VLBI structure of PSR B /LS2883 during the 2007 and 2010 periastron passages

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1 VLBI structure of PSR B /LS2883 during the 2007 and 2010 periastron passages Javier Moldón Marc Ribó Josep M. Paredes Simon Johnston, Ryan Shannon (ATNF -CSIRO) Adam Deller (NRAO/Berkeley) High Energy Phenomena in Relativistic Outflows III Barcelona June 28, 2011

2 Outline The radio emission from the gamma-ray binary PSR B VLBI radio observations from 2007 periastron passage. Campaign during the periastron passage. Discovery of extended emission from HESS J Gamma-ray binaries as seen by VLBI. Conclusions.

3 The gamma-ray binary PSR B

4 The binary system PSR B /LS 2883 PSR B /LS 2883 is a binary system located at 2.3 kpc that contains an Oe star and a 47.7 ms radio pulsar orbiting it every 3.4 yr in a very eccentric orbit. Non-thermal transient emission up to VHE is detected close to the periastron passage. [Moldón et al. 2011, ApJL, 732, L10 Negueruela et al. 2011, ApJL, 732, L11] [Aharonian et al. 2005, A&A, 442, 1]

5 Pulsed radio emission from PSR B PSR B shows a double radio pulse, and drastic changes in the Dispersion Measure (DM) and in the Rotation Measure (RM) of the pulses close to each periastron passage. [Johnston et al 2005,MNRAS, 358, 1069] Radio pulses disappear around 16 days before the periastron passage, and reappear 15 days after. Radio pulsar timing provides accurate values for some orbital parameters like f(mpulsar), e, P, t0, ω. [Wang et al. 2004, MNRAS, 351, 599]

6 Transient emission from PSR B /LS 2883 Radio eclipse The system displays transient unpulsed optically thin (α 0.6) radio emission up to 60 mjy close and after the periastron passage. The radio lightcurve shows two peaks centered around τ 10 and τ + 20 days. After the second peak the flux density of the unpulsed emission decreases continuously, and it has been detected up to τ days.

7 Gamma-ray binaries at AU scales

8 Radio emission in a binary pulsar An intense shock between the relativistic wind of a non-accreting pulsar and the stellar wind is produced. Particle acceleration at the termination shock leads to synchrotron and inverse Compton emission. The shocked material is contained by the stellar wind behind the pulsar, producing nebula extending away from the stellar companion. Adiabatically expanding flow produce the synchrotron emission from radio to X-rays UV photons from the companion star suffer inverse Compton scattering with the relativistic electrons from the pulsar wind

9 Expected behaviour at mas scales The cometary tail changes its direction continuously. The peak of the emission follows the path of an elliptic orbit. Astrometric and morphological changes expected [Dubus 2006, A&A, 456, 801] VLBI observations provide images at AU scales at ~2.5 kpc: mas AU

10 VLBI observations of PSR B /LS 2883 (The only gamma-ray binary with a confirmed pulsar)

11 PSR B (2007 periastron passage) We observed PSR B with Long Baseline Array (LBA) observations conducted during the 2007 periastron passage at three different orbital phases (T+1, T+21 and T+315). We used 5 antennas of the array. Observations at 2.3 GHz (13 cm). A B C Run Epoch Epoch Orbital phase A T B T C T

12 PSR B (2007 periastron passage) Observations during the 2007 periastron passage were conducted 1 and 20 days after the periastron passage to see variability changes of the transient emission, and 1 year later to find the pulsar position. Radio eclipse A B C Run Epoch Epoch Orbital phase A T B T C T

13 VLBI observations of PSR B (2007) We have just found extended emission from PSR B with Long Baseline Array (LBA) observations conducted during the 2007 periastron passage. [Moldón et al. 2011, ApJL, 732, L10] We confirm that non-accreting pulsars orbiting massive stars can produce variable extended radio emission at AU scales. The peak of the radio nebula is detected at distances between 10 and 50 AU from the binary system and with a total extension of 50 mas (120 AU). The discovery of such a structure in PSR B reinforces the link with the other known gammaray binaries, LS 5039 and LS I , for which the detection of pulsations is challenging.

14 Kinematical interpretation A simple kinematic model, using the approximation of a non-turbulent adiabatically expanding flow, as described in Kennel & Coronitti (1984). The flow speed depends only on the magnetization parameter σ when σ << 1. We only consider intertaction with an isotropic polar wind. This first approximation allow us to constrain the orientation of the orbit, given by the longitude of the ascending node, Ω, and the magnetization of the pulsar, σ. [Moldón et al. 2011, ApJL, 732, L10] The detected morphology can be accounted for if: 40º 0.005

15 2010 periastron passage of PSR B /LS 2883

16 VLBI observations of PSR B (2010) We have monitored the orbital variability of the nebula with the LBA during the 2010 periastron passage (Dec 15, 2010). We have 5 observations covering a wide range of true anomalies. A Radio eclipse B C D E Run Epoch Epoch A B T-21 T+0 Orbital phase C T D T E T

17 Pulsar gating The data are correlated with pulsar gating, only using the data during the on-pulse or the off-pulse. This technique allows us to enhance the pulsed emission or to eliminate it, and therefore, determine the position of the pulsar inside the nebula. When we receive the whole set of data, we will separate the pulsed emission from the transient emission for most of the runs. We will also obtain precise astrometry of the pulsar (binary system). We will obtain the relative position and distance between the pulsar and the nebula.

18 Discovery of extended emission from HESS J

19 HESS J HESS J is a gamma-ray binary composed by a compact object orbiting every 320±5 days the B0pe-type star MWC 148, located at 1.5 kpc. We conducted ToO e-evn observations triggered by the February 2011 X-ray outburst. Response timescale since VHE detection = 1 week (7 antennas in Europe, China and South Africa). Second ToO after 1 month to search for variability. Confirmed association with Be Star Confirmed the non-thermal nature of the radio source. Discovery of extended emission [Moldón et al., in preparation]

20 Gamma-ray binaries as seen by VLBI

21 LS I Dhawan et al. (2006), VI Microquasar Workshop

22 LS 5039 Orbital Phase (2007) (1999) (2000) [Ribó et al. 2008, A&A, 481, 17 Moldón et al. 2011, High-Energy Emission from Pulsars and their Systems Moldón et al., in preparation]

23 PSR B [Moldón et al. 2011, ApJL, 732, L10]

24 HESS J [Moldón et al., in preparation]

25 Gamma-ray binaries Pulsar VLBI LS 5039 Porb = 3.9 days? periodic orbital variability LS I Porb = 26.5 days? periodic orbital variability PSR B Porb = 3.4 years (periodic) orbital variability HESS J Porb = 26.5 days? Extended 1FGL J Porb = days??

26 Summary 1. We confirm that non-accreting pulsars orbiting massive stars can produce variable extended radio emission at AU scales. 2. The peak of the radio nebula is detected at distances between 20 and 50 AU from the binary system and with a total extension of 50 mas (120 AU). 3. VLBI radio observations can put constrains on physical parameters of the system. 4. During 2011 we will have the complete dataset to study the morphologic and astrometric changes of the radio emission, and precise astrometry of the binary system. 5. HESS J displays extended emission and peak displacement. 6. VLBI observations are a common link, useful to understand the behaviour of gamma-ray binaries.