Spindown measurement of PSR J1745-2900?

This is another update on the pulsar near Sgr A* that I’ve been writing posts about the last days. This piece of news comes again from the world of X-ray astronomy, after the last updates using radio observations.

Another X-ray detection (by Swift) and a spindown?

On Sunday, May 5, 2013, another ATel #5046 followed, this time from new X-ray observations using the Swift satellite following a Target of opportunity (TOO) request. The authors claim to have measured a preliminary spindown rate of the pulsar, which they find is

(…) Pdot = (2.5+/-1.1)e-11, implying a magnetic field B = 3e14 G, spin-down power Edot = 2e34 erg/s, and characteristic age P/2Pdot = 2.4 kyr, for the dipole model. (…)

All of this would place the pulsar squarely amongst the other known magnetars. The authors further caution that

(…) the value of Pdot may be significantly larger than the long-term spin-down rate due to the possibility of recovery from a glitch (…)

In other words, the large Pdot they claim to have measured might just appear large because the pulsar just has suffered a glitch and is now recovering from it. The long-term spindown of the pulsar could be much smaller, and would not be detectable over the very short time baseline.

A new measurement of the spin period

The ATel also has a new measurement of the spin period itself. The astronomers find a value of 3.7635603(68) s at MJD 56415.4186. Note that this epoch is a day earlier than the latest Effelsberg observations reported in ATel #5034. This is a good chance to update my table of the available spin period measurements of PSR J1745-2900, the name assigned in the new ATel:

Telescope  (EM range)  MJD        period (s)
NuStar     (X-ray)     56408.05   3.763542(8)
Effelsberg (8.35 GHz)  56414.10   3.76354(1)
Swift      (X-ray)     56415.42   3.763560(7)
Effelsberg (8.35 GHz)  56416.07   3.76343(2) 
Effelsberg (4.85 GHz)  56416.07   3.76343(2)

There’s something that looks, lets say interesting. The first spin periods measurements (one in the X-ray range, one in the radio range) at MJD 56408.05 and 56414.10 are perfectly consistent, no period change is apparent. The second X-ray measurement from MJD 56415.42 (just a day later) then shows a largely higher spin period. The radio measurements another day later then show a smaller spin period again.

Glitchy pulsar?

What’s going on here? I don’t know. There might be some systematic difference in how the spin periods are measured, which could explain the apparent changes. But let us assume that both radio and X-ray observations are correct. To make them consistent, one has to assume that the pulsar glitched between MJD 56415.42 and MJD 56416.07. The first epoch is that of the Swift measurement which made it look like the pulsar had spun down. The second epoch is that of the new Effelsberg measurements which look like the pulsar had spun up, due to the assumed second glitch.

I also not sure I understand what’s going on with the possible first glitch postulated by the new ATel. It should’ve happened somewhere between the first measurement by NuStar (the discovery) and the new Swift measurement, but the radio measurements in between are consistent with the discovery… So to me it really does not look like there could’ve been any first glitch at all. After all the spin period apparently stays constant for six day, but is suddenly larger only a day later.

It looks like we’ll have to wait for further measurements to see how this will pan out.

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