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Register a new userMultiwavelength evidence for a new flare-mode transitional millisecond pulsar
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We report the discovery of a new low-mass X-ray binary near the center of the unassociated Fermi GeV gamma-ray source 4FGL J0540.0-7552. The source shows the persistent presence of an optical accretion disk and exhibits extreme X-ray and optical variability. It also has an X-ray spectrum well-fit by a hard power law with a Gamma = 1.8 and a high ratio of X-ray to gamma-ray flux. Together, these properties are consistent with the classification of the binary as a transitional millisecond pulsar (tMSP) in the sub-luminous disk state. Uniquely among the candidate tMSPs, 4FGL J0540.0-7552 shows consistent optical, X-ray, and gamma-ray evidence for having undergone a state change, becoming substantially brighter in the optical and X-rays and fainter in GeV gamma-rays sometime in mid-2013. In its current sub-luminous disk state, and like one other candidate tMSP in the Galactic field, 4FGL J0540.0-7552 appears to always be in an X-ray flare mode, indicating that this could be common phenomenology for tMSPs.
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Weakly magnetic, millisecond spinning neutron stars attain their very fast rotation through a 1E8-1E9 yr long phase during which they undergo disk-accretion of matter from a low mass companion star. They can be detected as accretion-powered millisecond X-ray pulsars if towards the end of this phase their magnetic field is still strong enough to channel the accreting matter towards the magnetic poles. When mass transfer is much reduced or ceases altogether, pulsed emission generated by particle acceleration in the magnetosphere and powered by the rotation of the neutron star is observed, preferentially in the radio and gamma-ray bands. A few transitional millisecond pulsars that swing between an accretion-powered X-ray pulsar regime and a rotationally-powered radio pulsar regime in response to variations of the mass in-flow rate have been recently identified. Here we report the detection of optical pulsations from a transitional pulsar, the first ever from a millisecond spinning neutron star. The pulsations were observed when the pulsar was surrounded by an accretion disk and originated inside the magnetosphere or within a few hundreds of kilometres from it. Energy arguments rule out reprocessing of accretion-powered X-ray emission and argue against a process related to accretion onto the pulsar polar caps; synchrotron emission of electrons in a rotation-powered pulsar magnetosphere seems more likely.
We present simultaneous optical and near-infrared (IR) photometry of the millisecond pulsar PSR J1023+0038 during its low-mass X-ray binary phase. The r- and K_s-band light curves show rectangular, flat-bottomed dips, similar to the X-ray mode-switching (active-passive state transitions) behaviour observed previously. The cross-correlation function (CCF) of the optical and near-IR data reveals a strong, broad negative anti-correlation at negative lags, a broad positive correlation at positive lags, with a strong, positive narrow correlation superimposed. The shape of the CCF resembles the CCF of black hole X-ray binaries but the time-scales are different. The features can be explained by reprocessing and a hot accretion flow close to the neutron stars magnetospheric radius. The optical emission is dominated by the reprocessed component, whereas the near-IR emission contains the emission from plasmoids in the hot accretion flow and a reprocessed component. The rapid active-passive state transition occurs when the hot accretion flow material is channelled onto the neutron star and is expelled from its magnetosphere. During the transition the optical reprocessing component decreases resulting in the removal of a blue spectral component. The accretion of clumpy material through the magnetic barrier of the neutron star produces the observed near-IR/optical CCF and variability. The dip at negative lags corresponds to the suppression of the near-IR synchrotron component in the hot flow, whereas the broad positive correlation at positive lags is driven by the increased synchrotron emission of the outflowing plasmoids. The narrow peak in the CCF is due to the delayed reprocessed component, enhanced by the increased X-ray emission.
We report the discovery of a variable optical and X-ray source within the error ellipse of the previously unassociated Fermi Large Area Telescope $gamma$-ray source 4FGL J0407.7--5702. A 22 ksec observation from XMM-Newton/EPIC shows an X-ray light curve with rapid variability and flaring. The X-ray spectrum is well-fit by a hard power law with $Gamma = 1.7$. Optical photometry taken over several epochs is dominated by aperiodic variations of moderate amplitude. Optical spectroscopy with SOAR and Gemini reveals a blue continuum with broad and double-peaked H and He emission, as expected for an accretion disk around a compact binary. Overall, the optical, X-ray, and $gamma$-ray properties of 4FGL J0407.7--5702 are consistent with a classification as a transitional millisecond pulsar in the sub-luminous disk state. We also present evidence that this source is more distant than other confirmed or candidate transitional millisecond pulsar binaries, and that the ratio of X-ray to $gamma$-ray flux is a promising tool to help identify such binaries, indicating that a more complete census for these rare systems is becoming possible.
Transitional millisecond pulsars are accreting millisecond pulsars that switch between accreting X-ray binary and millisecond radio pulsar states. Only a handful of these objects have been identified so far. Terzan 5 CX1 is a variable hard X-ray source in the globular cluster Terzan 5. In this paper, we identify a radio counterpart to CX1 in deep Very Large Array radio continuum data. Chandra observations over the last fourteen years indicate that CX1 shows two brightness states: in 2003 and 2016 the source was the brightest X-ray source in the cluster (at L$_X sim 10^{33}$ erg s$^{-1}$), while in many intermediate observations, its luminosity was almost an order of magnitude lower. We analyze all available X-ray data of CX1, showing that the two states are consistent with the spectral and variability properties observed for the X-ray active and radio pulsar states of known transitional millisecond pulsars. Finally, we discuss the prospects for the detection of CX1 as a radio pulsar in existing timing data.
We report on radio timing and multiwavelength observations of the 4.66 ms redback pulsar J1048+2339, which was discovered in an Arecibo search targeting the Fermi-LAT source 3FGLJ1048.6+2338. Two years of timing allowed us to derive precise astrometric and orbital parameters for the pulsar. PSR J1048+2339 is in a 6-hour binary, and exhibits radio eclipses over half the orbital period and rapid orbital period variations. The companion has a minimum mass of 0.3 solar masses, and we have identified a $V sim 20$ variable optical counterpart in data from several surveys. The phasing of its $sim 1$~mag modulation at the orbital period suggests highly efficient and asymmetric heating by the pulsar wind, which may be due to an intrabinary shock that is distorted near the companion, or to the companions magnetic field channeling the pulsar wind to specific locations on its surface. We also present gamma-ray spectral analysis of the source and preliminary results from searches for gamma-ray pulsations using the radio ephemeris.
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