Gravitational waves from SGRs and AXPs as fast-spinning white dwarfs

Abstract

In our previous article we have explored the continuous gravitational waves (GWs) emitted from rotating magnetized white dwarfs (WDs) and their detectability by the planned GW detectors such as LISA, DECIGO and BBO. Here, GWs emission due to magnetic deformation mechanism is applied for Soft Gamma Repeaters (SGRs) and Anomalous X-Ray Pulsars(AXPs), described as fast-spinning and magnetized WDs. Such emission is caused by the asymmetry around the rotation axis of the star generated by its own intense magnetic field. Thus, for the first time in the literature, it is estimated the GWs counterpart for SGRs/AXPs described as WD pulsars. We find that some SGRs/AXPs can be observed by the space detectors BBO and DECIGO. In particular, 1E 1547.0-5408 and SGR 1806-20 could be detected in 1 year of observation, whereas SGR 1900+14, CXOU J171405.7-381031, Swift J1834.9-0846, SGR 1627-41, PSR J1622-4950, SGR J1745-2900 and SGR 1935+2154 could be observed with a 5-year observation time. The sources XTE J1810-197, SGR 0501+4516 and 1E 1048.1-5937 could also be seen by BBO and DECIGO if these objects have MWD ≲ 1.3M⊙ and MWD ≲ 1.2M⊙, respectively. We also found that SGRs/AXPs as highly magnetized neutron stars are far below the sensitivity curves of BBO and DECIGO. This result indicates that a possible detection of continuous GWs originated from these objects would corroborate the WD pulsar model.

Resolving dichotomy in compact objects through continuous gravitational waves observation

“…A similar analysis for SGRs and AXPs was recently carried out by Sousa et al (2020b). They considered all detected SGRs and AXPs and found that BBO and DECIGO should be able to detect them within 1 to 5 yrs of integration time if they are WDs, whereas no proposed detectors could detect them if they are NSs.…”

“…It is evident that a pulsar-like object can continuously emit GWs at two frequencies, Ω and 2Ω. This is another important consequence which is missing in the analysis of Sousa et al (2020b).…”

“…Earlier Sousa et al (2020b) considered an empirical formula for the ellipticity, 𝜖 = 𝜅(𝐵 2 s 𝑅 4 p /𝐺 𝑀 2 ) sin 2 𝜒, with 𝐵 s being the surface magnetic field and 𝜅 a distortion parameter. However, this formula is valid only for poloidal magnetic field configurations.…”

“…In addition, 𝜅 depends on the EoS of the star and the magnetic field configuration. However, Sousa et al (2020b) chose the same 𝜅 for their calculations for both WDs and NSs. We know that EoSs are very different for NSs and WDs.…”

“…In this paper, we consider only those sources that are not yet confirmed to be NSs and self-consistently calculate the signal-to-noise ratio (SNR) and thereby the necessary observation time, for the proposed GW detectors to detect these sources. Moreover, Sousa et al (2020b) assumed the magnetic fields inside these sources have a dipole-like configuration. However, Wickramasinghe et al (2014) showed that the magnetic field inside a compact object is likely to have a dominant toroidal field owing to the action of an Ω−dynamo at the time of its birth.…”