The search for radio emission from the exoplanetary systems 55 Cancri, υ Andromedae, and τ Boötis using LOFAR beam-formed observations
The search for radio emission from the exoplanetary systems 55 Cancri, υ Andromedae, and τ Boötis using LOFAR beam-formed observations
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使用 LOFAR 波束形成观测搜索来自系外行星系统 55 Cancri、υ Andromedae 和 τ Boötis 的无线电发射
DOI：
10.1051/0004-6361/201937201
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发表时间：
2020
影响因子：
6.5
通讯作者：
I. de Pater
中科院分区：
文献类型：
--
作者：
J. Turner;P. Zarka;J. Grießmeier;J. Lazio;B. Cecconi;J. Emilio Enriquez;J. Girard;R. Jayawardhana;L. Lamy;J. Nichols;I. de Pater
上下文探测系外行星的无线电辐射将开辟一个充满活力的新研究领域。观测行星极光射电辐射是探测系外行星磁场的最有前途的方法，对它的了解将为了解行星的内部结构、大气逃逸和可居住性提供有价值的见解。
目标。我们提出了LOFAR（低频阵列）低波段天线（LBA：10-90 MHz）的圆极化波束形成的外行星系统55 Cancri，仙女座α和τ牧夫座观测。所有这三个系统都被预测为寻找系外行星射电辐射的良好候选者。
方法.我们应用我们开发的BOREALIS管道来减轻射频干扰，并搜索缓慢变化和突发性无线电发射。我们的管道以前已经定量基准衰减木星无线电发射。
结果我们在14-21 MHz范围内探测到了牧夫座τ系统的圆偏振猝发辐射，其通量密度为~890 mJy，统计显著性为~3σ。对于该检测，我们在OFF波束中没有看到任何信号，并且我们没有发现可能导致误报的任何潜在原因。我们还尝试性地探测到牧夫座τ在21-30 MHz范围内的慢变圆偏振辐射，通量密度约为400 mJy，统计显著性>8σ。慢发射的结构在时间-频率平面，并显示出过剩的ON光束相对于两个同时关闭光束。虽然突发发射似乎相当强大，但仔细检查对缓慢变化信号的真实性产生了一些怀疑。我们详细讨论了所有的参数支持和反对一个实际的检测，并得出方法测试，也将适用于未来的搜索。此外，在14-38 MHz的一次爆发发射观测中，发现仙女座星系有一个~2σ的边缘信号，而在55 Cancri星系中没有检测到信号，我们对55 Cancri星系的观测时的通量密度设定了3σ上限73 mJy。
结论.假设检测到的信号是真实的，我们讨论他们的潜在来源。它们的来源可能是牧夫座τ行星系统，一个可能的解释是来自于系外行星牧夫座τ B通过回旋脉泽机制的无线电发射。假设一个行星的起源，我们推导出的行星极地表面磁场强度的限制，发现与理论预测兼容的值。需要用LOFAR-LBA和其他低频望远镜（如NenuFAR或UTR-2）进行进一步观测，以确认这可能是首次探测到系外行星无线电信号。
Context. The detection of radio emissions from exoplanets will open up a vibrant new research field. Observing planetary auroral radio emission is the most promising method to detect exoplanetary magnetic fields, the knowledge of which will provide valuable insights into the planet’s interior structure, atmospheric escape, and habitability.
Aims. We present LOFAR (LOw-Frequency ARray) Low Band Antenna (LBA: 10–90 MHz) circularly polarized beamformed observations of the exoplanetary systems 55 Cancri, υ Andromedae, and τ Boötis. All three systems are predicted to be good candidates to search for exoplanetary radio emission.
Methods. We applied the BOREALIS pipeline that we have developed to mitigate radio frequency interference and searched for both slowly varying and bursty radio emission. Our pipeline has previously been quantitatively benchmarked on attenuated Jupiter radio emission.
Results. We tentatively detect circularly polarized bursty emission from the τ Boötis system in the range 14–21 MHz with a flux density of ~890 mJy and with a statistical significance of ~3σ. For this detection, we do not see any signal in the OFF-beams, and we do not find any potential causes which might cause false positives. We also tentatively detect slowly variable circularly polarized emission from τ Boötis in the range 21–30 MHz with a flux density of ~400 mJy and with a statistical significance of >8σ. The slow emission is structured in the time-frequency plane and shows an excess in the ON-beam with respect to the two simultaneous OFF-beams. While the bursty emission seems rather robust, close examination casts some doubts on the reality of the slowly varying signal. We discuss in detail all the arguments for and against an actual detection, and derive methodological tests that will also apply to future searches. Furthermore, a ~2σ marginal signal is found from the υ Andromedae system in one observation of bursty emission in the range 14–38 MHz and no signal is detected from the 55 Cancri system, on which we placed a 3σ upper limit of 73 mJy for the flux density at the time of the observation.
Conclusions. Assuming the detected signals are real, we discuss their potential origin. Their source probably is the τ Boötis planetary system, and a possible explanation is radio emission from the exoplanet τ Boötis b via the cyclotron maser mechanism. Assuming a planetary origin, we derived limits for the planetary polar surface magnetic field strength, finding values compatible with theoretical predictions. Further observations with LOFAR-LBA and other low-frequency telescopes, such as NenuFAR or UTR-2, are required to confirm this possible first detection of an exoplanetary radio signal.
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