J. Moldón

The changing milliarcsecond radio morphology of the gamma-ray binary LS 5039

Context. LS 5039 is one of the few TeV emitting X-ray binaries detected so far. The powering source of its multiwavelength emission can be accretion in a microquasar scenario or wind interaction in a young nonaccreting pulsar scenario. Aims. To present new high-resolution radio images and compare them with the expected behavior in the different scenarios. Methods. We analyze Very Long Baseline Array (VLBA) radio observations that provide morphological and astrometric information at milliarcsecond scales. Results. We detect a changing morphology between two images obtained five days apart. In both runs there is a core component with a constant flux density, and an elongated emission with a position angle (PA) that changes by 12 ±

Discovery of Extended and Variable Radio Structure from the Gamma-ray Binary System PSR B1259–63/LS 2883

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Revealing the extended radio emission from the gamma-ray binary HESS J0632+057

between both runs. The source is nearly symmetric in the first run and asymmetric in the second one. The astrometric results are not conclusive. Conclusions. A simple and shockless microquasar scenario cannot easily explain the observed changes in morphology. An interpretation within the young nonaccreting pulsar scenario requires the inclination of the binary system to be very close to the upper limit imposed by the absence of X-ray eclipses.

Periodic morphological changes in the radio structure of the gamma-ray binary LS 5039

PSR B1259–63 is a 48 ms pulsar in a highly eccentric 3.4 year orbit around the young massive star LS 2883. During the periastron passage the system displays transient non-thermal unpulsed emission from radio to very high energy gamma rays. It is one of the three galactic binary systems clearly detected at TeV energies, together with LS 5039 and LS I +61 303. We observed PSR B1259–63 after the 2007 periastron passage with the Australian Long Baseline Array at 2.3 GHz to trace the milliarcsecond (mas) structure of the source at three different epochs. We have discovered extended and variable radio structure. The peak of the radio emission is detected outside the binary system near periastron, at projected distances of 10-20 mas (25-45 AU assuming a distance of 2.3 kpc). The total extent of the emission is ~50 mas (~120 AU). This is the first observational evidence that non-accreting pulsars orbiting massive stars can produce variable extended radio emission at AU scales. Similar structures are also seen in LS 5039 and LS I +61 303, in which the nature of the compact object is unknown. The discovery presented here for the young non-accreting pulsar PSR B1259–63 reinforces the link with these two sources and supports the presence of pulsars in these systems as well. A simple kinematical model considering only a spherical stellar wind can approximately trace the extended structures if the binary system orbit has a longitude of the ascending node of Ω ~ –40° and a magnetization parameter of σ ~ 0.005.

Binaries with the eyes of CTA

Context. After the detection of a 321-day periodicity in X-rays, HESS J0632+057 can be robustly considered a new member of the selected group of gamma-ray binaries. These sources are known to show extended radio structure on scales of milliarcseconds (mas). Aims. We present the expected extended radio emission on mas scales from HESS J0632+057. Methods. We observed HESS J0632+057 with the European VLBI Network (EVN) at 1.6 GHz in two epochs: during the January/February 2011 X-ray outburst and 30 days later. Results. The VLBI image obtained during the outburst shows a compact ∼0.4 mJy radio source, whereas 30 days later the source has faded and appears extended, with a projected size of ∼75 AU. The peak of the emission is displaced between runs 21 ± 5 AU, which is bigger than the orbit size. The position of the radio source is compatible with the Be star MWC 148, which sets the proper motion of the binary system below 3 mas yr −1 in each coordinate. The brightness temperature of the source is above 2 × 10 6 K. We compare the multiwavelength properties of HESS J0632+057 with those of the previously known gamma-ray binaries. Conclusions. HESS J0632+057 displays extended and variable non-thermal radio emission. Its morphology, size, and displacement on AU scales are similar to those found in the other gamma-ray binaries, PSR B1259−63, LS 5039, and LS I +61 303, supporting a similar nature for HESS J0632+057.

Detailed Radio to Soft Gamma-ray Studies of the 2005 Outburst of the New X-ray Transient XTE J1818-245

Context. Gamma-ray binaries allow us to study physical processes such as particle acceleration up to TeV energies as well as very high energy gamma-ray emission and absorption with changing geometrical configurations on a periodic basis. These sources produce outflows of radio-emitting particles whose structure can be imaged with very long baseline interferometry (VLBI). LS 5039 is a gamma-ray binary that has shown variable VLBI structures in the past. Aims. We aim to characterise the radio morphological changes of LS 5039 and determine if they are either repeatable or erratic. Methods. We observed LS 5039 with the VLBA at 5 GHz during five consecutive days to cover the 3.9-day orbit and an extra day to distinguish between orbital or secular variability. We also compiled the available high-resolution radio observations of the source to study its morphological variability at different orbital phases. We used a simple model to interpret the obtained images. Results. The new observations show that the morphology of LS 5039 up to projected distances of 10 milliarcsec changes within 24 h. The observed radio morphological changes display a periodic orbital modulation. Multifrequency and multiepoch VLBI observations confirm that the morphological periodicity is stable on timescales of years. Using a simple model we show that the observed behaviour is compatible with the presence of a young non-accreting pulsar with an outflow behind it. The morphology is reproduced for inclinations of the orbit of 60–75 ◦ . For masses of the companion star in the range 20–50 M� , this range of inclinations implies a mass of the compact object of 1.3–2.7 M� . Conclusions. The periodic orbital modulation of the radio morphology of LS 5039, stable over several years, suggests that all gamma-ray binaries are expected to show a similar behaviour. The changes in the radio structure of LS 5039 are compatible with the presence of a young non-accreting neutron star, which suggests that the known gamma-ray binaries contain young pulsars.

Radio continuum and near-infrared study of the MGRO J2019+37 region

The binary systems that have been detected in gamma rays have proven very useful to study high-energy processes, in particular particle acceleration, emission and radiation reprocessing, and the dynamics of the underlying magnetized flows. Binary systems, either detected or potential gamma-ray emitters, can be grouped in different subclasses depending on the nature of the binary components or the origin of the particle acceleration: the interaction of the winds of either a pulsar and a massive star or two massive stars; accretion onto a compact object and jet formation; and interaction of a relativistic outflow with the external medium. We evaluate the potentialities of an instrument like the Cherenkov telescope array (CTA) to study the non-thermal physics of gamma-ray binaries, which requires the observation of high-energy phenomena at different time and spatial scales. We analyze the capability of CTA, under different configurations, to probe the spectral, temporal and spatial behavior of gamma-ray binaries in the context of the known or expected physics of these sources. CTA will be able to probe with high spectral, temporal and spatial resolution the physical processes behind the gamma-ray emission in binaries, significantly increasing as well the number of known sources. This will allow the derivation of information on the particle acceleration and emission sites qualitatively better than what is currently available.

On the origin of LS 5039 and PSR J1825−1446

Context. XTE J1818−245 is an X-ray nova that experienced an outburst in 2005, as first seen by the RXTE satellite. The source was observed simultaneously at various wavelengths up to soft γ-rays with the INTEGRAL satellite, from 2005 February to September, during our INTEGRAL Target of Opportunity program dedicated to new X-ray novae and during Galactic Bulge observations. Aims. X-ray novae are extreme systems that often harbor a black hole, and are known to emit throughout the electromagnetic spectrum when in outburst. The goals of our programme are to understand the physical processes close to the black hole and to study the possible connection with the jets that are observed in the radio. Methods. We analysed radio, (N)IR, optical, X-ray and soft γ-ray observations. We constructed simultaneous broad-band X-ray spectra covering a major part of the outburst, which we fitted with physical models. Analyzing both the light curves in various energy ranges and the hardness-intensity diagram enabled us to study the long-term behaviour of the source. Results. Spectral parameters were typical of the soft intermediate states and the high soft states of a black hole candidate. The source showed relatively small spectral variations in X-rays with considerable flux variation in radio. Spectral studies showed that the accretion disc cooled down from 0.64 to 0.27 keV in ∼100 days and that the total flux decreased while the relative flux of the hot medium increased. Radio emission was detected several times, and, interestingly, five days after entering the HSS. Modeling the spectral energy distribution from the radio to the soft γ-rays reveals that the radio flares arise from several ejection events. Conclusions. XTE J1818−245 probably belongs to the class of low-mass X-ray binaries and is likely a black hole candidate transient source that might be closer than the Galactic Bulge. The results from the data analysis trace the physical changes that took place in the system (disc, jet/corona) at a maximum bolometric luminosity of 0.4−0.9 × 10 38 erg s −1 (assuming a distance between 2.8–4.3 kpc) and they are discussed within the context of disc and jet models.

LBCS : The LOFAR Long-Baseline Calibrator Survey

Context. MGRO J2019+37 is an unidentified extended source of very high energy gamm a-rays originally reported by the Milagro Collaboration as the brightest TeV source in the Cygnus region. Its extended emission could be powered by either a single or several sources. The GeV pulsar AGL J2020.5+3653, discovered by AGILE and associated with PSR J2021+3651, could contribute to the emission from MGRO J2019+37. Aims. Our aim is to identify radio and near-infrared sources in the field of the extended TeV source MGRO J2019+37, and study potential counterparts to explain its emission. Methods. We surveyed a region of about 6 square degrees with the Giant Metrewave Radio Telescope (GMRT) at the frequency 610 MHz. We also observed the central square degree of this survey in the near-infrared Ks-band using the 3.5 m telescope in Calar Alto. Archival X-ray observations of some specific fiel ds are included. VLBI observations of an interesting radio source were performed. We explored possible scenarios to produce the multi-TeV emission from MGRO J2019+37 and studied which of the sources could be the main particle accelerator. Results. We present a catalogue of 362 radio sources detected with the GMRT in the field of MGRO J2019+37, and the results of a cross-correlation of this catalog with one obtained at n ear-infrared wavelengths, which contains∼ 3× 10 5 sources, as well as with available X-ray observations of the region. Some peculiar sources inside the∼1 ◦ uncertainty region of the TeV emission from MGRO J2019+37 are discussed in detail, including the pulsar PSR J2021+3651 and its pulsar wind nebula PWN G75.2+0.1, two new radio-jet sources, the Hii region Sh 2-104 containing two star clusters, and the radio source NVSS J202032+363158. We also find that the hadronic scenario is the most likely in case of a s ingle accelerator, and discuss the possible contribution f rom the sources mentioned above. Conclusions. Although the radio and GeV pulsar PSR J2021+3651 / AGL J2020.5+3653 and its associated pulsar wind nebula PWN G75.2+0.1 can contribute to the emission from MGRO J2019+37, extrapolation of the GeV spectrum does not explain the detected multi-TeV flux. Other sources discussed here could contribute to the emission of the Milagro source.

Search for radio pulsations in LS I +61 303

Context. The gamma-ray binary LS 5039 and the isolated pulsar PSR J1825−1446 were proposed to have been formed in the supernova remnant (SNR) G016.8−01.1. Aims. We aim to obtain the Galactic trajectory of LS 5039 and PSR J1825−1446 to find their origin in the Galaxy, and in particular to check their association with SNR G016.8−01.1 to restrict their age. Methods. By means of radio and optical observations we obtained the proper motion and the space velocity of the sources. Results. The proper motion of PSR J1825−1446 corresponds to a transverse space velocity of 690 km s −1 at a distance of 5 kpc. Its Galactic velocity at different distances is not compatible with the expected Galactic rotation. The velocity and characteristic age of PSR J1825−1446 make it incompatible with SNR G016.8−01.1. There are no clear OB associations or SNRs crossing the past trajectory of PSR J1825−1446. We estimate the age of the pulsar to be 80‐245 kyr, which is compatible with its characteristic age. The proper motion of LS 5039 is µ� cos� = 7.09 and µ� =−8.82 mas yr −1 . The association of LS 5039 with SNR G016.8−01.1 is unlikely, although we cannot to discard it. The system would have had to be formed in the association Ser OB2 (at 2.0 kpc) if the age of the system is 1.0‐1.2 Myr, or in the association Sct OB3 (distance 1.5‐2 kpc) for an age of 0.1‐0.2 Myr. If the system were not formed close to Ser OB2, the pseudo-synchronization of the orbit would be unlikely. Conclusions. PSR J1825−1446 is a high-velocity isolated pulsar ejected from the Gal axy. The distance to LS 5039, which needs to be constrained by future astrometric missions such as Gaia, is a key parameter for restricting its origin and age.