Spin-down Rate Research Articles

Studies of Cepheus X-4 during the 2018 Outburst Observed with AstroSat

Abstract We present timing and spectral results for the 2018 outburst of Cepheus X-4, observed twice by AstroSat at luminosities of 2.04 × 1037 erg s−1 and 1.02 × 1037 erg s−1. The light curves showed strong pulsation and co-related X-ray intensity variation in the SXT (0.5–8.0 keV) and LAXPC (3–60 keV) energy bands. The spin period and spin-down rate of the pulsar were determined from two observations to be 65.35080 ± 0.00014 s and (−2.10 ± 0.8) × 10−12 Hz s−1 at epoch MJD 58301.61850, and 65.35290 ± 0.00017 s and (−1.6 ± 0.8) × 10−12 Hz s−1 at epoch MJD 58307.40211. Pulse shape studies with AstroSat showed energy- and intensity-dependent variations. The pulsar showed an overall continuous spin-down over 30 yr at an average rate of (−2.455 ± 0.004) × 10−14 Hz s−1, attributed to the propeller effect in the subsonic regime of the pulsar, in addition to variations during its outburst activities. Spectra between the 0.7 keV and 55 keV energy bands were well fitted by two continuum models, an absorbed compTT model and an absorbed power law with a Fermi–Dirac cutoff (FD-cutoff) model with a blackbody. These were combined with an iron emission line and a cyclotron absorption line. The prominent cyclotron resonance scattering features with a peak absorption energy of 30.48 − 0.34 + 0.33 keV and 30.68 − 0.44 + 0.45 keV for the FD-cutoff model and 30.46 − 0.28 + 0.32 keV and 30.30 − 0.34 + 0.36 keV for the compTT model were detected during two AstroSat observations. When compared with earlier results, these showed long-term stability of an average value of 30.23 ± 0.22 keV with wide variation in source luminosity. The pulsar showed pulse phase as well as luminosity dependent variations in the cyclotron line energy and width and in the plasma optical depth of its spectral continuum.

The impact of glitches on young pulsar rotational evolution

ABSTRACT We report on a timing programme of 74 young pulsars that have been observed by the Parkes 64-m radio telescope over the past decade. Using modern Bayesian timing techniques, we have measured the properties of 124 glitches in 52 of these pulsars, of which 74 are new. We demonstrate that the glitch sample is complete to fractional increases in spin frequency greater than $\Delta \nu ^{90{{\ \rm per\ cent}}}_{\mathrm{ g}}/\nu \approx 8.1 \times 10^{-9}$ . We measure values of the braking index, n, in 33 pulsars. In most of these pulsars, their rotational evolution is dominated by episodes of spin-down with n > 10, punctuated by step changes in the spin-down rate at the time of a large glitch. The step changes are such that, averaged over the glitches, the long-term n is small. We find a near one-to-one relationship between the interglitch value of n and the change in spin-down of the previous glitch divided by the interglitch time interval. We discuss the results in the context of a range of physical models.

Modelling spin-up episodes in accreting millisecond X-ray pulsars

ABSTRACT Accreting millisecond X-ray pulsars are known to provide a wealth of physical information during their successive states of outburst and quiescence. Based on the observed spin-up and spin-down rates of these objects, it is possible, among other things, to infer the stellar magnetic field strength and test models of accretion disc flow. In this paper, we consider the three accreting X-ray pulsars (XTE J1751–305, IGR J00291+5934 and SAX J1808.4–3658) with the best available timing data, and model their observed spin-up rates with the help of a collection of standard torque models that describe a magnetically threaded accretion disc truncated at the magnetospheric radius. Whilst none of these models is able to explain the observational data, we find that the inclusion of the physically motivated phenomenological parameter ξ, which controls the uncertainty in the location of the magnetospheric radius, leads to an enhanced disc-integrated accretion torque. These ‘new’ torque models are compatible with the observed spin-up rates as well as the inferred magnetic fields of these objects provided that ξ ≈ 0.1−0.5. Our results are supplemented with a discussion of the relevance of additional physics effects that include the presence of a multipolar magnetic field and general relativistic gravity.

Constraining mechanism associated with fast radio burst and glitch from SGR J1935

ABSTRACT The discovery of fast radio burst (FRB) 200428 from galactic SGR J1935+2154 makes it possible to measure rotational changes accompanied by FRBs and to test several FRB models which may be simultaneously associated with glitches. Inspired by this idea, we present order of magnitude calculations to the scenarios proposed. FRB models such as global starquakes, crust fractures, and collisions between pulsars and asteroids/comets are discussed. For each mechanism, the maximum glitch sizes are constrained by the isotropic energy release during the X-ray burst and/or the SGR J1935+2154-like radio burst rate. Brief calculations show that, the maximum glitch sizes for different mechanisms differ by order(s) of magnitude. If glitches are detected to be coincident with FRBs from galactic magnetars in the future, glitch behaviours (such as glitch size, rise time-scale, the recovery coefficient, and spin-down rate offset) are promising to serve as criterions to distinguish glitch mechanisms and in turn to constrain FRB models.

Constraints on ultra-low-frequency gravitational waves with statistics of pulsar spin-down rates. II. Mann–Whitney U test

Abstract We investigate gravitational waves with sub-nHz frequencies (10−11 Hz ≲ fGW ≲ 10−9 Hz) from the spatial distribution of the spin-down rates of millisecond pulsars. As we suggested in Yonemaru et al. (2018, MNRAS, 478, 1670), gravitational waves from a single source induce a bias in the observed spin-down rates of pulsars depending on the relative direction between the source and the pulsar. To improve the constraints on the time derivative of gravitational wave amplitude obtained in our previous work (Kumamoto et al. 2019, MNRAS, 489, 3547), we adopt a more sophisticated statistical method called the Mann–Whitney U test. Applying our method to the ATNF pulsar catalogue, we first find that the current data set is consistent with no gravitational wave signal from any direction in the sky. Then, we estimate the effective angular resolution of our method to be 66 deg2 by studying the probability distribution of the test statistic. Finally, we investigate gravitational wave signals from the Galactic Center (GC) and M 87 and, comparing simulated mock data sets with the real pulsar data, we obtain upper bounds on the time derivative as $\dot{h}_{\rm GC} < 8.9 \times 10^{-19}$ s−1 for the GC and $\dot{h}_{\rm M87} < 3.3 \times 10^{-19}$ s−1 for M 87, which are stronger than those obtained in Kumamoto et al. (2019, MNRAS, 489, 3547) by factors of 7 and 25, respectively.

Small glitches and other rotational irregularities of the Vela pulsar (Corrigendum)

Glitches are sudden increases in the rotation rate $\nu$ of neutron stars, which are thought to be driven by the neutron superfluid inside the star. The Vela pulsar presents a comparatively high rate of glitches, with 21 events reported since observations began in 1968. These are amongst the largest known glitches (17 of them have sizes $\Delta\nu/\nu\geq10^{-6}$) and exhibit very similar characteristics. This similarity, combined with the regularity with which large glitches occur, has turned Vela into an archetype of this type of glitching behaviour. The properties of its smallest glitches, on the other hand, are not clearly established. High-cadence observations of the Vela pulsar were taken between 1981 and 2005 at the Mount Pleasant Radio Observatory. An automated systematic search was carried out that investigated whether a significant change of spin frequency $\nu$ and/or the spin-down rate $\dot{\nu}$ takes place at any given time. We find two new glitches, with respective sizes $\Delta\nu/\nu$ of $(5.55\pm0.03)\times10^{-9}$ and $(38\pm4)\times10^{-9}$. In addition to these two glitch events, our study reveals numerous events of all possible signatures (i.e. combinations of $\Delta\nu$ and $\Delta\dot{\nu}$ signs), all of them small with $|\Delta\nu|/\nu<10^{-9}$, which contribute to the Vela timing noise. The Vela pulsar presents an under-abundance of small glitches compared to many other glitching pulsars, which appears genuine and not a result of observational biases. In addition to typical glitches, the smooth spin-down of the pulsar is also affected by an almost continuous activity that can be partially characterised by small step-like changes in $\nu$, $\dot{\nu,}$ or both. Simulations indicate that a continuous wandering of the rotational phase, following a red spectrum, could mimic such step-like changes in the timing residuals.

The X-ray evolution and geometry of the 2018 outburst of XTE J1810−197

ABSTRACT After 15 yr, in late 2018, the magnetar XTE J1810−197 underwent a second recorded X-ray outburst event and reactivated as a radio pulsar. We initiated an X-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using Swift, XMM–Newton, NuSTAR, and NICER observations. During the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of 2 change in its spin-down rate from ∼7.2 × 10−12 to ∼1.5 × 10−11 s s−1 after two months. Unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. Following the initial outburst, the spectrum of XTE J1810−197 is well modelled by multiple blackbody components corresponding to a pair of non-concentric, hot thermal caps surrounded by a cooler one, superposed to the colder star surface. We model the energy-dependent pulse profile to constrain the viewing and surface emission geometry and find that the overall geometry of XTE J1810−197 has likely evolved relative to that found for the 2003 event.

Constraints on wide-band radiative changes after a glitch in PSR J1452–6036

ABSTRACT We present high-sensitivity, wide-band observations (704–4032 MHz) of the young to middle-aged radio pulsar J1452–6036, taken at multiple epochs before and, serendipitously, shortly after a glitch occurred on 2019 April 27. We obtained the data using the new ultra-wide-bandwidth low-frequency (UWL) receiver at the Parkes radio telescope, and we used Markov chain Monte Carlo techniques to estimate the glitch parameters robustly. The data from our third observing session began 3 h after the best-fitting glitch epoch, which we constrained to within ∼4 min. The glitch was of intermediate size, with a fractional change in spin frequency of 270.52(3) × 10−9. We measured no significant change in spin-down rate and found no evidence for rapidly decaying glitch components. We systematically investigated whether the glitch affected any radiative parameters of the pulsar and found that its spectral index, spectral shape, polarization fractions, and rotation measure stayed constant within the uncertainties across the glitch epoch. However, its pulse-averaged flux density increased significantly by about 10 per cent in the post-glitch epoch and decayed slightly before our fourth observation a day later. We show that the increase was unlikely caused by calibration issues. While we cannot exclude that it was due to refractive interstellar scintillation, it is hard to reconcile with refractive effects. The chance coincidence probability of the flux density increase and the glitch event is low. Finally, we present the evolution of the pulsar’s pulse profile across the band. The morphology of its polarimetric pulse profile stayed unaffected to a precision of better than 2 per cent.

State-switching Mechanism of Intermittent Pulsars

Abstract The emission feature of intermittent pulsars is significant for understanding the pulsar emission mechanism. Using the observational evidence of radio emissions turning on and off and the corresponding spin-down rates in these two states of an intermittent pulsar, we will examine the polar-cap potential drop, gap height, and curvature radii of a few intermittent pulsars within the regime of the pulsar polar-cap emission theory by applying the current loss and energy flux of particle flow to pulsar braking, which are generally associated with radio emission from the polar cap. It is seen that the polar-cap parameters of the intermittent pulsars are almost equal to their maximum values, which is the main prediction of the pulsar polar-cap theory with respect to the breaking of the radio emission. It is also noticed that the intermittent pulsars are distributed near the dipole death line in the diagram, which is consistent with their emission features and the calculated polar-cap parameters. To further confirm the state switching of the intermittent pulsar, the relationships among spin-down rate, gap height, potential drop, and activity duty cycles of PSR B1931+24 are discussed. It is found that the gap height has an anticorrelation with the activity duty cycle, which indicates that the intermittency of the radio emission has a close connection to the gap height, as indicated by the pulsar polar-cap emission theory.

Spin-down and emission variations for PSR J0742−2822

PSR J0742−2822 is known for its quasi-periodic changes in the observed pulse profile and spindown rate. In this paper, we analyzed 13 years of timing data obtained with the Nanshan 25-m radio telescope and the Parkes 64-m radio telescope. We found that the average values of the spin-down rate () of this pulsar changed in four different states. We investigated the correlation between and W 50, and ascertained that the correlation changed in different states. Moreover, not all the changes in states and correlation can be associated with glitch activities. We examined the long term evolution of γ-ray flux (0.1–300 GeV) and the pulse profiles corresponding to the four different states using Fermi-LAT Pass 8 (P8R3) data from 2008 August 5 to 2019 October 1. We did not detect a significant change in γ-ray flux or the pulse profile. Our results suggest that the connection between pulsar rotation and emission is more complex than previously reported for this pulsar.

Small glitches and other rotational irregularities of the Vela pulsar

Context. Glitches are sudden increases in the rotation rate ν of neutron stars, which are thought to be driven by the neutron superfluid inside the star. The Vela pulsar presents a comparatively high rate of glitches, with 21 events reported since observations began in 1968. These are amongst the largest known glitches (17 of them have sizes Δν/ν ≥ 10−6) and exhibit very similar characteristics. This similarity, combined with the regularity with which large glitches occur, has turned Vela into an archetype of this type of glitching behaviour. The properties of its smallest glitches, on the other hand, are not clearly established. Aims. We explore the population of small-amplitude, rapid rotational changes in the Vela pulsar and determine the rate of occurrence and sizes of its smallest glitches. This will help advance our understanding of the actual distribution of glitch sizes and inter-glitch waiting times in this pulsar, which has implications for theoretical models of the glitch mechanism. Methods. High-cadence observations of the Vela pulsar were taken between 1981 and 2005 at the Mount Pleasant Radio Observatory. An automated systematic search was carried out that investigated whether a significant change of spin frequency ν and/or the spin-down rate ν̇ takes place at any given time. Results. We find two glitches that have not been reported before, with respective sizes Δν/ν of (5.55 ± 0.03) × 10−9 and (38 ± 4) × 10−9. The latter is followed by an exponential-like recovery with a characteristic timescale of 31 d. In addition to these two glitch events, our study reveals numerous events of all possible signatures (i.e. combinations of Δν and Δν̇ signs), all of them small with |Δν|/ν &lt; 10−9, which contribute to the Vela timing noise. Conclusions. The Vela pulsar presents an under-abundance of small glitches compared to many other glitching pulsars, which appears genuine and not a result of observational biases. In addition to typical glitches, the smooth spin-down of the pulsar is also affected by an almost continuous activity that can be partially characterised by small step-like changes in ν, ν̇ or both. Simulations indicate that a continuous wandering of the rotational phase, following a red spectrum, could mimic such step-like changes in the timing residuals.

NICER Discovery of Millisecond X-Ray Pulsations and an Ultracompact Orbit in IGR J17494-3030

Abstract We report the detection of 376.05 Hz (2.66 ms) coherent X-ray pulsations in NICER observations of a transient outburst of the low-mass X-ray binary IGR J17494−3030 in 2020 October/November. The system is an accreting millisecond X-ray pulsar in a 75-minute ultracompact binary. The mass donor is most likely a ≃0.02 M ⊙ finite-entropy white dwarf composed of He or C/O. The fractional rms pulsed amplitude is 7.4%, and the soft (1–3 keV) X-ray pulse profile contains a significant second harmonic. The pulsed amplitude and pulse phase lag (relative to our mean timing model) are energy dependent, each having a local maximum at 4 and 1.5 keV, respectively. We also recovered the X-ray pulsations in archival 2012 XMM-Newton observations, allowing us to measure a long-term pulsar spin-down rate of Hz s−1 and to infer a pulsar surface dipole magnetic field strength of ≃109 G. We show that the mass transfer in the binary is likely nonconservative, and we discuss various scenarios for mass loss from the system.

Parameter estimation of a two-component neutron star model with spin wandering

ABSTRACT It is an open challenge to estimate systematically the physical parameters of neutron star interiors from pulsar timing data while separating spin wandering intrinsic to the pulsar (achromatic timing noise) from measurement noise and chromatic timing noise (due to propagation effects). In this paper, we formulate the classic two-component, crust-superfluid model of neutron star interiors as a noise-driven, linear dynamical system and use a state-space-based expectation–maximization method to estimate the system parameters using gravitational-wave and electromagnetic timing data. Monte Carlo simulations show that we can accurately estimate all six parameters of the two-component model provided that electromagnetic measurements of the crust angular velocity and gravitational-wave measurements of the core angular velocity are both available. When only electromagnetic data are available, we can recover the overall relaxation time-scale, the ensemble-averaged spin-down rate, and the strength of the white-noise torque on the crust. However, the estimates of the secular torques on the two components and white-noise torque on the superfluid are biased significantly.

Comparing the galactic bulge and galactic disk millisecond pulsars

The Galactic Center Excess (GCE) is an extended gamma-ray source in the central region of the Galaxy found in Fermi Large Area Telescope (Fermi-LAT) data. One of the leading explanations for the GCE is an unresolved population of millisecond pulsars (MSPs) in the Galactic bulge. Due to differing star formation histories it is expected that the MSPs in the Galactic bulge are older and therefore dimmer than those in the Galactic disk. Additionally, correlations between the spectral parameters of the MSPs and the spin-down rate of the corresponding neutron stars have been observed. This implies that the bulge MSPs may be spectrally different from the disk MSPs. We perform detailed modelling of the MSPs from formation until observation. Although we confirm the correlations, we do not find they are sufficiently large to significantly differentiate the spectra of the bulge MSPs and disk MSPs when the uncertainties are accounted for. Our results demonstrate that the population of MSPs that can explain the gamma-ray signal from the resolved MSPs in the Galactic disk and the unresolved MSPs in the boxy bulge and nuclear bulge can consistently be described as arising from a common evolutionary trajectory for some subset of astrophysical sources common to all these different environments. We do not require that there is anything unusual about inner Galaxy MSPs to explain the GCE. Additionally, we use a more accurate geometry for the distribution of bulge MSPs and incorporate dispersion measure estimates of the MSPs' distances. We find that the elongated boxy bulge morphology means that some the bulge MSPs are closer to us and so easier to resolve. We identify three resolved MSPs that have significant probabilities of belonging to the bulge population.

Persistent Emission Properties of SGR J1935+2154 during Its 2020 Active Episode

Abstract We present detailed spectral and temporal characteristics of the persistent X-ray emission of SGR J1935+2154 based on our XMM-Newton and Chandra observations taken in the aftermath of its 2020 April burst storm, during which hundreds of energetic X-ray bursts were emitted, including one associated with an extraordinary fast radio burst. We clearly detect the pulsed X-ray emission in the XMM-Newton data. An average spin-down rate of 1.6 × 10−11 s s−1 is obtained using our spin period measurement combined with three earlier values reported from the same active episode. Our investigations of the XMM-Newton and Chandra spectra with a variety of phenomenological and physically motivated models concluded that the magnetic field topology of SGR J1935+2154 is most likely highly nondipolar. The spectral models indicate that surface field strengths in somewhat localized regions substantially exceed the polar value of 4.4 × 1014 G inferred from a spin-down torque associated with a rotating magnetic dipole.

NICER Observation of the Temporal and Spectral Evolution of Swift J1818.0−1607: A Missing Link between Magnetars and Rotation-powered Pulsars

Abstract We report on the hard X-ray burst and the first ∼100 days of NICER monitoring of the soft X-ray temporal and spectral evolution of the newly discovered magnetar Swift J1818.0−1607. The burst properties are typical of magnetars with a duration of T 90 = 10 ± 4 ms and a temperature of kT = 8.4 ± 0.7 keV. The 2–8 keV pulse shows a broad, single-peak profile with a pulse fraction increasing with time from 30% to 43%. The NICER observations reveal strong timing noise with varying erratically by a factor of 10, with an average long-term spin-down rate of s−2, implying an equatorial surface magnetic field of 2.5 × 1014 G and a young characteristic age of ∼470 yr. We detect a large spin-up glitch at MJD 58928.56 followed by a candidate spin-down glitch at MJD 58934.81, with no accompanying flux enhancements. The persistent soft X-ray spectrum of Swift J1818.0−1607 can be modeled as an absorbed blackbody with a temperature of ∼1 keV. Its flux decayed by ∼60% while the modeled emitting area decreased by ∼30% over the NICER observing campaign. This decrease, coupled with the increase in the pulse fraction, points to a shrinking hot spot on the neutron star surface. Assuming a distance of 6.5 kpc, we measure a peak X-ray luminosity of 1.9 × 1035 erg s−1, lower than its spin-down luminosity of 7.2 × 1035 erg s−1. Its quiescent thermal luminosity is ≲1.7 × 1034 erg s−1, lower than those of canonical young magnetars. We conclude that Swift J1818.0−1607 is an important link between regular magnetars and high-magnetic-field, rotation-powered pulsars.

High-cadence observations and variable spin behaviour of magnetar Swift J1818.0−1607 after its outburst

ABSTRACT We report on multifrequency radio observations of the new magnetar Swift J1818.0−1607, following it for more than one month with high cadence. The observations commenced less than 35 h after its registered first outburst. We obtained timing, polarization, and spectral information. Swift J1818.0−1607 has an unusually steep spectrum for a radio emitting magnetar and also has a relatively narrow and simple pulse profile. The position angle swing of the polarization is flat over the pulse profile, possibly suggesting that our line of sight grazes the edge of the emission beam. This may also explain the steep spectrum. The spin evolution shows large variation in the spin-down rate, associated with four distinct timing events over the course of our observations. Those events may be related to the appearance and disappearance of a second pulse component. The first timing event coincides with our actual observations, while we did not detect significant changes in the emission properties that could reveal further magnetospheric changes. Characteristic ages inferred from the timing measurements over the course of months vary by nearly an order of magnitude. A longer-term spin-down measurement over approximately 100 d suggests a characteristic age of about 500 yr, larger than previously reported. Though Swift J1818.0−1607 could still be one of the youngest neutron stars (and magnetars) detected so far, we caution using the characteristic age as a true-age indicator given the caveats behind its calculation.

Proper motion, spectra, and timing of PSR J1813-1749 using Chandra and NICER.

PSR J1813-1749 is one of the most energetic rotation-powered pulsars known, producing a pulsar wind nebula (PWN) and gamma-ray and TeV emission, but whose spin period is only measurable in X-ray. We present analysis of two Chandra datasets that are separated by more than ten years and recent NICER data. The long baseline of the Chandra data allows us to derive a pulsar proper motion yr-1 and yr-1 and velocity v ⊥ ≈ 900-1600 km s-1 (assuming a distance d = 3 - 5 kpc), although we cannot exclude a contribution to the change in measured pulsar position due to a change in brightness structure of the PWN very near the pulsar. We model the PWN and pulsar spectra using an absorbed power law and obtain best-fit absorption N H = (13.1 ± 0.9) × 1022 cm-2, photon index Γ = 1.5 ± 0.1, and 0.3-10 keV luminosity L X ≈ 5.4 × 1034 erg s-1(d/ 5 kpc)2 for the PWN and Γ = 1.2 ± 0.1 and L X « 9.3 × 1033 erg s-1(d/ 5 kpc)2 for PSR J1813-1749. These values do not change between the 2006 and 2016 observations. We use NICER observations from 2019 to obtain a timing model of PSR J1813-1749, with spin frequency ν = 22.35 Hz and spin frequency time derivative Hz s-1. We also fit ν measurements from 2009-2012 and our 2019 value and find a long-term spin-down rate Hz s-1. We speculate that the difference in spin-down rates is due to glitch activity or emission mode switching.

The Timing Behavior of the Central Compact Object Pulsar 1E 1207.4−5209

Abstract We present 20 yr of timing observations for 1E 1207.4−5209, the central compact object in supernova remnant PKS 1209−51/52, to follow up on our detection of an unexpected timing glitch in its spin-down. Using new XMM-Newton and NICER observations of 1E 1207.4−5209, we now find that the phase ephemeris can be well modeled by either two small glitches, or extreme timing noise. The implied magnitudes of the frequency glitches are and , at epochs 2010.9 and 2014.4, respectively. The updated timing solutions also rule out our previous suggestion of a large glitch in the frequency derivative . No other canonical pulsar with such a small spin-down rate ( Hz s−1) or surface dipole magnetic field strength ( G) has been observed to glitch; the glitch activity parameter of 1E 1207.4−5209 is larger than that of more energetic pulsars. Alternative parameterizations that do not involve glitches can fit the data, but they have timing residuals or a second frequency derivative that are orders of magnitude larger than in pulsars with similar spin-down parameters. These timing properties of 1E 1207.4−5209 further motivate the leading theory of central compact objects, that an initial B-field of normal strength was buried in the neutron star crust by fallback of supernova ejecta, suppressing the surface dipole field. The slow reemergence of the buried field may be involved in triggering glitches or excess timing noise.

Discovery of State Transition Behaviors in PSR J1124–5916

Abstract With the 12 year long observations by Fermi-LAT, we discover two pairs of spin-down state transitions of PSR J1124–5916, making it the second young pulsar detected to have such behaviors. PSR J1124–5916 shows mainly two states according to its spin-down rate evolution, the normal spin-down state and the low spin-down state. In about 80% of the observation time, the pulsar is in the normal spin-down state, in which the spin-down rate decreases linearly and gives a braking index of 1.98 ± 0.04. The two transitions to the low spin-down state are in MJD 55183–55803 and MJD 56114–56398, respectively, with fractional amplitudes both ∼0.4%. No significant difference between the γ-ray profiles of the two spin-down states is detected, which is similar to PSR B0540–69, the other young pulsar with state transition detected.