Gamma-Ray Detectability and Viewing Geometry of MSPs

Size: px

Start display at page:

Download "Gamma-Ray Detectability and Viewing Geometry of MSPs"

Daisy Kristina Perkins
7 years ago
Views:

1 Gamma-Ray Detectability and Viewing Geometry of MSPs (based on Guillemot & Tauris, MNRAS in press, 2014) Lucas Guillemot 5 th BONN workshop on the Formation and Evolution of Neutron Stars MPIfR / AIfA Bonn Uni., 05/03/14

2 Introduction The Large Area Telescope on Fermi (launched in June 2008) has detected pulsed γ-ray emission from 147 pulsars. About 40% (62) are MSPs (see +of+lat-detected+gamma-ray+pulsars) Detected γ-ray pulsars have the largest values of Ė (= 4π 2 I Ṗ/P 3 ) / d 2. Much larger fraction of MSPs with high values of Ė / d 2 are detected in γ rays, compared to the normal pulsar population. Nevertheless, some high Ė / d 2 MSPs escape detection in γ rays. Possible explanations: Figure 9. Radio and gamma-ray light curves for the four MSPs in our sample with Fermi LAT detections. Two pulsar cycles are shown for clarit profiles are based on 1.4 GHz observations conducted at Parkes, while the gamma-ray profiles GHz (red) were obtained light curves by selecting for Fermi PSR J LAT photons with re directions found within 5 of the MSPs, and with energies larger than 0.1 GeV. The photons were weighted by the probability that they origina pulsars as described in e.g. Kerr (2011). Photons with weights smaller than 0.01 were rejected. Horizontal dashed lines show the estimated backgr obtained by following the method described in Guillemot et al. (2012). The grey shadedfrom regionsng indicate et al., themnras OFF-pulsein intervals press used (2014). for the spect presented in Section 4.10, the ON-pulse regions being defined as the complementary intervals. underestimated distance (especially if based on NE2001), unfavorable orientation, leading to γ-ray beams not crossing our line of sight. expected energy fluxes for these pulsars much smaller than the L. Guillemot, lowest 05/03/14 value reported in Abdo et al. (2013) for an MSP, because of the generally large distance values, with the notable exception of LAT (blue, E > 0.1 GeV) and Parkes 1.4 its spin-down power into gamma-ray emission, or its g beams may not cross the Earth s line of sight. The high-ė 2 MSPs in this sample could contribute to the diffuse emi

3 γ-ray emission from MSPs VENTER, HARDING, & GUILLEMOT Increasing magnetic inclination α 814 Vol. 707 Figure adapted from Venter et al. (2009). Here: plots of the predicted radio and γray emission of MSPs under the Outer Gap model. Configurations with low ζ values and/or low α values are not detectable (no γ-ray emission at all, or very weak modulation of the signal). Figure 14. Sample light curves for an OG1 model with P = 2 ms. (A color version of this figure is available in the online journal.) Increasing viewing angle ζ 3

4 Can we constrain the geometry of MSPs? Methods for extracting pulsar geometry angles (namely, magnetic inclination α and viewing angle ζ): fits of radio polarization (Radhakrishnan & Cooke 1969). Generally does not work for MSPs radio and γ-ray light curve modeling (e.g. Johnson 2011), for γ-ray-detected MSPs only our approach: binary evolution. Key arguments: MSPs originate from LMXBs. The long time-scale ( yrs) of mass transfer in LMXBs should cause the spin axis to align with the orbital angular momentum vector: ζ = i. mass function: relation between the companion mass, the pulsar mass, the inclination i. How to calculate the companion mass? Tauris & Savonije (1999): unique relationship between Porb and MWD for MSPs orbiting He WD with 0.13 < MWD / M < f(m NS,M WD,i)= 4 2 G radio emission cone γ-ray emission fan beam (a p sin i) 3 P 2 orb = (M WD sin i) 3 (M NS + M WD ) 2 4

5 M WD - P orb relation (TS99) Adapted from Tauris and van den Heuvel, ApJ Lett. (2014). 5

6 Predicting viewing angles for a sample of MSPs We selected MSPs (P < 30 ms) in binary systems likely to be orbiting He WD companions (`BinComp parameter in the ATNF pulsar catalog), completed with other MSPs with ζ constraints from light curve modeling (Johnson 2011) or sin(i) measurements. For each MSP we calculated Ė / d 2, accounting for the Shklovskii effect whenever possible. Available sin(i) and MNS values were compiled. If MNS not known, assumed / M (average observed for MSPs with He WD companions, see Tauris et al. 2012). Viewing angles ζ were finally assigned to the MSPs in the sample in this order: for MSPs with ζ constraints from light curve modeling, the best-fitting value was used. if not, but constraints on sin(i) exist, use ζ = i. if none of the above, use ζ = i calculated using the TS99 relation. Obtained a sample of 70 MSPs with ζ estimates. 6

7 Example for PSR J

8 Example for PSR J TS99: MWD = M. In line with Bassa et al. (2003) (~0.2 M ). 7

9 Example for PSR J TS99: MWD = M. In line with Bassa et al. (2003) (~0.2 M ). Predicted ζ value: deg. 7

10 Predicted vs measured ζ values ζ from sin(i) measurements or TS99 if not available. ζ from light curve modeling analyses. 8

11 Predicted vs measured ζ values ζ from sin(i) measurements or TS99 if not available. Good overall agreement between ζpredicted and ζlc Modeling Potential outlier: J , with ζlc Modeling = 8. Would imply MWD = 1.9 M Alternative solution: ζlc Modeling = 32. Average difference: 8 with an rms of 8. Spearman coefficient: 0.88 ~ 1 (p-value = 8e-5). Our procedure provides a reliable method for estimating ζ. ζ from light curve modeling analyses. 8

12 Gamma-ray detectability metric green stars: gamma-ray MSPs. red circles: MSPs undetected in gamma rays. filled symbols: Ė corrected for the Shklovskii effect. γ-ray-detected MSPs occupy the upper part of the plot: Ė / d 2 is a good measure of γ-ray detectability. Define two samples: Ė / d 2 > 1.5e34: 75% of MSPs are detected in γ rays. Ė / d 2 > 8e32: 50% of MSPs are detected in γ rays. 9

13 Viewing angle distribution Distributions of the viewing angle ζ for the two pulsar samples. MSPs not detected in γ rays appear to be distributed towards smaller ζ values on average KS tests: p-value of 3% for both samples. Marginally significant effect, but we postulate that low ζ values are at least partly responsible for the non-detection of high Ė / d 2 MSPs. 10

14 Conclusions The viewing angles of binary MSPs are well described by the orbital inclination angles, confirming that spin axes align with the orbital momentum vector during recycling. We find evidence for slightly different viewing angle distributions for γ-detected and undetected MSPs: unfavorable orientations can prevent us from seeing γ-ray pulsations New Pass8 data: the LAT s sensitivity to pulsed γ-ray emission has increased substantially. Stay tuned for more MSP detections in γ rays 11

Neutron Stars. How were neutron stars discovered? The first neutron star was discovered by 24-year-old graduate student Jocelyn Bell in 1967.

Neutron Stars. How were neutron stars discovered? The first neutron star was discovered by 24-year-old graduate student Jocelyn Bell in 1967. Neutron Stars How were neutron stars discovered? The first neutron star was discovered by 24-year-old graduate student Jocelyn Bell in 1967. Using a radio telescope she noticed regular pulses of radio