be caused by either Doppler boosting effect with a mildly relativistic flow of the shocked flow or physical eclipse of the emission region. However, these two effects will not produce a significant variation in the spectrum. The observed amplitude (see Fig. 12) implies the Doppler factor is ~ 1 − 2, which does not cause a significant change in the spectral shape. Furthermore, the synchrotron cooling time scale of the particles that emit photons of energies < 10 keV is
We have also searched for the
[Fig. 13.] The background-subtracted 0.1？300 GeV γ-ray count map, smoothed with a Gaussian width of 0.3？, of the 2？×2？ region centered on PSR J1723-2837, whose radio timing position is indicated by the black cross. The blue circle indicates the error circle of the best-fit postion at the 68% confidence-level. The error eclipse of 1FGL J1725.5-2832, which is not regarded as a background source is shown. (cf. Hui et al. 2014)
For black widow/redback pulsars, the magnetospheric emissions and pulsar wind emissions produce the GeV gamma-rays. For the original black widow pulsar PSR B1957+20, Wu et al. (2012) suggested both the magnetospheric and the pulsar wind emissions contribute to the GeV emissions seen by
The shocked particles that emit the synchrotron photons will produce TeV photons through the inverse-Compton off the stellar photons, for which the effective temperature is
For identifying the promising MSP candidates, we first selected UFOs from 1FGL based on the following criteria:
1. Temporal variability;2. Galactic latitude, and3. γ-ray spectral shape.
We used the variability index in the 1FGL catalog to characterize source variability. Gamma-ray pulsars have always been found to be steady sources of gamma-ray emission (Abdo et al. 2010a, 2010b). This property can therefore be used to help identify which of the unidentified sources are probably pulsars. The 1FGL catalog defines a variability index, for which a value greater than 23.21 means that there is less than a 1% probability of being a steady source. We therefore selected sources with a variability index less than 23.
MSPs are in general older than energetic
Finally, we identified potential candidates from the gamma-ray spectra. Although only a power-law spectrum is listed in the first-year catalog, it also has a curvature index to indicate how good of a power-law fit. For
If a 1FGL source satisfied all four criteria, we shortlisted as a potential candidate. From the short list, we further searched for X-ray imaging data (
1FGL J2339.7-0531 has been observed by
We have also carried out an intensive optical monitoring campaign using the 1m telescope at the Lulin Observatory in Taiwan and the 0.8m Tenagra Telescope in Arizona. By using the Lomb Scargle periodogram on the combined optical data, we have found a period of 4.6342(9) hours. Fig. 14 shows the folded light-curves of all the optical data (upper panel) and
[Fig. 14.] Folded light-curve of optical and Chandra X-ray observations of 1FGL J2339.7-0531 with a best-fit period of 4.6342 hr. Optical colors (g′ ？I) obtained with MITSuME are also plotted. The phase zero is defined as 2010 October 31 (MJD 55500). Also plotted with the X-ray light-curve is the X-ray hardness ratio (1.5？8 keV/0.3？1.5 keV) with triangles. It is evident that both optical and X-ray light-curve show similar modulation. The X-ray hardness ratio exhibits some variability when the X-ray light-curve is at its minimum. Note that the Chandra data only cover about one orbital period (cf. Kong et al. 2012).
This system resembles the other aforementioned MSPs. The large orbital variation as seen in its optical lightcurve can be caused by the irradiation produces strong heating on the companion facing the pulsar. It is therefore suggestive that the companion is being evaporated by the high-energy radiation from the pulsar. Also, its X-ray/
Interestingly, this object can also be nice target for keen amateurs. The aforementioned orbital modulation in optical regime shows that it swings from classes